There is a fish that could be considered the patron saint of radical feminism. The female pipefish mates with a partner who fertilizes her eggs in the normal way, but then the male is literally left holding the babies. In this topsy-turvy species, it’s the males who have a pouch in which to carry the brood, and the females, as if trying to make up for the countless males across the animal world who leave their partners in the lurch, who carelessly wander off in an attempt to seduce another male with an empty pouch. And no, she didn’t phone the next day, either.
It’s a perfect twist in the sexual story. Females scour the territory, hungry for sex, while males selectively pick and choose the best genetic bet for their next pouch-load of eggs. Female pipefish get to play John Malkovich’s Valmont to the male’s Michelle Pfeiffer; the seducer becomes the seductress, without caring about the consequences.
In many species of pipefish the females are more brightly coloured than the males, advertising their fertility and health. The males are choosy about who they allow to put eggs in their pouch. The male pipefish is the brake on sexual activity; the female just wants to ‘impregnate’ as many males as she possibly can, and she has to compete for the privilege.
It all depends on who makes the most investment in the offspring. In most species, the females get saddled with the work and the males have to compete for females. And when there is a high level of competition among the males there are some interesting consequences, particularly with respect to size.
Consider a monkey, the mandrill. The male makes a big show of his masculinity; he has a brightly coloured face and he’s three times the size – and weight – of a female. Male hamadryas baboons and gorillas are also outsized compared to the female. But the species for which size really does matter is the elephant seal. The male is a whopping seven times the weight of the female (not that the females could be called petite themselves). From the female’s point of view, sex with a male elephant seal must be like having a lustful liaison with a Sumo wrestler – and nearly as dangerous.
This mismatch in male and female body sizes and forms is actually surprisingly revealing about the sexual habits of any particular species. Sexual dimorphism, as it’s officially known, is at its strongest among species that mate according to the harem system: one male and a number of females.
Each dominant male mountain gorilla is typically surrounded by a generous handful of females in any one group. There is bound to be fierce physical competition for the right to mate with a large clutch of females. In the past, those gorillas who just so happened to be bigger and stronger ended up having an edge in these macho contests, so the evolutionary pressure for the males to increase in size was born. Sheer bulk is clearly important where physical violence is concerned, but it’s certainly not the only weapon in the armoury. It turns out that some animals’ characteristics which we may assume have evolved as protection against predators are in fact more commonly used to fight off other males in the quest for sex. Deers’ antlers are a good example of this.
Among elephant seals, most calves are fathered by less than 5 per cent of the males. They fight, just like Sumo wrestlers, by rearing up and trying to knock each other over. The male who wins the fight gets the girl and the ultimate prize of siring offspring and passing on his genes, while the skinnier contenders have to wait, offspring-less, in the wings. Survival of the fattest, in this case.
So what about humans, who have so many of their genes in common with all other animal species? Human males also tend to be slightly bigger than females. That simple fact actually reveals something fascinating about our hominid ancestors. Our ancient male forefathers probably also had to fight for mating rights, suggesting that early human groups were commonly practising ‘polygyny’ – in other words, a proportion of the males would monopolize all the females. If everything else is equal, and if the theory is correct, we were living in a mild human version of the harem system, although not nearly to the same extent as the elephant seals or gorillas. But it is likely that there was some physical competition between males for access to the females. Men fought over women, inevitably, as they still do.
For the Yanomamo people of the Amazon jungle, fighting and warfare is a way of life. Up to one quarter of all Yanomamo men die in tribal battles, but the surviving heroes often then go on to be incredibly prolific in the mating game. The original founder of one group of villages claimed eight wives and forty-eight children as his own. As luck would have it, his sons also turned out to be good at strutting their stuff; in the end, some three-quarters of the entire population were descendants of the original founding male. The anthropologist Napoleon Chagnon spent most of his life observing and studying the Yanomamo people. In an attempt to understand the men’s strong leanings towards violence and fighting, and why they have among the highest rates of homicide in the world, he asked a group of them why it was so important in Yanomamo society. There were astonished looks all round. ‘What? Don’t ask such a stupid question! It is women! Women! We fight over women!’1
An ethnographic survey of 849 human societies across the globe, including all traditional tribal and hunter-gatherer groups, showed that 708 of these are polygynous or allow the powerful or wealthy males to indulge in polygyny. It seems that, from a strictly evolutionary point of view, the competitive nature which pushes a man to pursue power and status is strongly related to sex – and the number of women he can have sex with. That certainly puts modern politics into a whole new light, at any rate.
The anthropologist Laura Betzig maintains that tyrants and despots have kept harems whenever possible. When hunter-gatherers settled down around ten thousand years ago and started to build villages, cities and kingdoms, there was greatly increased centralization of power. Leaders tended to rule by excessive authority and sometimes terror, so there was much more potential for polygyny. The record stands with an Indian emperor called Udayama, who ruled in the fifth century BC and supposedly kept a harem of sixteen thousand women. (Even assuming Udayama had sex with two women every night, it would still have taken him almost twenty-two years to sleep his way through the entire harem.) One of the earliest biblical examples is Ahasuerus, King of Persia, who certainly flaunted his power by having as many attractive women in his harem as could be found throughout the 127 provinces of his empire. But he was still susceptible to female wiles, and his favoured (excessively beautiful and very young) queen, Esther, was able to exploit this vulnerability.
The Romans, despite having a reputation as monogamous and faithful lovers, kept slaves for the purposes of sex. Child slaves who were raised in the master’s home were called vernae, and traditionally historians have assumed their fathers were also slaves, but Betzig argues these children were almost certainly fathered by the master of the house. Children born of these illicit liaisons were often treated well, educated alongside the master’s ‘legitimate’ children, and even sometimes inherited parts of his estate.
There was a tradition in medieval Europe concerning a right called jus primae noctis, or ‘right of the first night’, an ancient privilege which gave the lord of the manor the right to bed a peasant’s bride on their wedding night. (It is unclear how often this privilege was claimed; possibly not all servants were as lucky and as wily as Susanna in her dealings with Count Almaviva in Mozart’s Marriage of Figaro.) And while the lord would generally have a wife, his household was set up as an unofficial harem of servant girls and lower-class young women for whom the manor served as a haven from poverty. Count Baudoin, a cad of the thirteenth century, was typical, according to Betzig. He fathered twenty-three illegitimate children under his own roof and saw to it that his bedchamber had interconnecting doors to the servant girls’ quarters, to the rooms of adolescent girls upstairs and even to what was known as ‘the warming room’ – the place where the infants of the house were suckled.
As the structure of Western society changed, as the landed gentry began to lose power and wealth and as life became more democratic and egalitarian, polygyny on this scale died out. Household staff were still employed by aristocratic families and the landed gentry, of course, but their numbers decreased dramatically, and they got on with the more mundane tasks of polishing the silver and serving the tea. Still, those who have seen the film Gosford Park will recognize that medieval male aristocratic practices and relationships below stairs did not die out.
But societies, ancient or modern, don’t always order their affairs according to human instinct. They might have invented ways, means and even traditions to try to repress instinct. The institution of marriage may in fact be a reaction to our innately unfaithful tendencies that are, for reasons we’ll consider later, thought to be undesirable or destructive.
The physical clues to human sexuality give us a mixed message. Size does matter, and I don’t only mean the relative height and weight of males and females. Believe it or not, the size of an animal’s testicles can tell us a lot about its sexual practices. Chimpanzees have extremely large testicles and produce prodigious amounts of sperm. The reason for this is that chimps have a very relaxed attitude to sex. Mating is relatively unregulated and there seems to be no awareness of the paternity of any of their offspring. Chimps, seemingly, could be regarded as the originators of the free-love hippy commune. When a chimp female is in her fertile ‘oestrus’ period, she has sex with a large number of males, not just the alpha male, who may try but often fails to dominate the mating game. Therefore each male’s sperm has to compete to fertilize the ovum, and the more spermatozoa a male produces, the more chance he has of becoming a father – literally by flooding the female’s genital tract with sperm.
Silverback gorilla males are large, aggressive and scary beasts, but they have very small testicles. This reflects the fact that Silverback alpha males take possession of harems of females and are secure in the knowledge that females rarely sneak off for an illicit rendezvous in the woods. Gorillas rarely have sex, because for those males with a harem sexual access is guaranteed; therefore they seem to need only a small amount of sperm.
Measured as a proportion of body weight, human testicles are four times the size of a gorilla’s but less than a third the size of a chimpanzee’s. What may we infer from this?
Human males tread the middle ground. They’ve acquired testicles big enough to combat a moderate amount of sperm competition from other males, but they certainly don’t need the heavy artillery of the chimpanzee gonads. Taking these measurements as indicators of our ancient sexual practices, we were probably a mixed bag. We had an element of the harem system, judging by the relative size of the male and female body, but we were also equipped for occasional female promiscuity too. It also seems that testicular size varies slightly across the human species. Even allowing for variations in body size, one study has shown that Japanese and Korean men tend to have rather smaller testes than Europeans, and the Chinese weigh in at half that of their Danish counterparts.
Penile size is also probably very important in primates. Once hominids were standing upright on two legs, their genitals would have been much more apparent to other members of the group. Moreover, the human penis is big. The average size is around five to six inches, compared with three for chimpanzees and perhaps half that in gorillas. A number of biologists have speculated about these dimensions, among them my friend the very eminent reproductive biologist Professor Roger Short. Various hypotheses exist; the long penis may be more attractive to women or it may be more threatening to competing males. I can certainly believe the latter to be true. As a young teenager, I remember being subjected to the sight of a sixteen-year-old adolescent who, in what was an undoubtedly aggressive display, exposed himself to me, fully erect, in the changing room of the local municipal swimming baths. What is interesting, in retrospect, is that this seemed at the time to be threatening behaviour rather than any mere show of potential sexual prowess. So, just possibly, a long, prominent penis frightens off other males who might compete for the same delights.
Lynn Margulis, Professor in the Department of Biology at the University of Massachusetts, Amherst, and her son, science writer Dorion Sagan, are a very well-known team in the biological sciences. In their book Mystery Dance: On the Evolution of Sexuality, they argue that a long penis might have been selected by evolution. They suggest that when a female mated with several males, the male who delivered his sperm closest to the uterine cervix – and therefore the site of fertilization, the Fallopian tubes – would have had the best chance of impregnating his partner and producing offspring.
Roger Short has argued, I think more cogently, that a thick penis (a notable feature in humans) may also be the result of natural selection. He points out that thickness is more likely to produce female sexual satisfaction, and that in consequence a male with a thicker penis would be at an advantage when it came to mating. (Incidentally, in comments reported elsewhere, Professor Short points out that having a penis and testicles is not entirely a bed of roses. Castrates live longer, as several studies show. Testosterone production continues at some cost to the host: for example, heart attacks are more common in men producing high levels of the hormone. For the fertile male, says Roger Short, ‘It’s a short life but a merry one.’)
It is also fair to emphasize that our sexual habits and evolutionary strategies have changed somewhat over the past five or six million years. In Lucy’s time (remember, Lucy was the petite Australopithecus afarensis female living around three million years ago), the difference in size between men and women was far greater. If we use size alone as an indicator, it appears that Lucy and her contemporaries were more likely to live according to the harem system.
But for modern Homo sapiens, there are two opposing forces: the attractions of the harem and the negative social and emotional aspects of promiscuity. The push and pull of sexual instinct might well have met somewhere in the middle. Perhaps monogamy is our ‘natural’ state of rest, the solution to the complex mating equation. But before all the incurable romantics and the deeply religious get too excited, this doesn’t necessarily entail lifelong monogamy. As we shall see, it may mean we are simply built for one sexual partner at a time – with lots of partners over the course of a lifetime.
The harem system is dangerous, particularly for males low in the pecking order. To be an undesirable, low-status male in a harem-led society is an unenviable fate (though possibly the fate of the male eunuch who traditionally looked after the ladies of the Eastern harem was even less desirable). In the main, our instincts for competition mean that males excluded from the system do not take their fate quietly and are likely to be a source of aggression and violence. This is why polygyny has tended to crop up in societies where power is most centralized; high status enables the Mao Tse-tungs of this world to control and repress any violent uprisings from the dispossessed males.
The harem system is also bound to cause problems among the wives. In Mormon families in which polygyny is still practised – although it is illegal and Mormons have officially renounced it, according to some reports there may still be as many as eighty thousand polygynists in the American state of Utah alone – wives have naturally reported feelings of jealousy and stress which emerge from conflicts between the women. In the USA, increasing numbers of conflicts are also reported by the courts. I do not know how typical is the case of Tom Green, a fundamentalist Mormon from Utah with five wives. He recently faced up to twenty-five years in prison on four counts of bigamy. He took his ‘head wife’ in 1986 when she was thirteen, and as a result he is also facing statutory rape charges. He had a job, but it did not pay enough to support his twenty-five living children, with another three on the way, so he has also been involved in a long-running welfare scam which allegedly netted fifty-four thousand dollars over a four-year period.
Our sexual psychology is rife with complications. The relative sizes of men and women suggest promiscuity, harems and polygamy, but there are excellent reasons for supposing that monogamy, or at least ‘serial’ monogamy, has always been the most popular family set-up. Practically, and in evolutionary terms, we can see this makes sense. On the savannah, when conditions were hostile, a male human or pre-human hominid might have found it difficult to maintain and protect more than one sexual partner and their respective children. Rarely might he have been in a position to commandeer enough resources to care for and supply a large group of females, as well as protect them from being snatched by rival males disgruntled at the lack of available sexual partners. To me, it seems quite probable that it wasn’t until humans constructed dwellings, with some sort of semi-permanence, that this pattern changed. So the picture of the biblical forefathers, such as Jacob, being able to support several wives and partners is a reflection of increasing stability in human society as we slowly ceased to be merely hunter-gatherers.
Where does plain, ordinary, monogamy fit in? Famously, birds do it, if not bees. Most bird species pair up to breed, both the mother and the father becoming involved in raising the chicks. Baby chicks are not dependent on the mother’s breast for their daily feeds, so male birds can take at least half the workload, unlike mammals. Most of the world’s mammal species do not pair off into couples; the amount that do is thought to be around only 5 per cent. Most are solitary beasts, and the brief moment of pleasure is more or less the only contact a male has with the mother of its offspring. Some species, like wolves or, as we have seen, chimpanzees, operate on a communal basis whereby a group takes care of the babies born within it, but the males do not seem to know or care which of the babies biologically belong to them.
Though monogamy is relatively uncommon in the animal kingdom, and rarely do the parents split the workload of raising their offspring, there are a few exceptions. Gibbons are monogamous for life, as are a few species of European and North American birds. And so are we, at least in theory; ‘till death us do part’ is still the defining moment in the modern marriage ceremony. But as we’ll see, our evolutionary past, our instinct for survival and even our biology shape our sexual morals too.
Even allowing for our high divorce rate, the Western world has been one of the most staunchly monogamous societies in history. ‘We are firmly attached’, said Saki (the Edwardian author H. H. Munro), ‘to the Western custom of one wife and hardly any mistresses.’
Monogamy does not mean, however, that we do not have affairs, because we do. Some estimates of the number of British married men or women who have extra-marital affairs can run as high as 50 per cent. We already know some of the genetic reasons why, it seems, men want to have affairs. They’re programmed for sexual variety, to spread their genes; if there’s a chance of impregnating another female – especially one who is already married and would therefore not have to be cared and provided for – then he may well have stumbled across the best evolutionary knock-down cut-price bargain of all time: all the benefits of the genetic legacy with none of the work or paternal investment in bringing up the child.
Alongside the chance of hitting the genetic jackpot is the rich cultural tradition of adultery and the pleasures one finds in illicit sex. The joy of adultery is even given a cryptic reference in the Bible: Proverbs tells us that ‘stolen waters are sweet, and bread eaten in secret is pleasant’. Extra-marital affairs are, in many hunter-gatherer societies today, as well as in more developed cultures, a rich source of gossip and an important aspect of communal life.
While it’s easy to see what a man may gain from ‘a bit on the side’, what, from the female perspective, is the evolutionary advantage to taking on a lover? We all know that Charles Darwin discovered the idea of sexual selection – the theory that some individuals are inherently more successful than others when it comes to competing for mates – but it seems he was rather innocent about sex itself. He made the assumption that the females of most species were naturally monogamous. It is also true that many of his contemporaries thought that women did not really enjoy sex, and they doubted the existence of the female orgasm. One notable Victorian doctor who specialized in giving sexual advice, Dr Acton, writing in 1860 in his book Disorders, regarded it as fact that female pleasure was an aberration and a rarity. And for decades afterwards, many biologists clung to this notion of natural female monogamy.
But as Victorian attitudes died, the reality was slowly revealed. Females of numerous species, not to mention women themselves, knew all about affairs. We now know that for women in all societies infidelity is a regular occurrence. Indeed, genetic studies in certain rural parts of the UK have thrown up the possibility that up to 15 per cent of children were not fathered by their ‘official’ father. But to be fair, these studies are based only on blood group evidence and possibly deserve re-evaluation.
One way of explaining female infidelity is that it’s a woman’s way of hedging her bets. The anthropologist Marjorie Shostak describes how Nisa, a !Kung woman from the Kalahari Desert, regularly sleeps with other men, despite being married. ‘A woman should have lovers wherever she goes,’ Nisa told her. ‘If she goes somewhere to visit and is alone, then someone there will give her beads, someone else will give her meat, and someone else will give her other food. When she returns to the village, she will have been well taken care of.’ Even in studies of more contemporary societies, this shines through. In the words of one woman, ‘Men are like soup – you always want to have one on the back burner just in case.’
The security of knowing there is more than one ‘provider’ for you and your children is not to be taken lightly, especially in a harsh desert environment where scarcity of resources is an ever-present and dangerous threat. In addition, a lover provides an extra insurance policy: if your husband dies or is killed on the hunting grounds, there is someone else to help you take care of the kids. So, from the point of view of material subsistence and wealth, lovers can be profitable. Nisa could also be lucky and snag a high-status male as an occasional lover, which could bring added benefits and respect from others within the group.
Adultery, for a woman, is also about dipping into the genetic pool. Your current husband may be infertile, or may simply carry poor genes. He may be weak, sickly or a poor hunter. Illicit adultery is one way of introducing different DNA into the litter without destroying the stability of the family structure; and the more desirable the man, the more desirable his genes, and vice versa.
The ubiquity of female adultery makes me wonder about the evolutionary psychologist’s portrait of the sexually coy, choosy female. The desire for sex is ever present. In societies with very relaxed morals about non-marital sex, the women are considered just as promiscuous as the men.
Biology also reveals fascinating insights into the long history of female infidelity. In the late 1960s, the entomologist Geoffrey Parker, now at the University of Liverpool, was sitting down to another long day of heady investigation into the behaviour of flies cavorting around dung pats when he made a startling observation. He noticed that the female flies regularly mated with more than one male. Subsequently, he was the first to recognize that sexual selection doesn’t actually stop at sex, it can continue even after copulation and right up to the point when fertilization occurs. This triggered off a whole field of investigation into what is now known as ‘sperm competition’.
Animal studies revealed that this type of competition is possibly a selective force in evolution, leading to a variety of behavioural, physiological and anatomical adaptations, each aiming to ensure the success of the individual male’s sperm. As we’ve seen, increased testicle size meant one male could flood a female with more sperm than another, increasing his genes’ chances of making it across the finishing line and to the egg first. The very fact that this adaptive mechanism has evolved confirms that female animals have not been monogamous over evolutionary history. Had they been, sperm competition in all its guises could not, by definition, exist.
Intriguing evidence of sperm competition and selection is certainly found in animal and insect studies. One of the most studied insects is the fruit-fly Drosophila. It may seem bizarre that a fly has such a major place in the annals of biology, but because of its large chromosomes, its breeding behaviour and the fact that it has many genes known to be very similar to those found in mammals, it has been of great interest to geneticists for a long time.
A recent study by Dr A. G. Clark in the USA shows that among female fruit-flies in the wild, 80 per cent clearly harbour sperm from more than one male, and that, curiously, the male who mated last is the insect who becomes the proud father. In some other species, too, the females, it seems, get a chance to be choosy about the father of their offspring, even as his sperm is on the way to her egg. Sometimes known as ‘cryptic female choice’, it seems there are dozens of ways a female may control who gets to be daddy after mating with more than one male has taken place. Female hens tend to fancy socially dominant males, but because all males are significantly bigger than the females, subordinate males often manage to mate with females anyway. A recent study showed that hens in a free-ranging wild population frequently ejected semen immediately after mating, and were more likely to do so after having copulated with a subordinate male. There’s also evidence of other female animals retaining the sperm of different males in different internal stores and choosing to release it selectively. While all these are observations from animal studies, it makes you wonder how much (unconscious) control human females may have in this extraordinary realm.
But what about human sperm? One survey carried out on a large group of women uncovered the fact that one in every thousand copulations was with a second male. According to the figures and calculations, this is eight times the frequency needed for full-scale, all-out sperm war to have evolved. Though I haven’t measured it myself, it’s been calculated that a man’s sperm volume relative to his body weight is in fact twice that found in primate species which are known to be monogamous. It seems that over time human females have indeed been doing their share of monkeying about too.
Long ago, I came across an extraordinary clinical situation where human sperm competition did not seem to work; and since that case, I have come across other rather similar stories at my infertility clinic.
Margaret B came for investigation of her infertility in her early thirties; IVF was not an option she wanted to consider. Exhaustive investigations failed to find the slightest thing wrong. She was fit, she ovulated, her uterus and tubes were normal, her hormones were fine, she had no immune problems and I could find sperm in her uterus on examination many hours after intercourse. Her husband, too, seemed in good health with an apparently excellent sperm count.
One day, some eight years after she had first come to me with the problem, and following another failed cycle of treatment when I had stimulated her ovaries and performed repeated insemination with her husband’s sperm, I said that perhaps the problem could just be with her husband. There was, I said, a small group of men with apparently normal sperm counts whose spermatozoa were incapable of fertilizing an egg. She looked at me for a long time, started crying and said, ‘No, it must be me.’
Eventually, her story poured out. She had been sleeping very regularly with her husband, but for the last six years she had been having very regular intercourse, sometimes on the same day and even when she was being treated by me, with her longstanding lover. ‘And he has three children, so I know he’s fertile,’ she told me. There was little I could say beyond offering her comforting noises about the possibility of her resolving which of her two partners she would eventually prefer to stay with, and gently suggested a temporary break in treatment might not be a bad idea.
Three months later, Margaret came to my clinic to tell me that she and her lover had taken a momentous decision. She had just seen him off at London airport – he had decided to leave England to settle in Canada. I arranged to see her again in two months’ time for another treatment, but before that, only five weeks after her final farewell at the airport, she phoned to say that she had just missed her period and the pregnancy test was positive. And this time, there was only one possible father.
Much publicity in the press has been given to human sperm competition. Most of it appears to be nonsense, but the stories persist, perhaps because the thought arouses other primitive instincts in those who buy newspapers. Even much of the academic data needs to be interpreted with care. At the University of Manchester, Robin Baker and Mark Bellis have argued that human spermatozoa come in different shapes and sizes because of this possible battle against a competing male’s sex cells. They claim that the most common sperm are the standard-issue ‘egg-getters’ with conical heads and long tails which are designed to swim for their lives. According to their studies, a different type of sperm is also ejaculated. They have coiled tails, so swimming certainly isn’t their forte; instead, they act as kamikaze sperm, wrapping themselves around foreign egg-getters thus hampering their progress to the all-important egg. These researchers are convinced that ‘sperm competition has been the main force to shape the genetic programme that drives human sexuality’. But I believe their views are fanciful; most of the unusual-looking sperm in human ejaculates are simply abnormal. They merely reflect the fact that humans produce many sperm that are incapable of fertilizing an egg because so many human sperm cells are improperly or incompletely formed, and therefore abnormal genetically or physically.
Whatever the truth of all this, it is possible that adultery is an evolutionary adaptation that has grown up alongside monogamy and long-term commitment. Adultery, especially for women, is a risk; the bloodshed and violence that comes from male jealousy would be disastrous for her and her children. It needs to be balanced by the long-term needs of a protective and helpful mate. The two forces are in the ultimate evolutionary tug-of-war. From our earliest ancestors onwards, the sex lives of humans were never going to be straightforward.
Let’s continue to examine the biological roots of our sexual inheritance. If early men and women did indeed have affairs and continually trod the difficult line between monogamy, with its much-needed bonus of security and adultery, with its potential advantage of improving the spread of their genes, what have been the means by which we’ve inherited these tendencies?
Can we find any evidence in our genes? The reality is that multiple genes control every single physiological process in our bodies, so we should certainly take care when seeking out single genes ‘for’ extremely complex behaviours. There is the vast impact of the social environment to consider as well. But that said, it’s possible that in some behaviours, just as in some physical processes, there are master genes much in charge of controlling the whole matter in question.
Tom Insel and Larry Young at Emory University have been doing some interesting work on the genetics of attachment and pair bonding among mice and prairie voles. Prairie voles are endearingly monogamous. After mating with their partner, the male forms a strong social bond with the female and spends more than 50 per cent of his time huddled up with her. They nest together, and the male turns out to be an exceptionally faithful partner and attentive father. Males spend as much time with their offspring as the females, and they’re also staunch defenders of the nest and their female partners, staying with them through each subsequent litter and usually for the rest of their lives. In contrast, the male mice they studied are out-and-out cads. They normally abandon the female instantly after mating and have no role in raising the offspring whatsoever.
Insel and Young looked at the brain chemistry of these two species; in particular, they measured the levels of the hormones oxytocin and vasopressin, which are known to be major contributing factors to pair bonding and attachment. When they compared the receptors where these two hormones have their effect in the two rodents’ brains, there was a difference. The prairie voles didn’t have higher levels of these hormones, but the receptors for them were laid out in a very different pattern from that found in the mouse brain. The researchers went a step further. They focused on and identified the key gene in the prairie vole which appears to control the hormone vasopressin; then, they injected this ‘vole’ gene into several embryos taken from their mice. These embryos were put into the womb of a female mouse and, eventually, they became successfully pregnant. Male mice born became normal adults in every respect except they had the vole pattern of brain receptors. Most impressive of all was the fact that they were unusually sociable and attentive to their mates, with only the very occasional lapse for the odd ‘extra-marital affair’.
Does this mean that each of us has a gene for monogamy or adultery? We have to be careful not to jump to too many conclusions. It is true that we have genes for oxytocin and vasopressin receptors in our brains, but it is also clear that a multitude of genes and environmental factors are undoubtedly involved in the evolution of behaviour. What is important about this kind of study is that it throws light on some of the fundamental links between our genes, the chemistry of our brains and complex social behaviour patterns we have inherited from our early human ancestors millions of years ago.
Whether or not there is a master gene inside us which guides how we bond with our partners is unknown; no-one has seriously looked for it as far as I know. We do know, however, that a closely related gene to that seen in voles, one which codes for the vasopressin-2 receptor in our own brains, varies from person to person. There’s also evidence that the action of the hormone oxytocin may have an important part to play in the way we form attachments to the opposite sex.
Have you ever wondered why after sex men are so happy to just roll over and go to sleep? Perhaps it is just the inevitable result of excessive exercise, but possibly oxytocin is the culprit. This hormone plays a number of roles in the body, including triggering uterine contractions during labour and helping a mother bond with her newborn baby. However, it’s also released during sexual arousal in both men and women. Levels rise during touching and cuddling, the extra oxytocin making you increasingly sexually receptive. In both sexes, the levels of the hormone peak at orgasm. But that’s where the similarity ends, because it seems that the combination of other hormones with oxytocin results in the age-old battle of the sexes running rife at this point. It’s believed that oestrogen somehow amplifies the effect of the oxytocin in women after orgasm, so most women become very affectionate, want to cuddle and feel that strong emotional bond with their partner. Men, on the other hand, mostly just want to snooze, and that may be because testosterone inhibits the bonding effects of oxytocin after orgasm.
This intriguing peek into the world of hormonal influences on our sexual behaviour reflects the importance of our biology in reinforcing the way in which women have evolved to become more easily emotionally attached to their partners. As we saw in the last chapter, women tend to find the idea of their partners becoming involved emotionally with other women much more abhorrent than their simply having purely sexual encounters with no strings attached, whereas for men the complete reverse is true. Men’s deepest fear is to find they have been cuckolded and tricked into raising another man’s child, whereas women don’t want to lose out on benefiting from their partner’s resources and that all-important security and protection when raising their children. It’s both fascinating and remarkable to find that these ancient differences between the sexes when it comes to fidelity are reflected in our very own biology. That should remind us that although our modern culture, society and its traditions have so much sway on our morals, we are also talking about behaviour that has been rooted deep within us for millions of years, and with good reason.
Some species of bird such as eastern robins, as well as other animals such as foxes, pair up for just one breeding season. They are serially monogamous, spending a predetermined length of time with their mates and helping to raise their young before splitting up. Robins will pair up in the spring and raise their young during the summer months. But when, in late summer, the chicks have flown the nest, the two parents amicably go their separate ways and join a flock. Similarly, a pair of foxes, having produced a litter together, will hunt for food and protect their helpless kits for the summer, and then, without ceremony, they will split up and leave their growing progeny to fend for themselves.
The evolutionary psychologist Helen Fisher suggests that humans pursue a similar strategy. We are designed, she submits, to be monogamous only for as long as it takes to raise a single child through infancy. Perhaps this is reflected in the amount of time our brains are receptive to those initial high levels of the chemical PEA (phenylethylamine), which, as we saw in the last chapter, launch us into those heady eighteen months to three years when we first meet a new partner and fall in love.
In the US and UK the figures are only too clear. Between 40 and 50 per cent of marriages end in divorce. But when is this separation most likely? Fisher has collected marriage and divorce statistics from almost sixty different countries. She has found a worldwide pattern that shows that divorce peaks at around four years into a marriage and then declines. It is a pattern that does not seem to alter despite different cultural norms, marriage practices, divorce procedures or relationship difficulties. According to this theory, every marriage is a divorce waiting to happen; and some understanding of the reasons for this may be found by studying the habits of the hunter-gatherers.
In many traditional societies such as the !Kung, there is a much longer period of breast-feeding than in the West. Breast-feeding is sustained until the child is as old as three or four. !Kung women keep their infants by their side day and night. They will use breast-feeding in the way parents in the West use a dummy or pacifier. This is probably the mechanism by which !Kung mothers space out their pregnancies. Although a woman can occasionally fall pregnant during the period when she is breast-feeding, during lactation ovulation tends to be suppressed by high levels of the milk hormone, prolactin. Births among the !Kung women are about four years apart. The same is true for several other hunter-gatherer cultures such as Australian Aborigines, Netsilik Eskimos and the Dani of New Guinea (although there are other contributory factors, such as their low-fat diets and their active lifestyles, that also have a bearing on post-natal fertility). We should not necessarily think that all human evolution can be deduced from these contemporary traditional cultures, even if they are apparently the closest modern-day comparisons to ancient human culture on the savannah. But there is collusion between cultural and biological factors to space out births over the years – which, after all, has the benefit of not overstretching resources and reducing the group’s vulnerability to scarcity or predators.
What are the reasons that force our marriages to break down? Is there any common cause? Interestingly, adultery is the most frequent reason, particularly adultery by the wife, and infertility is second. These two situations underline the fundamental point of human pair-bonding: we enter long-term monogamous relationships in order to have children, preferably our own biological children; anything that compromises this ambition is bound to be destructive in a marriage. As we’ve seen, jealousy is at its most powerful when a woman has sex with another man. The divisive aspects of infertility are self-evident; the fact is, even the most deeply felt love and respect for one’s wife or husband does not necessarily outweigh the instinctively felt need to produce one’s own children. And from my own disturbing experience of running an infertility clinic, childlessness is a very frequent cause of depression and severe marital disharmony. Divorce is particularly common among infertile couples.
Very few known societies have banned divorce. The Incas did not tolerate it, nor does the Catholic Church, but in the majority of cultures, both traditional and modern, divorce is commonplace. Couples in the modern Western world are starting to move away from the powerful Christian prescriptions that attempted to protect the sanctity of the lifelong marriage bond; ‘no-fault’ divorces and annulments are increasingly common methods of dissolving the marriage bond, and cultural squeamishness when it comes to divorce is slowly ebbing away. Some people welcome this trend; as George Burns once said, happiness is having a large, loving, caring, close-knit family in another city.
Whatever the individual reason for separation, we know that relationships do often break down, and the four-year mark is a statistical watershed, even when there are children in the mix. There is no instinctual force which keeps couples together for a lifetime; ‘till death us do part’ is more often than not a hope beyond hope, a tradition which from an evolutionary perspective has little relevance to the realities of human relationships. It seems that while we’re socially monogamous, the truth is, by nature, we’re sexually polygynous.
There’s a bad locker-room joke that says when a man’s wife reaches the age of forty he should be able to change her, like a bank note, for two twenties.
The reason you may find this offensive is because it chimes well with the reality of men’s sexual preferences. Men like younger women because they can have more babies. Although consciously the vast majority of modern men probably don’t see their love of younger women as being for the purpose of having babies at all, most studies suggest it is a powerful factor in modern mating patterns. For some men, the loss of sexual interest that corresponds with the ageing of one’s wife can be enough to fatally harm a twenty-year marriage, despite the bonds of love and companionship that have formed over the years. Cue the proverbial male ‘mid-life crisis’.
So what happens next? A man who is sufficiently attractive, or who has reached a position of high status or cachet, often ends up marrying a younger woman, and sometimes a succession of younger women. The phrase ‘trophy wife’ suggests the young, glamorous, sexy new woman in a man’s life is for the purposes of ego only, a symbolic catch to reinforce one’s status and virility – a bit like wearing the latest, biggest Rolex on your arm, just prettier. It seems the ageing cad is shouting out to the world, ‘OK, I may be older, but I’m so powerful that I can snag this pretty young specimen.’ Despite the attraction of this concept, it turns out that the ‘trophy wife’ phenomenon is far more deeply rooted than that, and it has real, practical implications. Men who marry a series of younger women – perhaps a bit like the Donald Trumps and Hugh Hefners of this world – are catching these women in their fertile prime; on an unconscious, biological level they may be striving to maximize their genetic legacy (and at the same time happily reminding everyone of their alpha maleness).
In fact, this small minority of men are practising a form of polygyny. Although they don’t keep more than one wife at a time, they’re marrying women in their prime and then discarding them, so the principle of polygyny holds. In many cases they will also have to support their wives and their ex-wives, just as in a polygynous harem. And, as in the harem system, they are excluding other men from the chance of mating with these young, fertile women.
So we are back to the sine qua non of human sexual activity – procreation. Whether or not we like the idea, we can nearly always trace the permutations and complexities of our relationships, our affairs and our sexual preferences back to the reason why we have sex in the first place – babies. This instinct really does define and shape our everyday lives.
There are several places in the Bible where the need to respect one’s parents is emphasized, but it is equally striking that nowhere in the Old Testament, which provides so much of the basis for Western views of morality, is there any suggestion of a requirement to respect one’s children. Almost certainly this is because it is automatic human behaviour to nurture one’s offspring, and this instinct means that any dictum of this kind is unnecessary. In my view, the worst bereavement any human can face is the loss of offspring, no matter what age. Our society may occasionally trivialize infant death and miscarriage, but I consider from personal clinical experience of many patients that a great number of women never fully recover from this kind of death in the family. Babies are at the heart of our family relationships, networks of kith and kin that go back hundreds of thousands, probably millions of years. They are also the focus of intense love and care among adults. Feelings of love for our children seem to be ingrained in our subconscious; babies do not just get attached to their parents, their parents also get attached to them.
The loving relationship between parent and child is, on the face of it, an obvious biological adaptation. Those children who are doted on and cared for are more likely to survive the ravages of childhood and last until they are sexually mature and able to reproduce themselves. Love, in this sense, follows the genetic pathway; our genes survive through our children, and so on through our children’s children.
In the rough and tumble of the natural world this fact has some disturbing consequences. Take the example of a pride of lions. Most prides are made up of juvenile cubs and adult females, all of whom are related – as sisters, mothers, aunts and so on. Alongside the female clan are a few non-related adult males; they will have ousted the previous group of males, who would have been too old or too weak to protect their place in the pride. When they mature, the young males desert the pride and go hunting for an adoptive family.
Lionesses nurse their cubs for a year and a half. During that time they cannot become pregnant and their fertility returns only once the cub is weaned. When a new group of males ousts the previous incumbents, it’s likely that one or more of the females will be nursing their young and thus unavailable for further reproduction. The new group is aware that the existing cubs were fathered by members of the previous group. On arrival, the new males swiftly and brutally wipe the slate clean; they seek out and murder every single cub in the pride. The females are once more free to mate and reproduce, and this time the new males will be the genitors.
The brutality and mechanical violence of animal behaviour still has the capacity to shock us. Yet we can find the same kind of behaviour in humans, if we look carefully enough.
The Ache Indians, a small hunter-gatherer society from the interior Atlantic forests of northern Paraguay, have an unusual tradition that mirrors the killing of the lion cubs. When a father dies, the villagers ceremonially kill one or more of the man’s children, even if the mother is still living. In the eyes of the tribe, the children are sacrificed to appease the gods, somewhat in a tradition shared by a number of human societies from the followers of Moloch, reviled by the ancient Hebrews, to the Aztecs. But what seems to us incredibly inhumane is in fact a practice reminding us of one of the fundamentals of evolution, which may well share its roots with the ruthlessness of the lions’ genetic cull. The Ache widow will remarry within the village, and it is not in the new husband’s genetic interests to support the dead man’s children. The child sacrifice may be a vestige of this brutal calculation, a symbolic gesture to maintaining the new husband’s genetic fitness.
In the case of the lions, however, biology is most definitely king, and we can be almost certain that their actions are a product of evolutionary adaptation. But why would lions have evolved such destructive behaviour? They are large mammals, with a long and slow reproductive cycle. It’s unlikely their numbers would ever have grown to the point where this kind of killing of cubs wouldn’t have damaged the success of the species. Their behaviour seems, from the point of successful maintenance of the species, completely deranged.
The answer to this conundrum lies in a truth the realization of which represented a seismic shift in our understanding of evolution and natural selection.
For years, evolutionary biologists built their theories on a largely faulty assumption. They believed, as had been originally suggested by Charles Darwin, that evolution would act on an entire species. This means that if a physical or behavioural trait, like bats’ ability to use ultrasound for navigation or the development of more powerful back legs in a frog, was of benefit for the species as a whole, then it would be a perfect candidate for natural selection. Evolution, from this point of view, had the interests of the entire species at heart. This theory of group selection was rife in the 1950s and early 1960s and seemed to explain many animal behaviours such as those seen in the lions.
Imagine a species of fish-eating birds that live around the shores of an inland lake. The birds have evolved to dive into the water and spear the fish with their beaks. But they are hampered by poor eyesight; they can only see the biggest fish, so their catching ability and therefore their population is limited. Say there is a mutation that brings an advantage, such as sharper eyesight. Now, the birds that have this mutation are able to spear the smaller fish too and because there are so many more fish to choose from, they thrive. The mutated birds are so successful that the mutation spreads quickly throughout the entire population. But there are only so many fish in this particular lake, and soon the supplies start to run dry. The birds are growing too fast, reproducing too quickly and too many young fish are eaten before they themselves have a chance to reproduce.
The ‘group selection’ camp of biologists would have said that the birds were not necessarily doomed. They might have argued that the species could evolve into a population that takes better care of its resources. In the long term, given that there are no other lakes nearby, there is no other option. Species can evolve to suit the limitations of their environment, which is why we see such precisely balanced eco-systems in which every species plays its allotted role.
They would, however, almost certainly have been wrong. In this case, the birds with the sharp eyesight should, individually, always do better within the group. There is no possibility that the ‘poor eyesight’ gene could spread throughout the population, even if the birds with poor eyesight are, in the long term, a much better bet for the species as a whole. Our flock of fish-eating birds is destined for evolution’s dustbin. The likely truth is that the individual will always strive for its evolutionary success over and above the group as a whole.
The idea of group selection was rapidly dropped by most scientists when it became clear that only in certain very rare circumstances would the group be the unit of selection. One area where things were much clearer as a result was sex selection and the ratio between males and females.
Just imagine if management consultants were employed to make human reproduction more efficient. They would quickly realize the human race would be far more productive if its population were made up of 10 per cent males and 90 per cent females. Even a male population as low as 1 per cent would be enough to fertilize the female majority on a regular basis. Instead of men lazing around having clocked off at the mating factory, every member of the species would be employed doing what they do best: making babies, in the most cost-effective, downsized manner possible. This harem system is an innovation of which Henry Ford would have been proud. So, if evolution is, as we know, nature’s very own management consultant, why on earth is 50 per cent of the human population male?
There is a practical consideration: despite the reductionist way we’ve been thinking about mating rituals, reproduction was certainly not the only activity on the savannah. Our hominid ancestors had to eat and fend off predators and scavengers, and the male members of a group must surely have helped significantly in this respect, although no-one can know for sure the extent of their contribution to the communal larder. But even if men were entirely useless in those respects, it turns out that there’d still be a sex ratio of 50:50 in the human population. The reason is that, as we have seen, natural selection does not appear to operate on groups, just individuals. A particular mutation or genetic trait has an evolutionary benefit only if it means that the chances of one single organism’s genes are passed on more effectively, or in greater numbers, to the next generation.
If a species is completely monogamous, then the sex ratio is self-evident; a male child has as much chance of siring offspring as a female child. But what if a species is polygynous? If Homo sapiens, as a species, was slightly polygynous in the past (males commandeering more than one female), would it not be a good strategy to give birth to more females, who are pretty much guaranteed always to have children, rather than males, who may sometimes be completely excluded from the mating process?
You may have already guessed the answer. Every human child has a biological mother and a biological father. Even if one male controls a harem of ten females (thus excluding nine other males), that one male will sire ten times the number of children. In a system like this, the odds are against most males reproducing, but when one male does, he hits the jackpot. So producing males is, in the long run, just as profitable as producing females.
From the point of view of evolution, we all play the long game. When mathematics is applied to the problem, modelling these different reproductive tactics over many generations, the reason why we are as we are becomes clear: a 50:50 sex ratio is a state of equilibrium. If more females than males are present in a population, then the best tactic for maximum genetic payoff is to produce more males. Natural selection, acting on individuals, will even out the ratio in the long run.
In the early 1960s, an unknown American graduate student called William Hamilton was becoming frustrated with traditional evolutionary biology, which tended to be rife with what he felt were mistakes about the notion of group selection. He attended lectures given by old-time biologists who believed firmly that that was the primary mode of evolution. After attending one of these lectures at the University of Chicago, Hamilton left, muttering under his breath, ‘Something must be done.’
In early 1963, Hamilton published a paper in a somewhat obscure biological journal. It was called ‘The Genetical Evolution of Social Behaviour’, a coldly mathematical analysis which outlined one of the most startling revelations in evolutionary biology. It began, ‘A gene is being favoured if the aggregate of its replicas forms an increasing proportion of the total gene pool.’ As Andrew Brown, author of The Darwin Wars, points out, Hamilton’s paper has the quality of a Turkish bath: one bathes in the warmth of his preface before plunging into the icy waters of the mathematics which make up the bulk of the paper. The mathematics, though, contain an inescapable conclusion: evolution acts on the gene, not on the level of the species, the group, or even the individual organism. This means that natural selection actually selects successful genes rather than successful people, and those genes will spread throughout the population. Hamilton realised that this truth has some extremely interesting implications.
If a gene is to be favoured by natural selection, if it is to survive and not die out, then it needs to enhance the survival and reproductive chances of the organism in which it exists. A gene that boosts the immune system and allows better resistance to disease is likely to grant a longer-than-average life to, and therefore a better-than-average number of offspring for, its animal host. Reproductive success means that the gene will be spread more quickly through the population than its predecessor, a less useful gene that leaves the animal host more susceptible to disease. This early version of the gene will eventually die out if the new gene is allowed to spread unhindered through the species. For any particular gene to survive, it must be dispersed through the population and the species. Its DNA must code for a physical or mental trait that increases the reproductive success of an individual animal. Even a gene that gives the animal a tiny percentage advantage in the resistance to disease could, over many hundreds or thousands of generations, find its way into each and every one of the genomes across the entire species. We can all trace our ancestry back to those with the successful genes; the rest were doomed to end at an evolutionary cul-de-sac, the dead branches of humanity’s family tree.
This has an all-important effect on our individual behaviour towards our kin. Our children carry our genes; in fact, we share 50 per cent of our genes with them. We also share 50 per cent of our genes with our siblings, because each brother or sister receives half of his or her genes from either parent. The percentage decreases as our blood relationship becomes more distant. We share one quarter of our genes with half-brothers or -sisters, the same as we do with first cousins. Because our children, siblings and cousins carry a proportion of our genes, it is in our interest to help them survive, prosper and reproduce. It doesn’t matter that these genes will find success in another person’s body. The fact that our own genes find success, whoever happens to be the host organism, is what is important. From a gene’s point of view, any organism that contains copies or partial copies of itself is worth preserving.
Hamilton’s breakthrough ideas became known as the theory of ‘kin selection’. Natural selection, according to this view, operates simultaneously on an individual and its kin, because all carry a proportion of the same genes. Hamilton’s equations lay bare some elegant truths about human and animal behaviour. This is a subject we shall return to, because it underlies a great deal of new thinking about human nature, altruism and our relationships with others.
So blood is thicker than water. But as an aside, it is interesting and alarming to see how often and how easily these instincts are used, and sometimes manipulated. I believe the feelings we have towards our family and our group go some way towards explaining xenophobia and racism – or at least, how some political figures exploit these strong feelings. As I write, there is a resurgence, it seems, of right-wing politics endeavouring to appeal to the baser instincts of the electorate. One old slogan of Monsieur Le Pen, the French fascist leader, will perhaps serve to illustrate what I mean: ‘I like my daughters better than my cousins, my cousins better than my neighbours, my neighbours better than strangers, and strangers better than enemies …’
There is some extremely interesting and to my mind reassuring scientific evidence about racism and our ‘racist’ instincts. Dr Robert Kurzban, working with John Tooby and Leda Cosmides, evolutionary psychologists at the University of California in Santa Barbara, has confirmed that automatic processes in people’s brains compute the race of those they encounter. One possibility is that these processes may be deeply ingrained and that people cannot help categorizing people by race. Given that categorizing people into groups nearly always leads to discrimination, this idea would be very discouraging. However, an evolutionary analysis would suggest a more hopeful view of human nature. During our history on the savannah, our ancestors would have lived in a world where the sex and the age of another person were likely to be very important in the immediate structure of their society. But hunter-gatherers travelled by foot, and residential moves of more than forty miles or so would have been uncommon, which makes it highly unlikely that the average hominid ever encountered people from populations that, genetically speaking, differed substantially from himself. Therefore, they argue, it’s unlikely that natural selection has left us with any preferential recognition of race.
Kurzban and his colleagues set out to try to show that, as a result, our attitudes to age and sex – ageism and sexism – are much more deeply ingrained than those of race. The experiment was complex and used extensive statistics, but the essence of it was simple enough. In a variety of experiments, they showed a series of volunteer undergraduates from a variety of racial backgrounds photographs of individuals wearing similar clothing from one of two rival basketball teams, with identically coloured shirts in some experiments and different colours in others. The photograph of each individual ‘basketball player’ was paired with a verbally given sentence which suggested allegiance with one or other basketball side – for example, ‘you were the ones that started the fight’. The volunteers were then asked to sort the basketball players into one of two ‘coalitions’ or groups. After sorting and analysis of their results, the experimenters found evidence that although recognition of race was soon forgotten by their volunteers, memories of the perceived age or sex of the subjects of the various photographs persisted. They concluded that attitudes towards gender or age are matters of much deeper instinct.
We can be almost sure that the genes carried by one particular male lion haven’t evolved by allowing another lion’s genes to flourish at their expense. There is no evolutionary sense in expending time and energy looking after cubs which are not your own; in fact, it’s an evolutionary disaster of a situation. Besides which we also know that the mathematics does not allow it. Genes that led their carrier to pursue such a hopeless strategy would simply not have been successful.
The harsh realities of the mathematics of evolution have some profound implications for human society. Although it’s sometimes difficult for us to think of human behaviours and relationships in this way, studies such as those by Margo Wilson and Martin Daly, the Canadian psychologists, have helped us to see how things work in our own everyday lives. As they say rather self-deprecatingly of themselves, they do field and laboratory work on desert rodents and human homicide which they ‘treat as a window on human passions and antagonisms’. Wilson and Daly described a phenomenon they call the ‘Cinderella effect’ which shows with startling clarity exactly how our family relationships are sculpted by genetic calculation.
Evil stepmothers have been a staple in myths and fables from every conceivable age and corner of the world. At the heart of the Cinderella story is the cruelty the heroine suffers at the hands of her wicked stepmother and stepsisters. In a Chinese version of the story, the Cinderella character, who is named Benizara, is sent with her stepsister to collect chestnuts, but her stepmother has cruelly torn a hole in the bottom of Benizara’s bag and when night falls the girl’s bag is still empty. Benizara is afraid to return home because she knows she will be punished. But, as in all good fairytales, there’s a happy ending around the corner. In the woods, Benizara meets an old woman, her fairy godmother, who gives her a magic box which later will be found to contain a beautiful gown which goes on to perform the same function as the glass slippers. There’s also a large bag of chestnuts to take home to the evil stepmother.
So entrenched is the theme of the wicked step-parent in stories around the world that it’s not surprising to find the pattern reflected in the grim realities of domestic murder and abuse. Wilson and Daly scrutinized the statistics of 147 cases in Canada between 1974 and 1983 in which children were killed by their parents or step-parents, and their conclusions were clear: very few of them were killed by their biological parents. The vast majority of children killed in these domestic situations are murdered by step-parents or adults who are in a sexual relationship with the child’s parent, whether or not they take a parental role in their adoptive family. In fact, their study showed that a child living with at least one non-biological parent was an incredible seventy times more likely to be murdered than a child living with both its biological parents. It is a simple fact of evolutionary biology: parents don’t want to waste their precious resources on children who are not their own genetic offspring, nor do they want to share the limited resources they have found for their own offspring with someone else’s if they can possibly help it.
The Cinderella effect is not just an American or Western phenomenon. Among the Ache Indians, if the children are not sacrificed after the death of their father, their prospects as stepchildren are not auspicious. Of a group of sixty-seven children raised by a mother and a stepfather, an extraordinary 43 per cent had died by the time of their fifteenth birthdays. Of children who were raised by both biological parents, 19 per cent had died by the the same age – still high, but the odds are definitely in their favour.
Daly and Wilson were careful to exclude any factors that could skew the result. They took into account the age of the parent or step-parent, whether or not they had a personality or mental disorder, and their social and economic status. And, rightly, they are quick to point out that the murder of children is, in most societies, a very rare phenomenon. Most step-parents are generally loving, peaceable and devoted. But the statistics reveal the sharp end of the truth about how kin selection really works. Without the genetic bond, the parental bond is bound to be weaker.
These harsh conclusions do not just apply to children and their step-parents. Domestic murder rates in general are excellent indicators of the mechanics of kin selection. Detroit used to have one of the highest murder rates in the Western world. Around a quarter of murders in 1972 took place in a private home or involved solely family members, but only a quarter of these killings involved two people who were biologically related; the rest involved lovers, husbands, wives or step-parents – family members, but not blood relatives.
As soon as we start to think about the basic mechanics of kin selection in any depth, we can’t help but notice that in our everyday human lives there appear to be a number of exceptions. We all know only too well that sometimes people do carry out selfless altruistic acts – directed at people who are not actually biologically related to them. How does this reality sit with the idea that we are lean, mean evolutionarily charged machines destined by our very nature to direct all our energies into furthering the spread and success of our own genes, not anyone else’s?
Such acts could range from simply giving a birthday card to the person in the office with whom everyone knows you’ve never seen eye to eye, to diving head first into a lake to rescue a drowning man you’ve never met before. The bottom line is we seem to be much nicer to other people than would be expected if this were simply to benefit our selfish genes. This is a theme to which I return at the end of this book, but two examples from the animal world which provide some insights are worth giving here.
When faced with a potential predator, one or two guppy fish usually swim away from the rest of the school towards the looming threat, so that they can inspect it. It seems that the brave scouts deliberately risk being eaten, but they’re also doing the rest of the group (most of whom will be unrelated) a favour by checking out whether or not the intruder is actually dangerous. Likewise, a female vampire bat who has recently eaten her dinner of warm blood taken from a sleeping pig will happily regurgitate the entire meal to save an unrelated nest mate from starvation. What on earth is going on? Are these fish and bats true altruists, and if so, how have they survived? Although it may seem that evolution has not made us all into selfish beasts, the truth is probably different. These creatures may be carrying out apparently selfless acts which will benefit other unrelated individuals, but they are also acting on a ‘tit-for-tat’ basis. The vampire bat will only perform her kind act of charity for another bat with whom she’s been a frequent roost mate, and she’s more likely to donate blood to a bat which has gone out of its way to help her in the past. And an ingenious experiment using mirrors which tricked a guppy fish into thinking its brave counterpart was either going all-out with him to inspect the predator or turning back at the last minute and leaving him all alone proved the same thing: the fish only behave co-operatively in their risky mission if they know for sure their scouting partner is going to back them up. If they get let down, they won’t risk being left in the lurch by the same individual again.
What does this say about humans? Mathematicians and evolutionary biologists believe that reciprocation is indeed a powerful driving force in our choice of actions. Perhaps that birthday card was given because the person in question had made the first move in the conciliation and brought you a cup of tea earlier, or even because the so-called kind act would make other people in the office think differently about you. Besides which, we’ve all learned, consciously or otherwise, as we grew up in this society that reciprocity leads to social cohesion – getting on better with everyone around you – and ultimately that’s got to be good for you and good for your genes. In the world of evolutionary biology, the bottom line is that selfish genes, in their many guises, lead to evolutionary success. Philanthropists be damned!
Despite our tit-for-tat leanings, in modern Western culture the family is still the main focus of social and personal life. Family relationships – how we act towards family members, how we help them, how we compete with them – are visible clues to age-old instinctual habits, and Hamilton’s theory of kin selection is an immensely powerful tool with which to investigate these habits.
As Hamilton’s theory goes, the most curious families in the animal kingdom are those of the social insects – bees, wasps and ants. Social insects have an enormous impact on the planet and the lives of other species, mainly because of their vast numbers. Any single hectare in west Africa is populated by an estimated average of twenty million ants. Insect colonies can grow to gargantuan proportions – cities which, measured by population, match the population of the largest human cities of the twenty-first century. The population of Tokyo, currently the largest city in the world, is around twenty-six million; the African driver ant Dorylus lives in colonies of up to twenty-two million workers. Their combined mass is more than fifty kilograms, and they feed off and protect a territory of a massive fifty thousand square metres.
It may seem strange to think that the behaviour of insects can tell us anything about ourselves, but their study is absolutely relevant because of the way their selfish genes are passed on to the next generation. The co-operation among these tiny creatures is legendary. They care for their young together; they hunt in crowds and group together to carry food, performing feats that would be beyond any single ant; the colony’s soldiers willingly engage potential predators in combat and sacrifice their lives for the good of the nest. Inside the colonies they maintain intricate systems of food distribution. Together they can manage the microclimate inside the nest. Polygerus ants in the Chirichaua Mountains in Arizona have even been known to raid nearby nests, kidnap the pupae and return home with their triumphant prizes. The enemy infants are raised as their own offspring and turned into ‘slaves’ who work ‘willingly’ for the good of their new hosts.
The family structure of social insects is called eusociality, and it’s evolved independently at least a dozen times across the millennia. Eleven eusocial species are in the insect order known as Hymenoptera, which comprises bees, wasps and ants. The other examples include termites and a few odd examples among aphids and beetles.2
The vast majority of eusocial societies are based around female workers; the males simply have to reproduce, to fertilize the queen, an outsized matriarch whose job it is to produce eggs. Workers, soldiers, drones, queens – all have their allotted roles to play. But the oddest thing about eusocial insects is that there is generally a caste of female workers who are sterile. This fact puzzled evolutionary biologists for years. How could natural selection have produced individuals who are completely unable to pass on their genes to the next generation? Why were they so entirely subservient to the cause of the community? In fact, although at first sight these insects appear to be a huge exception to Hamilton’s theory of kin selection, the idea actually goes a long way towards explaining it.
In most social insects, there is an unusual genetic difference between males and females. When a queen produces an egg that is unfertilized, it still develops – and always into a male adult. It is one of the examples of successful virgin birth, or parthenogenesis, in the animal kingdom. If the egg is fertilised in the normal way, then the result is a female. This means that males have genes only from the mother; they are what is known as haploid as opposed to diploid, with half the normal complement of genes.
Therefore, the basic rules of reproduction mean that the genetic relationship between sisters, brothers and offspring is significantly altered. If, as is often the case, the queen is fertilized by just one male, then all the daughters she produces will share 50 per cent of their genes with the queen; but they will share 100 per cent of the father’s genes, since all of his half (haploid) complement of genes are used in fertilization. We humans inherit 50 per cent of genes from our fathers and 50 per cent from our mothers, which means we also share 50 per cent of genes with our siblings, but because of the 100 per cent genetic inheritance from their father, ant, bee and wasp females share 75 per cent of genes with their sisters.
Were these females to have daughters themselves, they would share only 50 per cent of genes with their offspring, so from the point of view of gene survival and replication among these insects, helping to raise one’s siblings is a far more productive strategy than actually having babies. In technical terms, they ‘maximize their inclusive fitness’ by helping their siblings; bizarrely and quite uniquely, here sterility is a winning evolutionary strategy. So-called ‘honeypot’ ants will spend their lives lodged in a tunnel wall, bloated with liquid food which can feed the rest of the nest in time of famine. Others will be assigned the hard manual labour of nest excavation. In some species, soldier ants will enthusiastically conduct kamikaze missions, exploding on impact and killing themselves and their target, a predator and potential threat to the colony.
But while kin selection is a powerful theory, its explanation of the evolution of social insects doesn’t reveal the whole story. Consider the fact that if the queen mates with more than one male, the sisters are actually half-sisters, and therefore genetically they should be better off reproducing than helping their siblings survive at the expense of their own lives. Perhaps the queen, through adjusting and tweaking the gestation process, imposes her own evolutionary interests on the behaviour and fertility of her offspring; maybe the sterility of the worker castes came about through this process. This theory suggests that the queen could actually manipulate her offspring to act in ways that aren’t to their genetic advantage – a kind of natural selection by proxy. Like the kidnapped Arizona slave ants, the queen’s own children could have been duped into maximizing her own genetic fitness. It seems that other types of selection forces might have colluded to produce these extraordinary insect communities and their behavioural patterns.
Evolution is cruel, and it is endlessly creative. Its blindness to morality, pain and suffering has conjured up terrible fates for members of many different species: the lion cubs eaten by the new males of the pride, the kamikaze ants, the male spiders who are eaten mid-coitus, the ant soldiers who commit suicide. All are acting according to the mathematical equations. They are all simply doing their best for their genes.
Humans, too, are capable of deeds that appear brutal and amoral. One of the worst of these misdeeds is infanticide. How could our warm and tender feelings towards the smiling face of a newborn be shoved aside to allow a cold-blooded murder to take place? Is it really possible that the murder of a child is something for which we have an instinct? In fact, infanticide has been a feature of practically every known country, traditional or modern.
The Yanomamo, the people who live along the Orinoco River in Venezuela, practise occasional infanticide. In his studies of these people, Napoleon Chagnon reported that if a woman has a second child too soon after the first, she will often be forced to kill the newborn. The wife of a village leader admitted that she had done exactly that, and the reason, she said, was that she needed her breast milk for her two-year-old. The harsh practicalities of the decision did not diminish the grief she felt in having to carry out the killing. Love, even love for one’s offspring, does not conquer every other consideration. Whether or not evolution has built us in such a way as to regulate our family population in this manner is a difficult point to prove, especially because there are such vast differences in infanticide rates across different cultures and continents.
Traditional African cultures like the Kipsigis in Tanzania or the Lese in the Ituri forest rarely or never kill their children. However, South American cultures are much more prone to committing infanticide, the Yanomamo being a good example. Some think this is to do with the ecology of their respective environments and the ease with which they can provide for their families, but the argument is less than convincing. If we do have an instinct for infanticide, it seems that it only comes into play when there are powerful reasons – reasons that are generally a matter of survival, of life and death. Better to feed one child than have both at risk from starvation or malnutrition.
But there is one story of a child murder that goes against the grain of any practical calculation. It stands out as a cautionary tale, a warning not to ignore the startling power of human culture and beliefs over our nevertheless deeply rooted inherited instincts.
The village of Alinagar in the north Indian state of Uttar Pradesh is utterly unremarkable. Perhaps the most remarkable thing about this village is that it’s relatively close, about two kilometres away, to a police station, of which there aren’t many in this part of India. Basking in the oppressive heat of the dry season, there is a collection of ordinary, small, brick-built houses along rutted streets and an unmetalled town square. The village is surrounded by rich fields, a slow-moving river, buffaloes and plantations mostly filled with the long stems of sugar cane. For most of the long, hot, windless afternoons the villagers doze in the shade of their houses to the sporadic barking of pye-dogs in the background. Not obviously a place to recall the story of Romeo and Juliet.
Everybody in this tiny community knew about the romance between Vishal, a fifteen-year-old boy, and Sonu, a sixteen-year-old girl. Their families, after all, lived less than twenty feet apart. Stories in the Indian newspapers vary about what actually happened, but it seems that in August 2001 a neighbour caught them together as they chatted on the roadside next to a bush. She accused them of having ‘suspicious intentions’ and dragged them into her shed. And then she summoned their families. The teenagers had not been caught in flagrante delicto; they were not even holding hands. Their crime was far more primal and ancient: they were from different castes. Under India’s enduring system of social stratification, a relationship between the pair was unthinkable. Sonu was a lower-caste Jat, very much the predominant caste in these parts; Vishal was a Brahmin from the only upper-caste family in the village.
Sonu’s mother and father, Surender and Munesh, felt there was only one way to escape the terrible social humiliation their daughter had heaped upon them: they would have to kill her. Some say that they locked the two teenagers in a room, others in the roof of the house, before killing them, but the most credible account is that, aided by three neighbours, Sonu’s parents beat both children until they were bleeding profusely. The screams woke various neighbours but in spite of this they proceeded to strangle Sonu in a dark shed, with its abandoned bicycle and mattresses, in front of her terrified boyfriend. After that they got a rope, made a noose and hanged their daughter. Then they called on Vishal’s family, demanding that they kill their son. When the boy’s family refused, the girl’s parents took it upon themselves to hang the boy.
The entire village knew what was going on. According to local reports, villagers flung the teenagers’ bodies onto a buffalo cart and hid them under sacking. At three that morning, the villagers walked silently to the local cremation ground, ten minutes away. There, they burned Vishal and Sonu on a joint pyre made from cow dung. Sonu’s parents tossed on paraffin for good measure, against all the traditions of Hinduism, so the corpses would burn more quickly. They then surreptitiously threw the ashes into the Katha River.
Luke Harding wrote movingly about Vishal and Sonu in the Guardian on 14 August:
Alinagar is apparently now almost deserted. Most of the villagers have fled for fear of arrest. The buffaloes are unfed; Vishal’s house is ransacked and empty. We find only Sonu’s sister, Babita, and elderly aunt, Dagiyayi, sitting outside the family home. Neither sees much wrong in Sonu’s brutal demise. ‘I’m not happy. But Sonu was on the wrong path,’ Babita tells me, as she soaps a bucket full of clothes. ‘My parents did what they had to do. We were under compulsion.’ Did Sonu love her parents? ‘Sonu used to love her parents very much,’ she says.
The Alinagar case was at least the third such incident clearly documented in this region in ten years, and apparently the police have another forty-seven cases under investigation. The district magistrate, Manoj Singh, was quoted in the Indian newspapers as saying that the case of Vishal and Sonu was in accordance with local tradition: ‘There is a theory here in such cases which states: either you kill your son and I’ll kill my daughter or you kill my daughter and I’ll kill your son.’
It is extraordinary that such a tradition – and a tradition that has come in for vocal criticism within India as well as outside it – can triumph over a parent’s instinctive and apparent love for his or her children. The power of a belief system like that of the Indian caste restrictions is extremely deeply rooted and respected, sometimes above all other considerations. It shows us that culture has the capacity to transform or entirely dominate our biologically ingrained instincts, and that we should never underestimate its power.
In all but such tragic cases, however, our genes and their preservation provide us with intensely powerful motivations, and there is no area of human life where they are more deeply felt than in the family. Kin selection is a persuasive theory of how we have come to live lives which are defined by our parents, our children, our brothers and sisters. Polygyny, monogamy, marriage, children – all these relationships are inextricably bound up in our genetic heritage. Shadows of the savannah will always be present, cast over modern mores and ways of life, sometimes with a pleasing match between the way we act and the way we think we should act, but sometimes causing almost unbearable tension. Slowly, in certain fields of study – such as Daly and Wilson’s work on the ‘Cinderella effect’ – we are starting to grasp the very basic truths about human relationships, and not all of them are easy to accept.
1 But adverts like this suggest that women in the West may now be taking the whip hand: ‘I had a wonderful time at the submachine-gun course. The instructors were talented, knowledgeable, courteous and safety-conscious. This course is a must – especially for women!’ Michelle Martin, kindergarten teacher, on the Nevada Pistol Academy.
2 There is also one mammalian species that has many of the features of eusociality – the African naked mole rat. These small burrowing rodents live in huge cave complexes in groups of several dozen. A large queen sits in a central chamber surrounded by reproductive male drones and sterile female workers; the latter care for the queen and all her progeny. They co-operatively dig for tubers, with ‘chain gangs’ passing the food along the network of tunnels.