Auld Nature swears the lovely dears
Her noblest work she classes, O;
Her ’prentice han’ she tried on man,
An’then she made the lasses, O.
‘Green Grow the Rashes, O!’
Some five thousand years ago, Rameses the Great observed of his favourite wife, Nefertari, that her ‘buttocks are full but her waist is narrow . . .’ A timeless comment on the fact that women tend towards a natural hourglass shape – wide hips and chest, with a narrow waist. The narrow waist is not of itself that exciting, even though Victorian women famously endeavoured to exaggerate theirs by reining them in with corsets (even to the extent of surgically removing the lower set of ribs to produce their famous ‘wasp waists’). The real departure from the androgynous body shape of pre-puberty is the enlarged breasts and hips, created mainly by layers of fat deposited in these regions. We use a wide variety of physical cues to gauge the extent to which a prospective mate satisfies our various criteria. Many of these are facial, but by no means all. Body shape plays a role. These cues are often processed automatically and simply kick in a ‘yes’ or ‘no’ response.
Humans are unusually fat by primate standards. In normal, healthy, average-weight women, about 20 per cent of body weight is fat, with men closer to 15 per cent, compared to around 3 to 5 per cent in monkeys and apes.* The main reason that we are so fat is to fuel the costs of producing such large-brained infants. The costs of gestation and, especially, lactation are so enormous for us that women need to have massive reserves of fat that they can metabolise to convert into energy for the growing baby over the long haul of parental investment. In natural fertility populations, that amounts to nine months’ gestation plus as much as three to four years of lactation. Obviously, the costs are minimal during the first few months of gestation (when the embryo is tiny) and the last six months or so of lactation (when the infant is beginning to feed for itself). But the period in the middle, when everything the baby needs has to come through the mother, is massively expensive.
Under natural conditions, mothers find it hard to eat enough to replace the energy that is being drawn off by the baby, and they undergo considerable weight loss as a result. Without a significant store of fat to draw on, they would be driven below the point of no recovery and start to metabolise their own muscle mass to keep the baby alive. Evolution has seen fit to place these fat depots close to where they are most needed (the breasts and the womb), presumably to minimise the costs of moving energy around the body. The laying down of body fat is mediated by the amount of oestrogen being produced by the ovaries, and for this reason body fat in these key depots may be a good index of a woman’s fertility. However, excess body fat in other parts of the body has a negative effect on fertility, thus reinforcing the importance of the slim waist and the hourglass shape.
Because of the need for fat depots to provide energy reserves during pregnancy and lactation, men and women have very different body shapes. The natural male body shape – and that of girls up until puberty – is tubular, whereas after puberty that for women, thanks to the location of these fat depots, is hourglass-shaped. Sexual selection acted on this distinction to make the figure-of-eight shape sensually attractive to men, and in turn on women’s bodies to exaggerate their appearance. Conversely, the more tubular shape of males, with their broader, more muscular shoulders, is equivalently attractive to women. Body shape is conventionally indexed by the waist-to-hip ratio (WHR), waist size divided by hip size. In men, a WHR of ≈0.9 is rated by women as the most attractive (in other words, just enough waist to keep your jeans from falling down), whereas men find women with a WHR of ≈0.7 (the shape typical of centrefold models) to be the most attractive. A WHR of 0.7 is roughly equivalent to the more conventional 36-24-36.
The psychologist Dev Singh and his colleagues have obtained similar results from a variety of African, South East Asian and Polynesian cultures, suggesting that these preferences aren’t entirely the result of cultural influences from the Western media’s hang-ups with centrefold models. However, other studies suggest that, in traditional societies, sheer body mass may be a more common basis for choice. It seems likely that these apparent differences actually reflect the fact that males’ preferences for female body shape are fine-tuned to the best predictors of female fertility in the local environment. A plump Rubenesque figure may be a better index of a woman’s biological fitness in nutritionally stressed populations (i.e. most traditional small-scale societies), whereas WHR may be a better index in well-nourished populations. One likely reason for this is that a mother’s body mass affects how much spare energy she has to put into an infant, and in traditional societies this is probably the single most important determinant of infant growth and survival. In contrast, when survival pressure is markedly reduced, as it is in modern developed economies, the mother’s fatness may be much less important for her reproductive prospects than her fertility. In other words, men may switch back and forth between these two key traits as a function of economic circumstances.
By chance, Boguslaw Pawlowski and I turned up some evidence to support this suggestion. We showed that, in a sample of several thousand Polish women, WHR was the better predictor of neonatal weight (a key factor determining infant survival, and hence an index of the mother’s fitness) in larger-bodied women (those weighing over eight and a half stone, or about 120 pounds), but body mass index (weight divided by height, a widely used index of absolute fatness) was the better predictor in lighter women (those weighing less than eight and a half stone). Women in most small-scale traditional societies tend to fall at the lighter end of the weight distribution, and this might explain why males in these societies are more influenced by body mass when judging women’s attractiveness.
Irrespective of this, it remains a fact that the signals provided by a woman’s body shape do reflect her fertility. Women have to achieve a target ratio of fat to body mass both to undergo puberty and to ovulate. In addition, there is a relationship between WHR and fertility (the capacity to ovulate), reflecting the fact that fat deposition (in the hips, in particular) is regulated by oestrogen. Moderate levels of body fat also reflect general health. In a Finnish study, women with low WHR (i.e. tending towards 0.7, indicating a small waist and large hips) had lower blood pressure as well as lower cholesterol and triglyceride levels than women with high WHR (closer to 1.0). Similarly, in a sample of students in the slightly underweight to overweight (but not obese) range, facial fatness correlated negatively with the frequency and severity of chest infections and higher blood pressure, and positively with a greater likelihood of having taken antibiotics. Waist–hip ratio also has consequences for aspects of infant development. In a study of 16,300 American mother–infant pairs, the mother’s WHR was negatively correlated with her child’s IQ (i.e. women with WHR tending towards 0.7 had higher-IQ children). This was especially true of teenage mothers. The authors argued that this was due to the availability of resources during fetal development.
The object of these kinds of cue is not just to make the girls attractive to the boys, but also to attract the boys to the girl so that she can pick and choose among them. We use all kinds of other tricks of the trade to do this. Besides the perfumes that we talked about in Chapter 2, there is the use of cosmetics to exaggerate the signals we have to offer. Cosmetics have an ancient history that dates back at least to 1400 BC, when three ladies of the court of the Egyptian pharaoh Thutmose III were buried with various jars of cosmetics, including a cleansing cream made from a mixture of oil and lime. Women of their social class enhanced their eyes by lining them with dark kohl (made from galena, a dark-grey lead ore, and still much in use today in Africa) or powdered green malachite. Eye shadows appear to mimic a natural darkening of the eyelids around the time of ovulation (probably the result of an enriched blood supply), and so seem to signal heightened fertility. Similarly, the use of rouge (probably mainly in lighter-skinned people) seems to mimic the way the cheeks are flushed during arousal.
The cues even extend to how you walk, the clothes you wear and how you interact during conversations. Karl Grammer, from the University of Vienna, whose lab has been responsible for generating most of the research on human courtship in the last three decades, filmed young women walking away from the camera and showed that they swayed their hips significantly more when they were ovulating than during the rest of the menstrual cycle. These women were in nightclubs and dance halls, and so, as we might say, on the make. In addition to filming them, Grammer and his team also measured the extent to which the women’s dresses covered their bodies, and found that the amount of bare skin they were showing increased when they were ovulating.
Studies of human courtship carried out in singles bars have revealed a very distinct series of behaviours that women use when they are interested in a man they are talking to. These include leaning the upper body towards him, combined with a series of head tosses and hair flicks, and a distinctive pattern of laughter. These are often preceded by a momentary holding of the man’s gaze, during which the pupils visibly dilate, followed by a coy look away. In his research on courtship, Grammer noted that women’s behaviour seemed explicitly intended to control the development of the relationship, even though women seldom actually behaved negatively. If anything, women seemed to behave in a deliberately ambivalent way that gave the men few cues as to their real intentions. At the same time, they seemed to be probing for information to assess the man’s suitability as a mate. In effect, they seemed to be trying to keep their options open as long as possible, reflecting their greater concern over ensuring the quality of any male they finally chose to mate with. Grammer suggested that this may explain why men frequently overestimate a woman’s level of interest in them when they have only recently met.
However much we dismiss the importance of body shape, the fact is that we respond covertly to these cues in same-sex rivals as much as in members of the opposite sex. Doug Kenrick and his colleagues showed women subjects pictures of female models and measured their mood before and after doing so. After seeing the models, women’s mood fell significantly: they were much less content with their own appearance. Similarly, when men were shown pictures of the same female models, they rated their wives as much less attractive and were less content about their relationship with them.
It seems that the folk wisdom about being tall, dark and handsome may not be so wide of the mark either, at least in the case of height. Boguslaw Pawlowski and I analysed data on the life histories of 4,419 Polish men from the city of Wroclaw and showed that taller men were more likely both to be married than shorter men and to have more children, when education and place of birth were held constant. A similar relationship between stature and number of children was reported for the 1950 cohort of officer cadets at the prestigious US military academy at West Point (although in their case, the larger number of children was achieved by having more marriages). Daniel Nettle later looked for a similar effect in a very large cohort of UK males (the 1958 National Child Development Sample, which includes everyone born in England and Wales in one week in March 1958), but didn’t find one; however, he did find a relationship between stature and the likelihood of being married. In a traditional society that did not have modern contraceptive practices, that would of course translate directly into more children.
In short, it seems that women actively select for taller men. Some evidence for that was provided by our Polish sample. It turned out that there was a secular trend in the magnitude of the relationship between male stature and their reproductive output. During the Second World War, Poland lost more men per head of population than any other European country. As a result, when the teenage cohort that had been through the war began to marry during the next decade, the women didn’t have that many men to choose from. Consequently, evidence of a preference for tall men was virtually non-existent. But in each succeeding decade up to the 1980s, the effect became stronger and more explicit as women had more and more men to choose among.
When Nettle looked for the effects of stature in women in the 1958 UK birth cohort sample, he found a rather different pattern: a strikingly humped relationship in which shorter and taller women produced fewer children over their lifetimes than women of average height. The average height of women in this sample was five foot four inches, and the most fertile women were four foot eleven inches in height. This relationship held up even after controlling for socioeconomic class and the frequency of serious illness (since this is correlated with height in women). The reason for the low reproductive output at the extremes turned out to be that very short and very tall women were much less likely to be married or in a permanent relationship than women of average height. This suggests that humans are actually subject to conflicting selection pressures over height: women are selecting for taller men, but men are selecting for shortish women. Stabilising selection thus maintains our height as a species at a roughly constant level and prevents runaway natural selection driving it to either of the extremes.
There is, alas, an unexpected downside to the business of courtship. A recent study has shown that interacting with a member of the opposite sex impairs men’s (but not women’s) cognitive function. The test was a simple memory task: subjects were presented with a series of letters one at a time on a computer screen, and were asked to say whether or not each letter was the same as the last but one. Halfway through the task, they were asked to take a break in the room, where a male or a female confederate engaged them in conversation for several minutes. When they returned to the experiment and continued with the task, men who had interacted with a female confederate performed significantly worse than those who had interacted with a male confederate, and the decline was worse the prettier they rated the woman. The women subjects showed no such effect. Alas, it seems that men are easily distracted by a pretty girl and sent spiralling into a state of mental confusion.
Men and women show striking consistency in their preference for masculine faces in men and feminine faces in women, and this preference crosses cultural and racial boundaries. Dave Perrett at St Andrews University has made a career out of studying facial attractiveness. He and his students have designed some very smart software which allows them to create composite faces that average the facial features of many individuals of the same sex, and then, having done so, allows the faces to be masculinised or feminised. The two sexes’ preferred traits are quite distinct: women (and to some extent men) prefer men to have masculinised faces (a heavier jaw, wider cheekbones, more projecting lower face, larger eyes, prominent brow ridge, and darker skin tone, all traits influenced by high testosterone levels), whereas they prefer women to have feminised faces (smaller jaws, a narrow upper lip, less prominent brows, narrower cheeks, smaller eyes, lighter skin tones). You can subtly shift any face from more feminised to more masculinised, and back again, simply by adjusting these features in the appropriate direction.
The testosterone that drives these facial shifts along the masculine–feminine dimension is costly: it increases stress on the immune system and thus compromises immunocompetence, making males with higher testosterone levels less able to cope so well with attack by biological agents like viruses, bacteria and parasites. In that sense, masculinised features are hard-to-fake cues of good genes because they reflect the body’s capacity to cope with the intense stress that testosterone production places on it.
By comparison to men’s faces, women’s faces are more neotenised (i.e. more baby-like). Babies have a very distinctive facial structure with a high forehead and a foreshortened face. As we develop, our faces gradually change shape as the jaws drop and start to jut outwards, becoming more prognathous. The ratio of the distance between the eyes and the top of the head relative to the distance between the eyes and chin is large in babies (basically, they are all brain) and then declines steadily with age. After puberty, the shape of a male’s face changes faster, as the jaw line deepens and the lower face starts to protrude more to give the heavier, more angular male jaw.
In a study of women’s facial features, Doug Jones measured various facial proportions, combined them into a single composite index and then correlated this index against age to produce a shape-for-age relationship. He then used this to predict the facial shape that an average woman of a given age would be expected to have. Comparing this predicted shape to a woman’s actual shape gave a simple index of relative neoteny. When the faces were rated for attractiveness by subjects from five different cultural groups, he found that the ratings of female attractiveness increased as the difference between their predicted and actual ages increased: women whose predicted age was less than their actual age were considered more attractive. He then went on to do an analysis of the faces of magazine models. Compared against a standard shape-for-age graph, these women had faces that were the equivalent of a seven-year-old’s. Their faces were exaggeratedly neotenised. In effect, facial neoteny appears to be a supernormal cue of youth, presumably the result of males choosing younger and younger-looking faces. Half hidden beneath this would seem to be a rather disturbing, if sad, explanation for paedophilia.
Because humans are so long-lived, it is rather hard to relate appearance when young to how well people do over their lifetime. This is particularly problematic for evolutionary studies, because what you would really like to know is how many children and grandchildren someone produces over the course of their reproductive life. Sometimes, quirks of circumstance provide just what you need. Sociologists Ulrich Mueller and Allan Mazur rated photographs of the officer cadets at West Point and related indices taken from their faces to their future careers in the military and to the size of their families, all of which were meticulously recorded in their alumnus records. First, they showed that men with more dominant (i.e. more masculinised) faces ended their military careers in higher ranks than those with less masculinised faces. Then they showed that those who did better generally produced more children and grandchildren. However, there was a cost to competitiveness. Those who made it to the highest rank of full general typically had fewer children: it was as though they overdid themselves and went too far, or the regime within which they were competing meant that in order to win the highest prize they actually had to sacrifice something.
However, by far the most surprising finding of all this face research has been the way women’s preferences for male faces vary across the menstrual cycle. During the ovulatory (or follicular) phase, women prefer men with more exaggeratedly masculinised features, whereas at other times they prefer men with faces on the feminised side. They also prefer more symmetrical males during the follicular phase. Both highly masculinised traits and facial symmetry reflect gene quality – genes that are able to resist destabilising environmental effects during development. The explanation appears to reflect a ‘cads versus dads’ dimension (of which more in Chapter 7). In effect, when women are most likely to conceive, they shift their preference to men with cues of good genes, but at other times they prefer men with more feminised features because this reflects a more nurturing type. The strategy they seem to be playing is: conceive by the genetically best male you can find, and then get the most nurturing one to help bring it up.
One last curiosity discovered by David Perrett and his colleagues is that we tend to choose mates who resemble our opposite-sex parent, both in terms of facial shape and in terms of hair and eye colour. Similar results have also been reported from a Hungarian sample. Racial traits are an obvious case of parental resemblance since colour and hair form are such strong markers. Because of that, it provides a particularly conspicuous test case. And in fact, it turns out that children of mixed-sex marriages show a strong tendency to choose mates of the same race as their opposite-sex parent. In fact, so strong is this effect that children born of older parents even tend to prefer mates of about the age that their parents had been when they were young children. In other words, someone whose parents were, say, forty when he or she was born has a tendency to target older partners, irrespective of his or her own age, whereas someone who was born to parents who were in their twenties will be more likely to target someone younger. None of these effects are absolute, of course. Rather, they are statistical: but it happens significantly more often than you would expect if we really did choose our partners at random.
Faces do, of course, give multiple messages – cues of good genes in terms of testosteronised or feminised shapes, but also cues of relatedness and of personality. In a study of Canadians, Lisa DeBruine found that when people are asked to choose between photos that are opposite-sex transformations of their own face or averages of many unrelated individuals, they rated the self-similar faces as more attractive for a long-term relationship (the sharing, caring bit) but less attractive for a short-term fling (the good genes bit). This suggests that we can separate out the cues and use them in quite subtle ways to make choices for different purposes.
One of the more curious side effects of facial resemblance is what happens when babies are born. So great is the importance of bonding the father to the baby and persuading him that it really is his that the mother and the maternal grandparents will frequently comment on how the newborn baby resembles the father – at least, whenever the father is actually in the room. And this is a cross-cultural effect, having been demonstrated among North Americans as well as Mexicans. It’s invariably facial features that come in for comment: how baby has his father’s eyes, nose, mouth or chin; even foreheads and ears come in for their share of comment. Now the big issue here is that newborn babies don’t actually look like anyone in particular. Family resemblances are features that only develop later, as babies grow up. In fact, facial features are probably designed by evolution to be neutral and not to look like anyone’s in particular – just in case the father isn’t the father. By the time the baby develops its full panoply of adult features and comes to resemble its biological parents, it has been living with its ‘adoptive’ dad for a decade or more and it’s too late: ‘dad’ is well and truly bonded – which, of course, is why adoption works. The psychological pressure to make these comments is so strong you can observe it almost every time a new baby arrives. It’s as though the mother and her family are desperate to convince the husband that the baby really is his.
There may be some real substance behind this. My former colleague Steve Platek asked men and women to rate a series of five photographs of a two-year-old that had been morphed with between 3 and 50 per cent of their own personal facial traits, and to say which one they found most attractive and which one they would lend money to or consider adopting. Both men and women showed a striking tendency to prefer faces that contained at least 25 per cent of their own traits. However, the effect was much stronger for males, approximately double that for the women. By comparison, faces with 3, 6 or 12.5 percent of their traits were selected only at random. When asked afterwards, the subjects (all students at university) were unaware that any of the faces were morphs of their own. Their response seemed to be quite unconscious. In a subsequent brain imaging study, a small sample of men and women were shown photographs in which their own faces had been morphed on a fifty–fifty basis with either a two-year-old’s or an adult’s face. Men responded much more strongly to child morphs of their own faces (although women responded more strongly to child faces in general). In addition to the fusiform face area of the brain being active in both sexes (as one might expect on a face recognition task), men showed more activation in a number of frontal lobe regions (in particular) than women did. Not only did they seem more attuned to identifying their own traits in children’s faces, they also seemed to be engaged in more active processing than women were. Men thought about the face more carefully, perhaps searching for cues to reassure them that the child really was theirs.
I suppose the ultimate comment on how much faces mean to us is the extent to which some cultures go to hide women’s faces. In many traditional cultures around the world, including Western Europe in the not so distant past, women cover their heads with shawls. More familiar in the present day, of course, is the fact that in some of the more extreme Islamic cultures women must be veiled from head to toe in public. One obvious reason for doing this is to reduce the risk of attracting men’s attention by minimising the cues that are otherwise there to be read.
Symmetry of bilateral features (those parts of the body that have a mirror image, like the face, ears, arms or legs) is thought to be an important cue of gene quality. It seems to reflect how well the individual’s genes can produce an exactly symmetrical body despite all the traumas and insults that life throws at us as we grow up. Since every cough and sneeze, every brief period of food shortage, every infection destabilises the developmental processes, genes that can cope with these bad circumstances and still produce the perfectly symmetrical you must be good. The differences are often tiny, almost imperceptible – a difference in the length of the ear lobes, or of the right and left index fingers. Yet somehow we seem to be able to pick them up. Over the past three decades, many observational and experimental studies have been done on symmetry (paradoxically, always referred to in the technical literature as fluctuating asymmetry), and on mate choice in relation to symmetry, in birds and other animals, and these have produced extremely robust effects. In humans, men (but not women, interestingly) who are more symmetrical are rated as more attractive by both sexes. On the other hand, breast symmetry seems to be correlated with fertility in women: women with more symmetrical breasts produce more children. Symmetrical men and women also have higher intelligence, and, which is even worse news, men with higher intelligence have more fertile sperm, whether this is measured by count, concentration, or motility (all of which influence a male’s ability to sire offspring). Quite what causes this relationship is another matter.
Perfection, then, tends to come as everything or nothing, bad news though that may be for most of us. The bottom line, however, is that no matter what the evolutionary significance of symmetry may be, the reality is that it does seem to underpin human mate-choice decisions. And we are sensitive enough to it to be able to pick up extraordinarily subtle differences between individuals.
But it gets worse. Will Brown (then at Rutgers University in the USA) videoed men and women dancing in Jamaica and found a correlation between men’s symmetry (measured as the mean difference in the sizes of left and right ankles and wrists, finger and ear lengths) and how well they danced (as rated by both sexes), with a much stronger effect in men. Presumably reflecting this, both sexes had a greater preference for symmetry in male dancers than in female dancers, and this effect was stronger for female raters than for male raters. In this case, women seem to be exerting a significantly greater selection pressure on men than men do on women, forcing them to push themselves to the limit so as to sort the men from the boys.
Although no one has ever looked at it, I have often wondered whether symmetry and physical gracefulness might play an important role in women’s predilection for high heels. High heels have two important consequences for how women walk. One is that they force them to walk with knees slightly bent, because the heels tip them forward. That creates a tension right the way up through the body, and tension, like flushed cheeks, is indicative of arousal, and thus of one’s sexual interest. It just makes you more alluring. But heels also have another effect: women naturally have wider hips than men, and heels make them sway more as they walk in order to maintain balance. That’s difficult to do, especially when the heels are very thin, which perhaps suggests that heels might be a biological handicap. Walking elegantly and gracefully in high heels might be more difficult if you haven’t got close to perfect symmetry in leg length and good co-ordination (and a good sense of rhythm?), thereby providing direct proof of the quality of your genes. It might also be a cue of age – something that only those with a supple, youthful body can do.
In an extensive series of studies carried out at the University of Albuquerque by Steve Gangestad and Randy Thornhill, more symmetrical men (those with low fluctuating asymmetry) were found to have had more sexual partners and to have engaged more often in extra-pair relationships. Neither the men’s current resources and wealth nor their anticipated future salary predicted the frequency of extra-pair matings. Similarly, they found that more symmetrical men were more likely to be chosen as extra-pair mates by women who were in permanent relationships. As if it couldn’t get worse, women are more likely to achieve orgasm with more symmetrical men. Symmetrical men even smell more attractive to women. And perhaps, given what we have already learned, it will be no surprise to learn that women found symmetrical men most appealing when they were ovulating.
• • •
So much for mate choice and our romantic relationships. But, as I mentioned in Chapter 1, we have other kinds of relationships with adults that, in their different ways, can be just as important for us and so shouldn’t be overlooked. These are, of course, friendships and our relations with kin. In the next chapter, we’ll explore these in more detail.
* Monkeys and apes can, of course, become obese in captivity, but only when they are fed an excessively rich diet and allowed too little exercise.