THE HUMAN LIFE CYCLE INCLUDES SEXUALITY and family life. It is not always easy, though, to investigate these subjects. One problem is that, where people’s sexual behavior is concerned, there are limits to the scientific approach. We can’t do controlled experiments the way we can with diet or teeth-brushing habits. Another is that the subject can be a sensitive one. Scientists did not begin to study human sexuality seriously until recently, and it still can be difficult to view the subject with a scientific attitude.
Most people regard their relationships with loved ones—family relationships such as parent-child ties, and also romantic and sexual relationships—as deeply meaningful, personal, and private. It can seem cold-blooded, even harsh, to put those relationships under a microscope, so to speak, and look at them with a scientist’s eye. Some people might even be offended to see their interactions with the people in their lives compared with the childrearing behavior of apes or the courtship habits of birds.
As you read this chapter and those that follow, keep two important points in mind. First, we are looking at the human life cycle through the particular framework of evolutionary biology. It does not necessarily explain everything about why people do the things they do—it is simply one of many tools that can help us understand ourselves. Second, our focus is on the entire human species, not on specific examples. There are always many exceptions to every rule, and many people behave very differently than science might predict. Our concern is with general trends, not with people as individuals.
In spite of the challenges of studying human sexuality, we are beginning to understand how it is intertwined with other human characteristics, such as tool use, large brains, and child-rearing practices. Our shift from being just another species of big mammal to being uniquely human involved changes not just in our bones and skulls but also in our family lives and sex lives.
To understand how human sexuality got to be the way it is, we have to understand the evolution of our diet and our society. From the vegetarian diet of our ape ancestors, we separated within the last several million years to eat meat as well as plant foods, yet our teeth and claws remained those of apes, not of tigers. Instead of on teeth and claws, our success in hunting depended on large brains. By using tools and hunting in organized groups, our ancestors could hunt, and they regularly shared food with one another. We started using tools to gather roots and berries as well, so even the vegetarian part of our diets required large brains.
Human children took years to acquire the information and experience they needed to be efficient hunter-gatherers, just as today they take years to learn how to be farmers or computer programmers. For many years after they are weaned—after they stop nursing on milk from the mother’s body and start to eat food—they are too ignorant and helpless to take care of themselves. Human children depend on parents to bring food to them. This seems natural to us, but it is exceptional in the primate world. Baby apes gather food for themselves as soon as they are weaned.
Human infants are terrible food gatherers for two reasons. One reason is mechanical. Making and using the tools needed to find food requires fine finger coordination that children take years to develop. Just as my sons couldn’t tie their shoelaces when they were four years old, four- year-old hunter-gatherer children can’t sharpen a stone axe or build a dugout fishing canoe.
The second reason is mental. More than other animals, we depend on brainpower when we look for food, because we have a much more varied diet and more complicated food-gathering techniques. New Guineans whom I work with typically have separate names for about a thousand different plant and animal species in their vicinity. For each species, they know something about where to find it, whether it is edible or useful, and how to capture or harvest it. All this information takes years to acquire.
Weaned human infants not only need adults to feed them; they need adults to teach them for a decade or two. As with so many human hallmarks, these needs occur in other species. Young lions, for example, must be taught how to hunt by their parents. Chimpanzees, like humans, have a varied diet and use a number of techniques to obtain food. Chimpanzee parents do help their young find food, and common chimps also make some use of tools, although bonobos do not. But for humans, the necessary survival skills, and the burden on the parents, are much greater than for lions or chimps.
Family life in the species Homo sapiens is organized around a simple fact: human children cannot fend for themselves at birth or for a long time afterward. They need parental care, not just for food, shelter, and protection, but to learn the skills they will need to survive in society.
That parental burden means that care by the father as well as the mother is important if the child is to survive. Orangutan fathers provide their offspring with nothing. Gorilla, chimpanzee, and gibbon fathers do more, providing some protection for their young. Human hunter-gatherer fathers do even more, providing some food and much teaching. Our complex food-gathering habits require a social system in which a male has a longterm relationship with a female, so that he can help rear their child. Otherwise the child will be less likely to survive, and the father will be less likely to pass on his genes.
A Social System That Meets Our Needs
The orangutan system, in which the father simply departs as soon as he and his female partner have mated, wouldn’t work for us. The chimpanzee system also wouldn’t work for us. Among chimpanzees, a female who is ready to be fertilized and become pregnant is likely to mate with several adult males within a short time. As a result, a chimpanzee male has no idea which infants in the troop he has fathered. This is no big loss to a chimp father, because males don’t do a great deal for the troop infants. A human father, though, spends a lot of time and energy on the care of his child. From the point of view of evolution, a human male had better have some confidence that the child is his, or his child care contributions may help pass on some other man’s genes.
Confidence about fatherhood would be no problem if humans, like gibbons, were scattered across the landscape in isolated couples, so that a female almost never saw a male other than her mate. Almost all human populations, however, have consisted of groups of adults. Hunting and gathering often involve cooperative group efforts among men, women, or both. Groups also offer protection against predators and enemies, especially other humans.
To meet our need for both confidence in fatherhood and group living, humans evolved a social system that seems normal to us, although it is strange by ape standards. Adult orangutans are solitary. Adult gibbons live as solitary male-female pairs. Gorillas live in harems consisting of several adult females and usually one dominant adult male. Common chimpanzees live in communities of scattered females plus a group of males, in which individuals mate with more than one partner. Pygmy chimps, or bonobos, form colonies of both sexes that are even more promiscuous, meaning that individuals have multiple sex partners.
Human societies resemble none of these primate societies. Like our food habits, our social system resembles more that of lions or wolves. We live in bands containing many adult males and females. Among lions, however, any male can and does mate with any female, meaning that the fatherhood of lion cubs is unknowable. Among humans, males and females are paired off with each other. The closest thing to our social system in the animal world is large colonies of seabirds, such as gulls and penguins, which also organize into male-female pairs.
Officially, at least, in most modern political states human pairing is more or less monogamous, meaning that each individual has a single partner. Among hunter-gatherer bands, which are better models for how humans lived over the last million years, most men can support only a single family, but a few powerful men have several wives. The huge harems some human rulers have maintained weren’t possible until the rise of agriculture and centralized government let a few princes tax everyone else to feed the royal harem’s babies.
Why Men Are Bigger Than Women
Adult men are, on average, slightly bigger than women of the same age. Although there are many individual exceptions, across a whole population men weigh about 20 percent more than women and are about 8 percent taller. Why is this the case? The answer lies in our social and sexual organization.
The typical hunter-gatherer social system, in which most men have one partner but a few men have several wives, can be called “mildly polygynous.” (Polygynous means “with multiple wives.”) Because humans were hunter- gatherers for many millennia before the rise of agriculture, that particular social organization explains why men are bigger than women.
Among polygynous mammals, the average difference in size between males and females is related to the number of females that mate with a single male, and only with that male. The more females in a male’s harem, the greater the size difference between the sexes. The biggest harems are seen in species with males much larger than females. Three examples from the animal world show how this works.
Surrounded by his harem of smaller females, a large male elephant seal basks on the not-so-balmy beach of King George Island, Antarctica.
Gibbons are monogamous. Each individual has only one partner. The male gibbon has no harem, and there is no average size difference between the sexes. Males and females are the same size. Male gorillas, on the other hand, typically have harems of three to six wives. This is reflected in a size difference: a male weighs about twice as much as a female. The average harem of a southern elephant seal is forty-eight females. With such a big harem size, you would expect a big difference in size between the sexes, and you would be right. A three-ton (six-thousand-pound) male seal dwarfs his seven-hundred-pound wives.
The explanation is that in a monogamous species, every male has the opportunity to win a female. In a very polygynous species, such as the elephant seal, many males go without mates, because a few dominant males have succeeded in rounding up all the females into their harems. The bigger the harem, the fiercer the competition among males—and the more important it is for a male to be big, because the bigger male generally wins a fight.
We humans, with our slightly bigger males and slight polygyny, fit this pattern. At some point in human evolution, though, male intelligence and personality came to count for more than size. Big men don’t tend to have more wives than smaller ones.
Our Unusual Sex Lives
Human sexual activity is freakish by the standards of other mammals. For one thing, most mammals are sexually inactive most of the time. They copulate, or engage in sex, only when the female is estrous. That means that she has entered estrus, the part of her biological cycle when she is ovulating—her ovaries are preparing to release an egg. During this time she can be fertilized and become pregnant.
Depending upon the species, females enter estrus at various time intervals, from every few weeks to a few times a year. Females of some species enter estrus just once a year. Then and only then are they willing to copulate with males, and they advertise this fact to males through behavior and, sometimes, changes in their appearance.
Human sexual cycles are quite different. Instead of being limited to a short estrus phase, a sexually mature woman, like a man, may choose to engage in sexual activity whenever she decides to do so. Women ovulate once a month, but unlike other primate females, they do not advertise this fertile phase to males with changes in their appearance or behavior. In fact, human ovulation is so well concealed, from women as well as men, that doctors only began to understand its timing in the 1930s.
Concealed ovulation, together with the fact that women have the ability to be sexually active when they want and not just during the time of the month when they are fertile, means that most sexual encounters by humans are at the wrong time for conceiving a child. Whatever the main biological function of human sex might be, it isn’t to produce children. In no species besides humans has sex become so unconnected to conception.
For animals, copulation is a dangerous luxury. Animals in the act of mating burn energy and neglect opportunities to obtain food. They are also vulnerable to predators who want to eat them or rivals who want to take over their territory. Copulation is something to be accomplished in the minimum time needed for fertilization.
If we regarded human sex as a means of achieving fertilization, it would be a colossal failure in evolutionary terms. It gets the job done, but it consumes a great deal of time and energy, because humans engage in much sexual activity at times when fertilization is impossible or unlikely. If we had kept an estrus cycle like other mammals, including our close primate relatives, our hunter-gatherer ancestors could have spent that wasted time butchering more mastodons and gathering more berries.
The most hotly debated question in the evolution of human sexuality is how we wound up with concealed ovulation, and what good all those mistimed copulations do us. Sex is pleasurable, but evolution made it that way. If our species weren’t getting some evolutionary benefit from our sexual activities, the world would have been taken over by mutant humans who evolved not to enjoy sex.
Closely related to the question of concealed ovulation is that of concealed copulation. All other animals that live in groups—whether individuals are monogamous or have multiple partners— copulate in full view of the other animals in their group. Why are humans unique in our strong preference for keeping our sexual activity private?
Biologists are currently arguing over theories to explain the origin of concealed ovulation and copulation in humans. From an evolutionary point of view, some factor or combination of factors caused us to evolve these traits in the distant past. The traits would not still be present today if some factors weren’t keeping them alive. The factors responsible for concealed ovulation and copulation today don’t necessarily have to be the same factors that caused those traits to appear in the first place. Three of the explanations biologists have suggested for the origin of our unusual sex lives, however, seem to me to be still in operation today.
Those explanations are:
* Concealed ovulation and copulation evolved to reduce aggression and increase cooperation among males;
* Concealed ovulation and copulation strengthen the bonds between particular couples, laying the foundations of the human family; and
* Women evolved concealed ovulation to encourage men to bond permanently with their partners, which in turn makes men more confident that they are the fathers of the children their partners bear.
All these explanations reflect a key feature of human social organization. A man and woman who want their child (and their genes) to survive must cooperate with each other for a long time to rear that child. At the same time, they must cooperate economically with other couples living close by. Regular sexual relations between the man and woman create a bond that is closer than the couple’s ties to their friends and neighbors. These close bonds between couples are a kind of social cement, not just a mechanism of fertilization. Our sex lives take place in private to emphasize the difference between sexual and nonsexual partners within the same close group.
The Science of Adultery
The human mating system is based on male and female pairs that rear children together and form a lasting bond. Another primate species also forms lasting pairs: the little apes called gibbons. Their mating system is different from that of humans, however. Gibbon couples live alone, apart from other gibbons, not in groups or communities. Gibbons who have formed a pair bond do not have sex outside the bond.
Humans live in social groups, not solitary pairs. They also, at times, engage in sexual activity with people other than their partners. Sexual activity by married people outside their marriage, called adultery or extramarital sex, is an exception to our “normal” pattern of married sex.
Adultery can be a heartbreaking, life- wrecking matter. Why, then, do humans do it? Like other behaviors, adultery can be examined from the point of view of evolution. Remember that when we use the framework of evolutionary biology, we look at patterns across whole species, and evolution is only one of the forces that drive human behavior.
If life is viewed as an evolutionary contest, the winners are those who leave the greatest number of offspring. Different species have different strategies for winning the contest. Some species are purely monogamous, others are highly promiscuous (meaning that individuals may mate with many partners), and some follow a mixed strategy: monogamous, with exceptions.
Within any species, the best strategy for males may not be the same as the best strategy for females. That is the case with humans. For men, the minimum effort needed to produce an offspring is copulation, which requires only a little time and energy. For women, the minimum effort is copulation plus nine months of pregnancy—and, throughout most of human history, several years of nursing as well. That’s a huge investment of time and energy. As a result, a man can potentially produce far more offspring in his lifetime than a woman can. The record lifetime number of offspring for a man is 888, sired by Emperor Ismail the Bloodthirsty of Morocco. The record for a woman is 69 children, born to a nineteenth-century Russian woman who had multiple sets of triplets. Few women have topped twenty children, while some men in polygynous societies easily do so.
This biological difference means that a man can potentially gain much more from extramarital sex than a woman can—if the only measure of success is the number of offspring. That could be one reason a man might seek sex outside marriage. Why would a woman? Research in many parts of the world suggests that women’s motives for sex outside marriage often include dissatisfaction with their marriages and a desire to find a new lasting relationship.
Does all this mean that extramarital sex is “only natural” and should be accepted? Not at all. Understanding and explaining a behavior is not the same thing as defending or accepting it. The goal of all human activity can’t be reduced to an evolutionary drive. We humans are able to choose other goals. Many people are simply not interested in relationships with people other than their partners. For others, goals such as honoring a promise to be faithful to one’s partner, staying true to religious and moral beliefs, or protecting the family are stronger than the urge to engage in extramarital sex. Among our species, success and happiness are not measured only by the number of offspring we leave. For evidence, consider the fact that many successful and happy women and men choose to limit their family size, or not to have children. Consider also that many humans form same-sex bonds, or have gender identities beyond traditional male and female roles.
MONOGAMOUS BIRDS—OR ARE THEY?
IN THE ANIMAL WORLD, THE CLOSEST MATCH to the human mating system is found in the nesting colonies of certain birds. Herons and gulls, for example, breed and rear their young in dense colonies of male-female pairs that appear monogamous. Successful chick rearing calls for two parents. One bird cannot raise a chick alone, because an unguarded nest will probably be destroyed while the parent is off gathering food, and a male cannot feed and guard two nests at the same time.
In a study of great blue herons and great egrets in Texas, observers watched males who were left guarding their nests while their mates went offto find food. For the first day or two after pairing with their mates, the males often courted other passing females, although they did not copulate with them. The courting seemed to be a kind of “insurance,” in which a male tried to line up a backup mate in case his female deserted him (which happens in 20 percent of cases). The passing “backup” female is a single bird seeking a mate. She does not know the male already has a mate until his spouse returns to chase her away. Eventually the male gains complete confidence that his spouse will not desert him, and he stops courting the passing females.
Ardea herodias or great blue herons, and their nests.
Herring gulls follow a different strategy. A study of these birds in Lake Michigan found that 35 percent of the mated males engaged in extramarital sex. All mated female gulls, however, rejected the advances of males other than their mates, and they never flirted with neighboring males when their mates were away. All the male gulls who committed “adultery” did so with single females. At the same time, these males were “good providers”—they brought their mates plenty of food.
Studies such as these have shown that so-called “monogamous” birds are not always monogamous. In some species, adulterous males try to have it both ways: keeping their spouses faithful while siring and their nests. offspring with other females.
We are not mere slaves to the traits we have evolved, not even to those encoded in our genes. Modern civilization is fairly successful at ending ancient behaviors such as stealing brides from rival tribes or murdering children. While the evolutionary viewpoint is valuable for understanding how human social and sexual practices originated, it is not the only way to understand the way we behave today.
Once human culture was firmly in place, it acquired new goals. Questions of sexual faithfulness or promiscuity are not decided simply by our evolutionary heritage. They are also ethical questions, involving our ideas and beliefs about right and wrong behavior. Like other animals, we evolved to win at the contest of leaving as many descendants as possible, but we have also chosen to pursue ethical goals, which can direct our behavior in different ways. Having that choice among goals is one of the biggest differences between us and other animals.
How We Pick Our Mates
One final piece of the puzzle that is human sexuality concerns the mystery of attraction. What draws us to one possible partner instead of another? How do we choose our mates?
Psychologists have tackled this question by examining many married couples, measuring everything conceivable about them and then trying to make sense out of who married whom. Not surprisingly, most husbands and wives turn out to share the same ethnic background (although racially and ethnically mixed marriages are on the rise), religious beliefs, and political views. Spouses also tend to match each other reasonably well in intelligence and in personality qualities such as neatness.
What about physical appearance? It turns out that if you measure enough couples, you make an unexpected discovery. There are many exceptions, but on average, spouses resemble each other slightly—but enough to be statistically significant—in almost every physical feature. This is true for the obvious physical features we think of first when describing people: height, weight, and color of eyes, hair, and skin. But it is also true for dozens of less obvious traits, such as breadth of nose, length of earlobe or middle finger, distance between eyes or around the wrist, and lung volume! Experimenters have made this finding for people as far apart as Poles in Poland, Americans in Michigan, and Africans in Chad. In each case, spouses were not identical, but they were more alike than they would have been if they had been randomly paired.
In spite of the old saying “opposites attract,” people on average tend to marry people who are more like them than they are different. One reason for this is that we tend to spend a lot of time around people who are similar to us. Many people live in neighborhoods defined by ethnic background, or religion, or social and economic status. We meet people of the same religion in church. Family friends often share our own family’s interests, political views, and social and economic status.
Those contacts give us many opportunities to meet and fall in love with someone similar to ourselves. But we don’t live in neighborhoods grouped by length of earlobe, so there must be some other reason spouses tend to be matched that way, too. The answer lies in physical attraction based on appearance. All kinds of traits—the obvious ones such as height and hair color, and the less obvious ones such as earlobe length and eye spacing—come together to form a search image, a mental picture of our ideal mate. We may not be consciously aware that we have such an image, but it is what makes us feel that “He’s my type” or “She’s not my type” when we meet someone new.
We are attracted to people who look somewhat like us because our search images are based on people who share half our genes: our parents and siblings. We begin to develop our search image of a future partner when we are very young, between birth and age six. The image is heavily influenced by the people of the opposite sex whom we see most often. For most of us, that’s our mother or father, sister or brother, and close childhood friends.
We may have evolved to form an early search image of the ideal mate, but researchers have found time and time again that factors such as personality, intelligence, and religion have more influence than physical appearance on our choice of spouses. Like other aspects of our social and sexual lives, our feeling of attraction to possible romantic partners, and our eventual choice of partners, whether of the opposite sex or our own, is driven only partly by our evolutionary heritage. It is also determined by our life experiences, values, and goals.
