Most psychologists agree that the evolutionary history of our species has made certain kinds of learning either difficult or easy. Most acknowledge that simple behaviors, such as smiling or preferring sweet tastes, resemble instincts (behaviors that are relatively uninfluenced by learning and that occur in all members of the species). And most agree that human beings inherit some of their cognitive, perceptual, and emotional capacities. But social scientists disagree heartily about whether biology and evolution can help account for complex social customs, such as warfare, cooperation, and marriage. Nowhere is this disagreement more apparent than in debates over the origins of male–female differences in sexual behavior, so we are going to focus here on that endlessly fascinating topic.

In 1975, one of the world's leading experts on ants, Edward O. Wilson, published a little book that had a big impact. It was titled Sociobiology: The New Synthesis, the “synthesis” being the application of biological principles to the social and sexual customs of both nonhuman animals and human beings. Sociobiology became a popular topic for researchers and the public, generating great controversy.

Sociobiologists contend that evolution has bred into each of us a tendency to act in ways that maximize our chances of passing on our genes, and to help our close biological relatives, with whom we share many genes, do the same. In this view, just as nature has selected physical characteristics that have proved adaptive, it has selected psychological traits and social customs that aid individuals in propagating their genes. Customs that enhance the odds of such transmission survive in the form of kinship bonds, dominance arrangements, taboos against female adultery, and many other aspects of social life.

In addition, sociobiologists believe that because the males and females of most species have faced different kinds of survival and mating problems, the sexes have evolved to differ profoundly in aggressiveness, dominance, and sexual strategies (Symons, 1979; Trivers, 1972). In many species, they argue, it is adaptive for males to compete with other males for access to young and fertile females, and to try to win and then inseminate as many females as possible. The more females a male mates with, the more genes he can pass along. But according to sociobiologists, females need to shop for the best genetic deal, as it were, because they can conceive and bear only a limited number of offspring. Having such a large biological investment in each pregnancy, females cannot afford to make mistakes. Besides, mating with a lot of different males would produce no more offspring than staying with just one. So females try to attach themselves to dominant males who have resources and status and are likely to have “superior” genes.

In this view, the result of these two opposite sexual strategies is that males generally want sex more often than females do; males are often fickle and promiscuous, whereas females are usually devoted and faithful; males are drawn to sexual novelty, whereas females want stability and security; males are relatively undiscriminating in their choice of sexual partners, whereas females are cautious and choosy; and males are competitive and concerned about dominance, whereas females are less so.

Evolutionary psychologists generally agree with these conclusions, but they rely less on comparisons with other species than sociobiologists do, focusing instead on commonalities in human mating and dating practices around the world. In one massive project, 50 scientists studied 10,000 people in 37 cultures located on six continents and five islands (Buss, 1994; Schmitt, 2003). Around the world, they found that men are more violent and more socially dominant than women. They are also more interested in the youth and beauty of their sexual partners, presumably because youth is associated with fertility (see Figure3.3). According to their responses on questionnaires, they are more sexually jealous and possessive, presumably because if a man's mate had sex with other men, he could never be 100 percent sure that her children were also genetically his. They are quicker than women to have sex with partners they don't know well and more inclined toward polygamy and promiscuity, presumably so that their sperm will be distributed as widely as possible. In contrast, women tend to emphasize the financial resources or prospects of a potential mate, his status, and his willingness to commit to a relationship. On questionnaires, they say they would be more upset by a partner's emotional infidelity than by his sexual infidelity, presumably because abandonment by the partner might leave them without the support and resources needed to raise their offspring. Many studies have reported similar results (Buss & Schmitt, 2011).

Evolutionary views of sex differences in dating and mating have become enormously popular. Many academics and laypeople are persuaded that males have an evolutionary advantage in sowing their seeds far and wide, and females have an evolutionary advantage in finding a man with a good paycheck. But critics, including some evolutionary theorists, have challenged this conclusion on conceptual and methodological grounds:

1. Stereotypes versus actual behavior. In reality, the behavior of humans and other animals often fails to conform to the stereotyped images of sexually promiscuous males and coy, choosy females (Barash & Lipton, 2001; Birkhead, 2001; Fausto-Sterling, 1997; Hrdy, 1994; Roughgarden, 2004). In many species of birds, fish, and mammals, including human beings, females are sexually ardent and often have many male partners. The female's sexual behavior does not seem to depend only on the goal of being fertilized by the male: Females have sex when they are not ovulating and even when they are already pregnant. And in many species, from penguins to primates, males do not just mate and run. They stick around, feeding the infants, carrying them on their backs, and protecting them against predators (Hrdy, 2009; Snowdon, 1997).

   

   Human sexual behavior, especially, is amazingly varied and changeable across time and place. Cultures range from those in which women have many children to those in which they have very few; from those in which men are intimately involved in childrearing to those in which they take no part at all; and from those in which women may have many lovers to those in which women may be killed for having sex outside of marriage (Hatfield & Rapson, 1996/2005). In many places, the chastity of a potential mate is much more important to men than to women, but in other places, it is important to both sexes—or to neither one (see Figure3.4). In some places, just as evolutionary theory predicts, a relatively few men—those with the greatest wealth and power—have a far greater number of offspring than other men do; but in many societies, including some polygamous ones, powerful men do not have more children than men who are poor or who are low in status (Brown, Laland, & Mulder, 2009). Sexual attitudes and practices also vary tremendously within a culture, as is immediately apparent to anyone surveying the panorama of sexual attitudes and behaviors within the United States and Canada (see Chapter 12).

2. What people say versus what they do. Evolutionary psychologists have tended to rely on data from questionnaires and interviews, but critics observe that people's responses can be a poor guide to their actual choices and actions. When people are asked to rank the traits they most value in a sexual partner or someone they'd like to go out with, sex differences appear, just as evolutionary theory would predict (Kenrick et al., 2001): Heterosexual women say that they'd ideally like a man who is rich and handsome, and men say they'd like a woman who is good-looking and sexy. But those preferences are hypothetical; their actual choices about whom to date, love, and marry often tell a different story. That's why people who are plain, pudgy, smart, foolish, rich, poor, gorgeous, or goofy all usually manage to find partners.

   Likewise, when you ask people which would upset them more, their mate having sex with someone else or their mate falling in love with someone else, women are usually likelier than men to say that emotional infidelity would be worse (although cultures vary widely on this reaction). But when one researcher asked people about their actual experiences with infidelity, men and women did not differ at all in the degree to which they had focused on the emotional or sexual aspects of their partner's behavior (C. Harris, 2003). In fact, men, supposedly the more sexually jealous sex, were significantly more likely than women to have tolerated their partner's sexual unfaithfulness, whereas women were more likely to have ended the relationship over it. As research continues to disentangle when and how evolutionary theory would predict sex differences in this area, the distinction between reported and actual behavior will be prominent (Kato, 2014; Sobraske, Boster, & Gaulin, 2013).

3. Convenience versus representative samples.As we saw in Chapter 2, “convenience samples” of undergraduates sometimes produce results that do not apply to nonstudents, and this may well be the case in much of the evolutionary research on attitudes toward sex and marriage. In a national study, researchers at the Centers for Disease Control and Prevention (CDC) interviewed more than 22,000 American men and women ages 15 to 44 about sex, living together, marriage, divorce, and parenting (Groves et al., 2009). The agency had conducted similar surveys from 1973 to 2002, but only with women. Starting in 2002, the researchers asked a question that in retrospect seems obvious: What about men? Thus, more recent surveys were finally able to draw conclusions about male and female attitudes based on a sample that was far more representative of the general population than college students are. What the researchers found casts a different light on evolutionary notions of sex differences.

   As we've seen, in the evolutionary view, women on the whole value commitment to a relationship more than men do and are more dedicated to parenting. Yet 66 percent of the men, compared to only 49 percent of the women, agreed that “It is better to get married than go through life being single.” Furthermore, 68 percent of women and 75 percent of men agreed that “It is more important for a man to spend a lot of time with his family than be successful at his career.” Among fathers living in the same household as their children, the majority spent considerable time feeding and bathing their kids, helping with homework, and taking them to activities. Some other results were in the stereotypical direction, but taken as a whole, the CDC findings suggest that American men today are just as interested in serious family relationships as women are (Jones & Mosher, 2013).

   If you are thinking critically, you may be wondering whether these questionnaire results are any more reliable than those on dating preferences. Good question! The answer is yes. From studies that had people keep diaries of how they spend their time each day, we know that men's behavior has changed along with their attitudes. Women still do more housework and childcare than men do, but since the 1960s, the time men spend on housework has more than doubled, and since the 1980s, the time they spend on primary childcare has nearly tripled (Raley, Bianchi, & Wang, 2012; Wang & Bianchi, 2009).

4. The Fred Flintstone problem. Finally, some scientists have questioned evolutionary psychologists' emphasis on the Pleistocene Age, which extended from about 2 million years to about 11,000 years ago. Analysis of the human genome in Africans, East Asians, and Europeans suggests that during the past 10,000 to 15,000 years, natural selection has continued to influence genes associated with taste, smell, digestion, bone structure, skin color, fertility, and even brain function (Voight et al., 2006). Some of these changes may have begun when humans abandoned hunting and gathering in favor of agriculture, a switch that made certain genetic dispositions more adaptive and others less so. David Buller (2005), a philosopher who was captivated by evolutionary psychology until he took a closer look, concludes that “There is no reason to think that contemporary humans are, like Fred and Wilma Flintstone, just Pleistocene hunter-gatherers struggling to survive and reproduce in evolutionarily novel suburban habitats.”

Even if the Pleistocene period did strongly influence human mating preferences, those preferences may differ from the ones usually emphasized by evolutionary theory. Our prehistoric ancestors, unlike the undergraduates in many mate-preference studies, did not have 5,000 fellow students to choose from. They lived in small bands, and if they were lucky, they might get to choose between Urp and Ork, and that's about it; they could not hold out for some gorgeous babe or handsome millionaire down the road. Because they had only a small range of potential partners, there would have been no need for the kinds of sexual strategies described by most evolutionary theorists (Griskevicius, Haselton, & Ackerman, 2015). An alternative explanation is that evolution instilled in us a tendency to select a mate based on similarity (in looks, intelligence, and so forth) and proximity (the person is standing right there, available). And indeed the evidence supports this view, as we will see in Chapter 12.

Ultimately, what evolutionary scientists and their critics are quarreling about is the relative power of biology and culture. In On Human Nature (1978), Edward Wilson argued that genes hold culture on a leash. The big question, replied paleontologist Stephen Jay Gould (1987), is: How long and tight is that leash? Is it too short and tight to allow much change, or is it long and flexible enough to permit many possible customs? To sociobiologists, the leash is short and tight. To evolutionary psychologists, it is elastic enough to permit culture to modify evolved biological tendencies, although those tendencies can be pretty powerful (Kenrick & Trost, 1993). To critics of both sociobiology and evolutionary psychology, cultural variations mean that no single, genetically determined sexual strategy exists for human beings. What evolution has bestowed on us, they say, is an amazingly flexible brain. Therefore, in matters of sex and love, as in all other human behaviors, the leash is long and flexible. Learn more about the strengths and weaknesses of the evolutionary perspective in psychology by watching the video Evolutionary Psychology 3.