Having faced many similar challenges throughout history, males and females have adapted in similar ways. We eat the same foods, avoid the same predators, and perceive, learn, and remember in much the same way. It is only in areas where we have faced differing adaptive challenges—most obviously in behaviors related to reproduction—that we differ, say evolutionary psychologists.

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And differ we do. Consider sex drives. Both men and women are sexually motivated, some women more so than many men. Yet, on average, who thinks more about sex? Masturbates more often? Initiates more sex? Views more pornography? The answers worldwide—men, men, men, and men (Baumeister et al., 2001; Lippa, 2009; Petersen & Hyde, 2010). To see if you can predict such gender differences, take the quiz in TABLE 4.2.

Sources: (1) Pryor et al., 2005; (2) Bailey et al., 2000; (3 and 4) Research from Laumann et al., 1994; (5) Dugan, 2015

Many gender similarities and differences transcend sexual orientation. Compared with lesbians, gay men (like straight men) report more responsiveness to visual sexual stimuli and more concern with their partner’s physical attractiveness (Bailey et al., 1994; Doyle, 2005; Schmitt, 2007). Gay male couples also report having sex more often than do lesbian couples (Peplau & Fingerhut, 2007). And (also like straight men) gay men report more interest in uncommitted sex. Although men are roughly two-thirds of the gay population, they are only 36 percent of same-sex legal partners in marriages, civil unions, or domestic partnerships (Badgett & Mallory, 2014).

Natural selection is nature selecting traits and appetites that contribute to survival and reproduction. Thanks to random genetic mutations, our ancestors were born with varied traits, some of which helped them to survive and reproduce. Eventually, these characteristics became widespread. Evolutionary psychologists use this natural selection principle to explain how men and women differ more in the bedroom than in the boardroom. Our natural yearnings, they say, are our genes’ way of reproducing themselves. “Humans are living fossils—collections of mechanisms produced by prior selection pressures” (Buss, 1995).

Why do women tend to be choosier than men when selecting sexual partners? Women have more at stake. To send their genes into the future, a woman must—at a minimum—conceive and protect a fetus growing inside her body for up to nine months. And unlike men, women are limited in how many children they can have between puberty and menopause. No surprise, then, that heterosexual women prefer stick-around dads over likely cads. Partners who stick around can offer their joint offspring support and protection. Heterosexual women are attracted to tall men with slim waists and broad shoulders—all signs of reproductive success (Mautz et al., 2013). And they prefer men who seem mature, dominant, bold, and wealthy (Asendorpf et al., 2011; Conroy-Beam et al., 2015; Gangestad & Simpson, 2000). One study of hundreds of Welsh pedestrians asked people to rate a driver pictured at the wheel of a humble Ford Fiesta or a swanky Bentley. Men said a female driver was equally attractive in both cars. Women, however, found a male driver more attractive if he was in the luxury car (Dunn & Searle, 2010). If you put a man in a mating mind-set, how will he try to show he is a “catch”? He’ll buy showy items, express aggressive intentions, and take risks (Baker & Maner, 2009; Griskevicius et al., 2009; Shan et al., 2012; Sundie et al., 2011).

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The data are in, say evolutionists: Men pair widely; women pair wisely. And what traits do straight men find desirable?

For heterosexual men, some desired traits, such as a woman’s smooth skin and youthful shape, cross place and time (Buss, 1994). Mating with such women might increase a man’s chances of sending his genes into the future. And sure enough, men feel most attracted to women whose waist is roughly a third narrower than their hips—a sign of future fertility (Perilloux et al., 2010). Even blind men show this preference for women with a low waist-to-hip ratio (Karremans et al., 2010).

There is a principle at work here, say evolutionary psychologists: Nature selects behaviors that increase genetic success. As mobile gene machines, we are designed to prefer whatever worked for our ancestors in their environments. They were predisposed to act in ways that would produce children, grandchildren, and beyond. Had they not been, we wouldn’t be here. And as carriers of their genetic legacy, we are similarly predisposed.

Why might “gay genes” persist? Same-sex couples cannot naturally reproduce. Evolutionary psychologists suggest a possible answer: the fertile females theory. The theory goes like this. As we noted earlier, women relatives of homosexual men have tended to have larger-than-normal families. Perhaps, then, the genes that dispose women to be strongly attracted (or attractive) to men—and to have more children—also dispose some men to be attracted to men (LeVay, 2011). Thus, there may actually be biological wisdom to genes that dispose some men to love other men.

Most psychologists agree that natural selection prepares us for survival and reproduction. But critics say there is a weakness in the reasoning evolutionary psychologists use to explain our mating preferences. Let’s consider how an evolutionary psychologist might explain the findings in a startling study (Clark & Hatfield, 1989), and how a critic might object.

In this experiment, someone posing as a stranger approached people of the other sex and remarked, “I have been noticing you around campus. I find you to be very attractive.” The “stranger” then asked a question, which was sometimes “Would you go to bed with me tonight?”

What percentage of men and women do you think agreed to this offer? An evolutionary explanation of genetic differences in sexuality would predict that women would be choosier than men in selecting their sexual partners. If so, the women in this experiment should be less willing to hop into bed with a complete stranger. Indeed, not a single woman agreed—but 70 percent of the men did. A repeat of this study in France produced a similar result (Guéguen, 2011). The research seemed to support an evolutionary explanation.

Or did it? Critics note that evolutionary psychologists start with an effect—in this case, that men are more likely to accept casual sex offers—and work backward to explain what happened. What if research showed the opposite effect? If men refused an offer for casual sex, might we not reason that men who partner with one woman for life make better fathers, whose children more often survive?

Other critics ask why we should try to explain today’s behavior based on decisions our ancestors made thousands of years ago. Don’t cultural expectations also bend the genders? Behavior differences between men and women are smaller in cultures with greater gender equality (Eagly, 2009; Eagly & Wood, 1999). Such critics believe that social learning theory offers a better, more immediate explanation for these results. We all learn social scripts by watching and imitating others in our cultures. Women may learn that sexual encounters with strangers are dangerous, and that casual sex may not offer much sexual pleasure (Conley, 2011). This explanation of the study’s effects proposes that women react to sexual encounters in ways that their modern culture teaches them. And men’s reactions may reflect the social scripts taught them: “Real men” take advantage of every opportunity to have sex.

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A third criticism focuses on the social consequences of accepting an evolutionary explanation. Are heterosexual men truly hard-wired to have sex with any woman who approaches them? If so, does this mean that men have no moral responsibility to remain faithful to their partners? Does this explanation excuse men’s sexual aggression—“boys will be boys”—because of our evolutionary history?

Evolutionary psychologists agree that much of who we are is not hard-wired. Our destiny is not written in our genes. “Evolution forcefully rejects a genetic determinism,” insisted one research team (Confer et al., 2010). Evolutionary psychologists also remind us that men and women, having faced similar adaptive problems, are far more alike than different. Natural selection has prepared us to be flexible. We humans have a great capacity for learning and social progress. We adjust and respond to varied environments. We adapt and survive, whether we live in the Arctic or the desert.

Evolutionary psychologists also agree with their critics that some traits and behaviors, such as suicide, are hard to explain in terms of natural selection (Barash, 2012; Confer et al., 2010). But they ask us to remember evolutionary psychology’s scientific goal: to explain behaviors and mental traits by offering testable predictions using principles of natural selection. We may, for example, predict that people are more likely to perform favors for those who share their genes or can later return those favors. Is this true? (The answer is Yes.) And evolutionary psychologists remind us that the study of how we came to be need not dictate how we ought to be. Understanding our tendencies can help us overcome them.