Written by Lila Chrysikou

Consider the following questions.  What do they have in common?
How many sexual partners do you want over your lifetime?
Would you go to bed with an attractive stranger of the opposite sex?
Would you be more upset if your romantic partner had passionate sexual intercourse with someone else or formed a deep, emotional attachment with that person?
How important is physical attractiveness in a potential romantic partner?
How many pairs of shoes do you own?
OK, the last one was a joke (though see the literature on female self-ornamentation as a function of the chance to conceive).  Aside from being unfortunate choices for first-date conversation, the other questions can reliably distinguish between men and women.  A number of studies have documented robust gender differences in how men and women approach sexual behavior and what they seek in romantic partners.  For instance, data suggest that men are more distressed by physical than emotional infidelity, and female chastity is prized the world over.  Further, while polygamous societies (where a husband has more than one wife) exist, when was the last time you heard of a polyandrous arrangement (where a wife has more than one husband)?
Studies also show that men are more likely than women to say “yes” to an indecent proposal from an attractive stranger.  For instance, in one study, male and female students were approached by an attractive confederate of the opposite sex, whom they have never seen before, and asked: “Would you go to bed with me tonight?”  Virtually all women refused, while 75% of men acquiesced (compare that to the response rate to a request for a date, where 50% of both men and women agreed; Clark & Hatfield, 1989). In general, men are more likely to seek short-term mates (i.e., one-night stands, brief affairs, etc.), have less stringent standards for shot-term mates, and need less time to consent to sex.  But what is the source of these gender differences in sexual behavior?

Psychologists who study human behavior from an evolutionary perspective have proposed that mating preferences and reproductive strategies are the result of selection pressures (just like your opposable thumbs, the ability to learn a language, or the preference for sweet and fatty foods).  Among other recurring problems, such how to avoid toxins or select nutritious food, our ancestors were faced with the challenge of successful reproduction: they needed to manufacture as many as possible genetically-related, healthy children, who could survive until reproductive age and make babies of their own.  This task involves at least two facets: one has to choose a good mate and one has to decide how much time, effort, and resources to invest in a particular mate (i.e., in a single offspring) instead pursuing other reproductive opportunities.  From an evolutionary perspective, the ancestors who chose mates well and allocated their resources wisely were more likely to pass on their genetic material to future generations…and eventually to us.  Thus, we inherited their genetically-encoded mating preferences, much in the same way that we inherited their sweet tooth (albeit, the latter does not serve us so well anymore).
How does the addition of evolutionary considerations help explain gender differences in sex-related behavior?  Consider the relative costs of having a child for men and women.  A woman must carry the child for nine months: she is not free to have other children during this time period and she needs surplus energy and nutrients during pregnancy and lactation.  In addition, a woman has a limited number of ova and, even under ideal conditions, her body can support no more than 12-13 pregnancies.  In comparison, males can replenish sperm easily, copulation requires only a few minutes, and they can move on to impregnate the next female almost immediately.  Given this stark contrast, it makes sense that women would be much pickier about their selection of mates, more oriented toward long-term mating (i.e., relationships), and would spend more time raising the offspring: after all, they only have a handful of chances to get this “reproduction thing” right.  Men, on the other hand, may be better off focusing on quantity over quality of offspring, betting that sheer numbers will win out in the end.  Thus, men are less likely to be choosy about mates, prefer a short-term mating strategy, and invest less in caring for each child, in favor of begetting more offspring.
In addition to differences in mating strategies, cross-cultural data have suggested that men and women look for different qualities in potential romantic partners.  Although both genders look for kindness and intelligence, men value physical attractiveness more than women and women value ambition, industriousness, and wealth (i.e., access to resources) more than men.   How can evolutionary psychology explain this pattern of results?  Given the physical demands of pregnancy and child rearing, women ought to be on the lookout for partners who can provide material resources (e.g., food, shelter, and protection).  Since men remain fertile throughout their life, women should have little reason to prefer the younger candidates.  On the other hand, since women can only have children during a brief window and their health status affects the success of the pregnancy, men should be particularly attuned to youth and beauty.
Evolutionary psychologists even have an explanation for gender differences in sexual jealousy.  A woman is certain that a child is her own, but paternity is always uncertain.  Consequently, men should react more negatively to physical infidelity and jealously guard female chastity: spending resources on an offspring who does not share one’s genetic material is a very costly mistake.  Women, on the other hand, are more distressed by emotional infidelity, since a new bond might mean competition for material resources.
Is the evolutionary account of gender differences “correct?”  Can another theory explain the observed pattern of results just as succinctly? Perhaps our mating preferences are just social or cultural constructions.  After all, we are all socialized into culture-specific gender roles from the time we are babies (e.g., studies show that parents perceive and treat identical female and male infants differently), the standards of appropriate female and male behavior are widely known, and deviations from the accepted social norms can be socially disastrous (e.g., imagine the response of classmates to a boy coming to school in a dress).
One alternative approach explains the observed gender-specific mate preferences as a social adaptation to a particular social context.  For instance, biological differences in physical attributes between men and women (e.g., men are stronger; women carry and nurse children) may have led to a division of labor such that men had more opportunities to gain status and power while women were left at home as caretakers.  Thus, men ended up with more control over material resources and, in order to survive, women had to become sensitive to the ability of a mate to provide such resources.  This theory would predict less emphasis on the ability to provide, as a desirable male quality, in societies where the power disparity between genders is smaller, and data from a cross-cultural analysis show this exact pattern (Eagly & Wood, 2002).
What about the evidence in favor of the evolutionary approach? Evolutionary psychology is difficult to test empirically: you cannot randomly assign participants to experience selection pressures in a laboratory.  Some evidence comes from the fact that many of the sex differences we described are near-universal.  We can also test evolution-based theories by generating and evaluating unique predictions.  For instance, a number of researchers have found evidence for changes in female preferences as a function of the chance of conception (“cyclical preferences”).  Several groups of researchers tested whether women would be particularly sensitive to signs of “good genes” during the phase of their menstrual cycle when they are particularly likely to conceive.  A (debated) marker of gene quality is physical symmetry; we are programmed to develop symmetrically but various stresses during development lead to deviations (in reality, no one is symmetrical).  A number of studies have already found a preference for symmetrical faces. One group of researchers wanted to test whether women could pick out the more symmetric men by their scent.  Women were asked to sniff t-shirts worn by different men and rate the scent for attractiveness.  As predicted, women who were near the peak fertility of their cycle found the scents of the more symmetric men more attractive.
Another group of researchers showed that preference for masculine-looking faces varies cyclically.  More masculine features signal reproductive readiness and more testosterone; since testosterone stresses the immune system, masculine features may indicate that a male is healthy enough to withstand such stress.  Researchers placed photographs of male faces that were altered to look more masculine or more feminine in a magazine and asked female readers to pick the most attractive face and to indicate the phase of their menstrual cycle.  Women in the most fertile phase were more likely to choose a masculinized face.
Given that data bear out unique predictions, we have to pay attention to the claims of evolutionary psychologists.  Few researchers doubt that our brains, and the mental tools “embodied” within, have been subject to selection pressures.  On the other hand, social norms shape our preferences and decisions every day.  Evolutionary psychologists would respond that the content of our social norms is not arbitrary (they are shaped by evolutionary forces as well)… but that is a debate for another day.

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