5 Mating
So far we have explored a number of rather abstractly theoretical issues regarding the evolution and structure of the mind and haven’t paid much attention to the contents of the mind, the specific ways that people think and feel. But what makes Evolutionary Psychology so fascinating is how it applies its abstract theoretical principles to generate specific hypotheses about human psychology. For it holds the promise of revealing the nature of, and evolutionary reasons for, the psychology underlying our intimate relationships with others—why we desire sex with some people but not others, why we marry or cohabitate with the people we do, why we are sometimes unfaithful, why infidelities elicit jealousy, and why we care so deeply for our children. Unlike the more abstractly theoretical issues we have so far considered, these claims concern issues that occupy the overwhelming majority of our daily lives.
This is the first of three chapters that will examine Evolutionary Psychology’s specific hypotheses, and the evidence offered in their support,regarding the psychology of mate choice, infidelity, jealousy, and parental care. This chapter will focus on the psychology of mate choice.
Evolutionary Psychology has offered a number of interesting hypotheses regarding sex differences in the psychology of human mating. But this chapter will focus exclusively on two core hypotheses that have become shibboleths of Evolutionary Psychology. Men, Evolutionary Psychologists claim, have an evolved preference for mating with young women, and women have an evolved preference for mating with high-status men. These preferences are supposedly implemented in evolved modules that are also designed to detect signs of youth and status, respectively.
Evolutionary Psychologists claim to have gathered overwhelming empirical evidence that confirms both of these hypotheses, and this chapter will examine that evidence. Since chapter 4 argued that we don’t have evolved modules for all the functions Evolutionary Psychologists claim, I will not
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be concerned here with evaluating any evidence for modularity. My focus,instead, will be on the preferences themselves—on whether men have evolved to detect and prefer young women and whether women have evolved to detect and prefer high-status men—regardless of the kind of mechanism that implements them. The question is: How good is the evidence for Evolutionary Psychology’s core hypotheses about male and female mate preferences? But before examining the evidence let’s briefly examine the theoretical foundation of the hypotheses.
“The Evolution of Desire”
As we saw in chapter 1, life (in the biological, not existential, sense of the term) is all about reproductive success—how many copies of one’s genes one contributes to future generations via the bodies of one’s offspring. We also saw in chapter 1 that many activities have fitness costs and benefits,which respectively diminish and enhance fitness. Producing offspring, the very sine qua non of fitness, is no exception. Indeed, producing offspring is a costly endeavor.
First of all, barring very recently invented reproductive technologies (which are too new to have affected evolved motives and preferences), in sexually reproducing species such as ours, you’ve got to have sex with a member of the opposite sex in order to produce offspring. But, unfortunately, members of the opposite sex don’t have sex with you just because you want them to. They’ve got to be enticed into it, one way or another,and the cost of enticement can range from the metabolic costs of producing a come-hither wink to the costs of building a bower or obtaining and presenting gifts over an extended period. Once a partner has been enticed,the sex act costs the energy involved in doing it (plus the contents of an ejaculate if you’re male). Then, if sex results in conception and you’re a female, you’ve just begun to pay. If you’re a human female, you pay the costs of a nine-month gestation, which exacts an enormous physiological toll on your body. And, throughout most of our evolutionary history,ancestral women paid the additional metabolic costs involved in breastfeeding for several years.
So here is one of Nature’s great inequities. If you’re a woman, the absolute minimum cost for producing a single offspring is quite high. Not only do you pay the costs of gestation and lactation, but you also pay the cost of forgoing any other possible reproductive opportunities with males other than the father of your offspring—possibly better males than the father of your offspring—during the period of pregnancy and lactation. If
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you’re a man, on the other hand, the absolute minimum cost for producing a single offspring is the energy expended in copulation and the contents of a single ejaculate (an inexpensive 300 million sperm and three milliliters of semen). After a fruitful copulation, a man can get up and pursue reproductive opportunities with other women, whereas a woman is committed to the costly act of childbearing. This is a radical asymmetry in the minimum costs the sexes must pay in order to produce a single offspring.
Although the costs are real, it’s not like flushing money down the toilet,since you do get an offspring out of the deal. So these expenditures are really an investment—what is called parental investment. Parental investment is standardly defined as any characteristics or behavīors of a parent that enhance the ability of an offspring to survive and reproduce at a cost to the parent’s fitness, including diminishment in the parent’s future abilities to mate or care for other offspring. Thus, one way of describing the above asymmetry between the sexes is that the minimum obligatory parental investment for women is vastly higher than that for men.
Evolutionary Psychologists derive their hypotheses about evolved mate preferences from this fact about minimum obligatory parental investment,and the derivation begins in the work of the evolutionary anthropologist Robert Trivers. In a classic article, Trivers argued that, when there is a sex asymmetry in parental investment, selection will tend to make the higherinvesting sex choosier in the mating market, because that sex stands to lose more by making a poor choice of mate. This greater choosiness on the part of the higher-investing sex will force members of the other sex to compete among one another to be chosen. As a result, the higherinvesting sex will appear more cautious in the mating market, while the lower-investing sex will appear more eager and more intensely competitive in its attempts to attract mates. For example, if males invest nothing beyond the act of copulation and an ejaculate, leaving females to cover all costs of parental care, females will be very selective in choosing a mate.Under these circumstances, males are little more than sperm transport, so a male’s quality is solely a function of the genes he can provide. Females will then hold out for males who show signs of having “good genes”—signs such as good health and bodily symmetry (a purported sign of developmental stability). And males will compete among themselves to be chosen by females, attempting to present the best advertisements of “good genes.”
Trivers’s theory is supported by observations of the mating habits of many species. Some of the strongest support for the theory comes from
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species in which males provide greater parental investment than females,since males in those species tend to be more selective in choosing mates and females are more competitive. But Trivers’s theory also predicts that,if the parental investment in both sexes is relatively high, both sexes will be highly selective in choosing mates, holding out for mates who demonstrate the ability to provide a fairly high level of parental investment.
Humans are among a small minority of species in which both sexes invest heavily in offspring. Of course, as we’ve seen, the physiological investment by females vastly exceeds that by males. But, as we saw in chapter 1, merely bringing offspring into the world is no guarantee of genetic immortality. In some species, offspring are born sufficiently developed that they can survive on their own almost immediately. But human offspring are heavily dependent on parental care for many years after birth. Indeed, in the early years they are entirely incapable of caring for themselves, requiring very intensive parental care. Since reproductive success requires that offspring themselves survive to reproduce, human offspring need to be nurtured at least until they’re able to survive on their own.
And this, according to Evolutionary Psychology, is where male parental investment comes in. During our evolutionary history, Evolutionary Psychologists argue, a female who had to spend all her days tending to a suckling infant would not have been able to adequately provide for herself and her infant. So it was necessary for males to provide their mates and offspring with food, shelter, and protection. Further, the demands of survival among our ancestors required learning the skills involved in foraging for food and making shelter, and the demands of reproduction required learning the skills involved in negotiating one’s social group. So males could also enhance the survivability and subsequent reproductive success of their offspring by playing a role in teaching them such skills. On average, then, the rate of survival and subsequent reproductive success of offspring of ancestral “single mothers” would have been lower than that of offspring who enjoyed both maternal care and a high level of male parental care. Thus, Evolutionary Psychologists argue, the extraordinarily heavy dependence of human offspring on parental care created strong selection pressure for a fairly high level of male parental investment. (There are reasons, which I will discuss in chapter 6, for believing that this is not why male parental investment evolved.)
Despite the relatively high level of male parental investment in our species, however, the postnatal parental investment provided by females still
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our species. With internal fertilization, a female can always be 100 percent certain that the offspring she births are hers. But no male can be 100 percent certain that the offspring birthed by his mate are his. For we are a species in which internal fertilization is coupled with concealed ovulation. This contrasts with other primates, such as chimpanzees. When a chimpanzee female is ovulating, her genitals swell and become red, a clear sign to chimpanzee males that it is time to take action. If a chimpanzee male wants to sire an offspring, he merely needs to ensure that he sexually monopolizes a female during her fertile period. Ancestral human males, in contrast, had no idea when females were ovulating, so they could never be sure whether they were inseminating a fertile female or not. So,in order to sire an offspring, they had to copulate with ancestral females round-the-month. But a lot can happen in a month. The demands of survival would have required frequent periods during which mates were out of one another’s sight foraging, for example. A female who had been out of sight for a mere twenty minutes could have been carrying internally the inseminate of another male. Even if her mate copulated with her immediately upon their reunion, there was never any sure way to know exactly what was going on in there. As a result, any issue from her womb was of uncertain provenance from a male’s perspective. This is known as the problem of paternity uncertainty.
Given the possibility that a male’s putative offspring are not truly his own, there is always the chance that the male is investing in another male’s offspring, thus squandering resources that could be better spent in a competition to fertilize other females. A female, in contrast, never faces the potential problem of squandering her parental investment on offspring she mistakenly believes to be hers. Thus, Evolutionary Psychologists argue,since human male parental investment can be misspent in a way that human female parental investment cannot, selection should have designed males to deliver a lower level of parental investment than females as a hedge against the possibility of misspending it. In fact, Evolutionary Psychologists further predict, the degree of male parental investment should be a function of the degree to which a male feels confident in his paternity of offspring.
Nonetheless, because human males provided a fairly high level of parental investment throughout our evolutionary history, they, like human females, have evolved to be very selective in choosing a female with whom they will jointly invest in offspring. However, because the two sexes provided different forms of parental investment throughout human evolutionary history, Evolutionary Psychologists argue, each sex has evolved to prefer as mates those members of the opposite sex who
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show signs of being able to provide the forms of parental investment in which that sex specialized in human evolutionary history.
As the Evolutionary Psychologists Douglas Kenrick and Richard Keefe put it: “Males invest relatively more indirect resources (food, money,protection, and security), and females invest relatively more direct physiological resources (contributing their own bodily nutrients to the fetus and nursing the child). For this reason, females who are choosing mates are assumed to pay particular attention to a male’s ability to provide indirect resources, and males are assumed to pay special attention to signs of a female’s apparent health and reproductive potential.”1 Thus, females should have evolved to prefer males who can provide indirect resources,whereas males should have evolved to prefer females of peak reproductive
potential.
But this poses a “detection problem” for both sexes: How can each sex detect the members of the opposite sex who possess the preferred qualities?A male’s ability to provide indirect resources cannot be directly detected in the way the length of his nose can. Similarly, as David Buss says: “The number of children a woman is likely to bear in her lifetime is not stamped on her forehead. It is not imbued in her social reputation.Even women themselves lack direct knowledge of their reproductive value.”2 Therefore, Evolutionary Psychologists argue, women should have evolved to be attracted to detectable qualities of men that are correlated with
the ability to provide indirect resources, and men should have evolved to be attracted to detectable qualities of women that are correlated with peak reproductive potential.
Women, according to Evolutionary Psychologists, solved their detection problem by evolving a preference for high-status males. For, as the Evolutionary Psychologist Bruce Ellis says: “In general, the higher a male is in status (i.e., the higher the level of esteem and influence accorded to him by others), the greater his ability to control resources across many situations.. . . Since control of positional resources is both a sign and a reward of status, natural selection could be expected to have favored evaluative mechanisms in women designed to detect and prefer high-status men.”3 Hence Evolutionary Psychology’s core hypothesis about female mate preferences.
Men, on the other hand, needed to solve the problem of detecting peak reproductive potential. Reproductive potential involves two things. On the one hand, it involves fertility, which is a measure of the likelihood of being able to conceive and carry a pregnancy to term, and a human female’s fertility typically peaks in her mid-twenties. On the other hand, reproductive
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potential involves reproductive value, which is a measure of the remaining number of offspring that a female can produce. The younger a fertile woman is the greater is her reproductive potential, since the greater is the number of years she has remaining in which to produce offspring. So women with the highest fertility don’t have the greatest reproductive value and vice versa. But women in their very early twenties are near the peaks of both fertility and reproductive value, so they have the highest overall reproductive potential—that is, the greatest ability to immediately provide the physiological resources necessary for bearing many offspring.
Of course, as Buss notes, “even age must be inferred, as it cannot be assessed directly.”4 To further complicate matters, this preference for women of peak reproductive potential evolved well before calendars and even before counting, so it wasn’t possible to simply ask about a woman’s age during the evolution of these preferences. Males, Buss argues, also had to evolve a solution to the detection problem for age. Thus, “according to evolutionary psychologists, the evolutionary model predicts that what men desire is not youth per se, but rather features of women that are associated with reproductive value or fertility.”5 These features are “full lips [since lips thin with advancing age], clear skin, smooth skin, clear eyes,lustrous hair, good muscle tone and body fat distribution.”6
The qualities of full lips, good muscle tone, and so on, are perhaps self-explanatory. But body fat distribution requires some comment. Before puberty, Evolutionary Psychologists argue, boys and girls are shaped much alike, with a waist-to-hip ratio of roughly 0.90 (which means that the girth of the waist is 90 percent of that of the hips). At puberty, however, the release of estrogen in females causes fat to be deposited on the hips and upper thighs. As a result, females’ hips become even wider after puberty, with the waist-to-hip ratio decreasing to around 0.70. Pregnancy,however, often leaves a lasting deposit of fat on the waist, increasing the waist-to-hip ratio. Further, as women approach middle age and undergo menopause, more body fat gets deposited in the waist, thereby further increasing the waist-to-hip ratio. Thus, according to Evolutionary Psychologists,a waist-to-hip ratio of around 0.70 indicates a fertile female who
has yet to bear a child; and, throughout much of human evolutionary history a fertile, yet childless, female would have been very close to her peak reproductive potential. So, Evolutionary Psychologists conclude, males have evolved a preference for females with waist-to-hip ratios around 0.70.
We see, then, how Evolutionary Psychologists arrive at the hypotheses that women have an evolved preference for high-status men and that men
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have an evolved preference for young women (that is, women with physical features that are correlated with peak reproductive potential).These hypotheses are derived from general theoretical considerations regarding the nature of parental investment in our species.
It is worth noting, however, that these preferences are for the qualities of long-term mates. According to Evolutionary Psychologists, when people are in the market for short-term mates (one-night stands, for example),their preferences shift. Men still like fertile women as short-term mates,Evolutionary Psychologists claim, but men’s standards for short-term mating typically drop so low that they’re willing to copulate with pretty much anything that is self-moving (since, after all, sperm is cheap).Women, on the other hand, are less interested in status and more interested in intelligence and good looks when seeking a short-term mate. In what follows, I will not examine these claims about short-term mate preferences,but will focus exclusively on the two core hypotheses regarding long-term mate preferences.
Each of the hypotheses about long-term mate preferences is separable into two independent claims. One is a claim about what people prefer, and the other is a claim about why they prefer it. Each hypothesis, that is, contains a claim that a particular universal preference has evolved in each sex and a claim that that universal preference evolved because of selection for it in our evolutionary past (that it is an adaptation). These claims are typically not separated, because empirical studies in Evolutionary Psychology are presumed to test both claims simultaneously.
To illustrate, consider the male preference for youth. As Buss says,“because male reproductive success in humans depends heavily on mating with reproductively capable females, selection over thousands of generations should favor those males who prefer to mate with reproductively capable females.”7 Here a hypothesis about what males prefer (reproductive capability) is derived from a hypothesis about how selection has acted during human evolutionary history, which would explain why males have that preference (it is an adaptation). If we get confirmation of the derived (preference) hypothesis, it seems to be simultaneous confirmation of the hypothesis (about past selection) from which it was derived. So, if the evidence shows that males indeed prefer youth, that appears to confirm the hypothesis that the preference is an adaptation.
Universality enters the picture because, for the reasons discussed in chapters 2 and 3, Evolutionary Psychologists believe that adaptations are,of necessity, species universals. This is why, in attempting to confirm hypotheses about evolved mate preferences, Buss conducted a massive
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cross-cultural study (to be discussed below) to determine whether the predicted preferences are indeed universal. According to Buss, the evidence shows that “men universally prefer younger women as wives” and that “women worldwide desire financial resources in a marriage partner.”8 Thus,Evolutionary Psychologists believe that the evidence shows that male preference for young females and female preference for high-status males are adaptations.
I’ve belabored the distinction between a hypothesis about what people prefer and a hypothesis about why people have those preferences because it helps clarify two different ways in which Evolutionary Psychology’s hypotheses about mate preferences can be questioned. On the one hand, one could ask: How good is the evidence that male preference for youth and female preference for high status are adaptations? That is, how good is the evidence for Evolutionary Psychology’s claims about why people have these preferences? A number of critics of Evolutionary Psychology have asked this question and answered it in the negative, arguing that Evolutionary Psychology falls far short of providing convincing evidence that these preferences are adaptations. Indeed, this is the line of argument that Gould consistently urges against Evolutionary Psychology. According to this line of argument, selection isn’t the only explanation for the existence of these preferences, so merely finding the preferences doesn’t confirm that they are adaptations, since it doesn’t rule out nonadaptationist explanations of the preferences.
But this line of argument presupposes that Evolutionary Psychologists have provided convincing evidence that males indeed prefer youth and that females indeed prefer high status. So, on the other hand, one could ask: How good is the evidence that males prefer females of peak reproductive potential and females prefer high-status males? That is, how good is the evidence about what people prefer in mates? I believe that this question has not received the attention it deserves, and it will be the focus of the sections to follow. I will argue that there is no convincing evidence for either hypothesized universal mate preference.
Before turning to these arguments, however, a comment is in order on the notion of universality. We have already discussed some of the complexities of this notion in Evolutionary Psychology, and we have seen that when Evolutionary Psychologists use the term “universal” they are implicitly referring to a developmental program shared by all “normal” human beings, not to manifest or observable preferences, beliefs, attitudes, or behavīors. So, if push came to shove, Evolutionary Psychologists would admit that their claims regarding mate preferences do not mean that
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each and every human male prefers young women and that each and every female prefers high-status men. It is always possible that certain individuals have unusual developmental experiences and end up not possessing the predicted preferences. But, if there is a truly universal developmental program that has been designed by selection to reliably produce a preference for young females in men and a preference for high-status males in women, that developmental program should produce those preferences across a very wide range of conditions. Thus, Evolutionary Psychologists would maintain, to say that those preferences are “universal” means that they are observable in all cultures, all historical periods, all economic or political systems, all social classes, all religious groups, all “races” or ethnicities, and all relevant ages of the life cycle. It is this more restricted sense of “universal”that is operative when Buss claims that female preference for highstatus,resource-holding mates is universal. As Buss says, “women across all continents, all political systems (including socialism and communism), all racial groups, all religious groups, and all systems of mating (from intense polygyny to presumptive monogamy) place more value than men on good
financial prospects.”9
I will argue that, even in this more restricted sense of “universal,” the data on human mate preferences fail to provide convincing support for claims of a universal male preference for youth and a universal female preference for high status. Indeed, I will argue, the mate preferences in which Evolutionary Psychologists are interested tend to vary with age and social class, among other things. If this is right, then something is wrong with the hypotheses about human evolution from which Evolutionary Psychology derives its claims about mate preferences. Let’s turn now to the evidence for Evolutionary Psychology’s core mate-preference hypotheses,
focusing on the studies that are standardly cited in support of those hypotheses.
Men Seeking Women
In collaboration with a bevy of social psychologists from around the world,David Buss gathered survey data about mate preferences from 4,601 men and 5,446 women (a total of 10,047 subjects), who comprised thirty-seven survey samples from thirty-three countries located on six continents and five islands. The sheer scale of this study is remarkable, and the study has become an exemplar of empirical research in Evolutionary Psychology.
Among other things, Buss’s survey asked subjects to give the age at which they’d prefer to marry, and he found that, on average, males preferred to
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marry at 27.49 years of age. He also asked subjects to give the preferred age of their mates relative to their own ages. So males were asked to state how much younger or older than themselves their ideal mate would be.He found that in every one of the thirty-seven samples males indicated a preference for younger mates, with average preferences ranging from 0.38 to 7.38 years younger. Pooling all the samples, Buss found that, on average,males preferred a mate who was 2.66 years younger. “By subtracting the mean age difference preferred between males and their mates (2.66 years)from the age at which males prefer to marry (27.49 years), it can be inferred that males in these samples prefer to marry females who are approximately 24.83 years old. This age preference is closer to peak female fertility than to peak reproductive value.”10
Without splitting hairs about peak fertility versus peak reproductive value (or peak reproductive potential, which incorporates both), an average preferred age of 24.83 years is clearly near the height of female reproductive potential. Given the large cross-cultural scale of Buss’s study, this appears to show that male preference for females with high reproductive potential is universal. And this, in turn, appears to confirm the hypothesis that selection has designed male mate preferences to be highly sensitive to female reproductive potential.
As Buss recognizes, however, males may indicate preferences on a survey questionnaire that don’t accord with the actual decisions they make in choosing a mate. In addition, offspring are produced not by preferences for mates with certain qualities, but by actual matings. Consequently,selection cannot have acted on male preferences unless males throughout human evolutionary history actually mated in accordance with their preferences. In particular, a preference for fertile young women could not increase in frequency in a population unless there was a strong correlation between that preference and actually mating with fertile young women.
Thus, there can’t have been past selection for a male preference for young women unless males with that preference actually produced more offspring,by actually mating with fertile young women, than did males with alternative preferences.
To confirm that the preferences for young women that males reported on his questionnaire are (and presumably were in our evolutionary history)reflected in actual mating behavīor, Buss compared the age-preference data with the actual ages at marriage of men and women in thirty of his thirtyseven samples. He found that the average age at marriage was 28.2 years for males and 25.3 years for females, only slightly higher than males’ preferred ages of 27.49 years and 24.83 years respectively.
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Of course, it takes two to mate. So, while males may prefer mates who are 24.83 years old, females have their own preferences, and females expressed a preference to marry at 25.4 years to a man of 28.8 years. Thus,the discrepancy between males’ preferred ages of self and spouse at marriage and the actual ages at marriage appears to be a product of compromise with female preference. Indeed, Evolutionary Psychologists argue, we should expect all actual mating decisions and behavīors to differ from the preferences of both sexes, since the preferences of the sexes will typically differ; and, when preferences of the mating parties differ, actual mating
decisions and behavīors will reflect a compromise between the preferences.Despite the expected compromise, however, the actual average age of women at marriage is very close to the male preference, so male preferences for young women do indeed appear to be reflected in actual mating behavīor. Therefore, Buss concludes, the preference data together with the marriage data provide strong “support for the evolution-based hypothesis that males both prefer and choose females displaying cues to high reproductive capacity.”11
But Buss’s analyzed data do not clearly confirm this hypothesis. Buss’s analysis of his preference data consists in subtracting the average preferred age difference between male respondents and their female mates (2.66 years) from the average age at which his male respondents said they preferred to marry (27.49 years). Since the average age of his male respondents was 23.49 years, this shows only that young men say that they prefer to marry relatively younger women and to do so at a fairly young age. As Buss recognizes, what males say they want in a mate stands in need of a validity check, which his analysis of his marriage data purportedly provides. But
Buss’s analysis of his marriage data consists only of comparing the average age of males at marriage (28.2 years) with the average age of females at marriage (25.3 years). While this does show that on average males marry fairly young women, it also shows that on average the males marrying them are themselves young—only 2.9 years older than their brides.
If males both prefer and choose young women as mates, however, this preference should be present across the male life cycle. Older males should exhibit a preference for young women just as young males do. Since Buss’s analysis employs only the averages from his samples, it doesn’t show that older males prefer and choose young women as mates. The mate preferences of older males disappear into the averages, and the averages present a profile of the mate preferences of relatively young males. But, to confirm that males have an evolved preference for young women, it is not enough to show that young men prefer young women.
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The reason is that there is a large body of sociological evidence that shows the most robust mate-choice phenomenon to be what social scientists call homogamy. Homogamy is the tendency for people to mate with those similar in race or ethnic background, age, socioeconomic status,educational background, and religious orientation. Homogamy is a form of what biologists call assortative mating, which is preferential mating with other organisms with like phenotype(s). In the case of homogamy,mating is assortative with respect to social characteristics rather than
morphological or behavīoral phenotypes. And a very recent large-scale study of mating in the United States, conducted by the sociologist Edward Laumann and his colleagues, found that similarity in age is even more important in mate choice than similarity in religious orientation.
But why should age similarity be important in mating? The Evolutionary Psychologists Douglas Kenrick and Richard Keefe argue that there may have been selection for assortative mating by age in our evolutionary past.“Extended interactions over long periods between mates would have been easier if the partners had similar expectations, values, activity levels, and habits. A preference for similarity in age, all else being equal, would have made the long-term cooperation of mates more feasible and thus adaptive.. . . Thus, humans may have evolved with a preference for similar mates,including similarly aged mates, because of the advantage to parenting effort this would have contributed.”12 This simple hypothesis of age homogamy—that human mating is assortative by age—appears sufficient to explain Buss’s finding that young men prefer young women.
Of course, assortative mating by age doesn’t explain why Buss found a consistent age difference in both his preference data and his marriage data.If people merely seek similarly aged mates, in a very large sample such as Buss’s we should expect the average male preference to be for similarly aged mates (rather than for mates 2.66 years younger) and the average age difference between spouses at marriage to be close to zero (rather than 2.9 years). Why do males consistently prefer and mate with younger women, while women prefer and mate with older men? According to Buss, this age difference reflects the fact that men seek young women as mates, because
of their reproductive capacity, and women seek older men as mates,because older men tend to have greater resources than younger men. Thus,the consistent age difference between mates appears to tell in favor of the hypothesis that males have an evolved preference for young women and against the hypothesis of age homogamy.
But, if Buss is right, why should the average age difference be as small as it is? Why shouldn’t twenty-eight-year-old males on average prefer
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twenty-year-old females, who have greater reproductive potential than twenty-five-year-old females? Similarly, why shouldn’t twenty-two-yearold females prefer thirty-five-year-old males, since they tend to have greater resources than twenty-five-year-old males (whom they actually prefer), yet still have a fairly long life ahead of them in which to provide resources for a female and her offspring? Age similarity does seem to be a factor in mate choice. Perhaps some variation on the hypothesis of age homogamy would account for the age difference that Buss found, while providing a better explanation of Buss’s data than the hypothesis that males simply have an evolved preference for young women.
Consider the following variation on the hypothesis of age homogamy.Let’s begin by supposing that selection favored assortative mating by similar age for the reasons that Kenrick and Keefe suggest (although, in chapter 6, I will present reasons for thinking that this preference was driven by sexual selection rather than natural selection). We need now to explain why, within this general constraint of age similarity, there should be a consistent age difference of just a few years between mates. The zoologist Janet Leonard suggests that this relatively small average age difference between mates is due partly to the fact that human males achieve reproductive maturity later than females. In fact, males lag behind females in reaching puberty and full adult growth by two years, on average. In addition, she argues, because competition among males for mates is slightly greater than competition among females, males require more time than females after reaching physiological maturity to hone their competitive skills and become successful at acquiring mates. This would further increase the age difference between mates. So, if humans paired up strictly as a function of similar age (for the reasons Kenrick and Keefe suggest), but offset similarity in age by sex differences in the achievement of reproductive maturity (for the reasons Leonard suggests), males would be a few years older, on average, than the females with whom they pair. And this corresponds closely with the average age difference Buss found in his marriage data.
This age difference could be the result of evolution’s having equipped males with a simple preference for females who are a little younger and females with a simple preference for males who are a little older. These preferences could have been adaptive in our evolutionary past by helping to ensure matings between individuals of comparable reproductive maturity at the point in life at which reproduction typically began, which in turn helped ensure extended cooperation in providing parental care.
Let’s call this alternative hypothesis the hypothesis of adjusted age homogamy. Like Buss’s hypothesis, this hypothesis makes a prediction
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about mate preferences and provides an evolutionary explanation of those preferences. The hypothesis of adjusted age homogamy predicts that males and females both prefer similarly aged mates, but that the preferred ages are adjusted for sex differences in age at reproductive maturation. This entails that males prefer females who are a few years younger than themselves and that females prefer males who are a few years older. This hypothesis thus explains why Buss found that males prefer and mate with females who are a few years younger. More interestingly, however, it also explains why age similarity—albeit adjusted age similarity—would be such a robust effect in human mating.
We have, then, two hypotheses to consider. One is Evolutionary Psychology’s
hypothesis that males have an evolved preference for young women because young women have the greatest reproductive potential.The other is the hypothesis of adjusted age homogamy, according to which males have an evolved preference for females who are, on average,a few years younger than themselves. These hypotheses make competing predictions regarding the preferences of forty- or fifty-year-old males.Evolutionary Psychology’s hypothesis predicts that males of all ages should
prefer—and, when possible, mate with—women in their early twenties.The hypothesis of adjusted age homogamy, on the other hand, predicts that forty-year-old males should prefer women in their late thirties, while fifty-year-old males should prefer women in their late forties. Both hypotheses, however, predict that males in their late twenties should prefer mates in their early midtwenties. Buss’s finding that marriages occur between twenty-eight-year-old males and twenty-five-year-old females, on average, and that this accords closely with the stated preferences of young males, is actually compatible with both hypotheses. Thus, Buss’s findings
don’t actually confirm Evolutionary Psychology’s hypothesis, since they don’t rule out the competing hypothesis of adjusted age homogamy.But Kenrick and Keefe conducted a study that does appear to show that males do, indeed, have a preference for young women, not simply for slightly younger women. Rather than averaging the ages at marriage of all the subjects in their samples, Kenrick and Keefe examined the average age differences between spouses at marriage for separate age groups—for individuals who married in their teens, twenties, thirties, forties, fifties,and sixties.
Kenrick and Keefe expected that age similarity would be a large factor in mate choice, for the reasons already discussed, but expected that males would also prefer females of peak reproductive potential. Consequently,they hypothesized that males weigh both age similarity and reproductive
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potential in mate choice, with the result that actual choices of mate strike
a balance between the two potentially competing considerations.
This has some interesting implications. For a male in his twenties, like
Buss’s average respondent, similarly aged females are also those near their
peak reproductive potential, so males in their twenties should prefer
females in their early twenties. But, as males age, similarly aged females
are increasingly further from their peak reproductive potential, so older
males must trade off the increasingly competing considerations of age
similarity and reproductive potential. Thus, Kenrick and Keefe predicted,
“whereas aging males should prefer progressively older women (because of
similarity), they should also prefer women progressively younger than
themselves (to maximize reproductive opportunities).”13 That is, as males
get older, the average age difference at marriage between self and spouse
should gradually increase. While the age difference at marriage should be
relatively small for males in their twenties, it should be fairly large for older
males, who must choose females no older than their forties in order to
have mates with some remaining, albeit small, reproductive potential.
Kenrick and Keefe examined all the marriages that took place in Seattle
in January 1986 and a sample of those in Phoenix in January and May
1986. To ensure that their results would not simply be an artifact of 1980s
America, they also examined a sample of one hundred marriages in
Phoenix in 1923. And to further ensure that these combined results would
not simply be an artifact of American culture, they examined all marriages
on the Philippine island of Poro between 1913 and 1939.
Kenrick and Keefe found the same pattern in all their samples. The 1986
samples were virtually identical. In these samples, on average, males who
married in their twenties married females a year or so younger; males in
their thirties married females a few years younger; males in their forties
married females about six years younger; males in their fifties married
females about nine years younger; and males in their sixties married
females about ten years younger. The sample of Phoenix marriages in 1923
showed the same pattern for males in their twenties and thirties, but there
were even greater age differences between older males and their spouses.
In 1923 Phoenix, males in their forties married females about thirteen
years younger, and the age difference between spouses increased a year for
each decade of male age after that. Finally, in Poro, on average, males in
their twenties married females three years younger; males in their thirties
married females about nine years younger; males in their forties married
females about twelve years younger; males in their fifties married females
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fifteen years younger; and males in their sixties married females a full
twenty years younger.
Although these data appear to provide straightforward confirmation of
Kenrick and Keefe’s hypothesis that males weigh both age similarity and
reproductive potential in selecting a mate, thus striking a balance between
the two considerations, things are not quite that simple. First, as the
psychologist Kim Wallen points out, the principal period of fecundity for
women is between the ages of twenty and forty, and the average age of
menopause is fifty. But the data show older males, on average, marrying
women who are past the period of principal fecundity and much older
males marrying women who are in their postreproductive years or very
nearly so. If reproductive potential is a significant factor in male mate
choice at all, regardless of the male’s age we should not find males marrying
women who are at or very near the end of their reproductive careers.
Of course, males aren’t the only ones doing the choosing. It may be that
males in their late fifties and older are unable, for the most part, to attract
and marry women with significant remaining reproductive potential. So
the fact that older males marry women with little or no reproductive
potential could simply be a result of compromise in the mating market.
Perhaps older men would rather marry significantly younger women, but
they can’t, so they settle for women who are postmenopausal or very
nearly so.
But Kenrick and Keefe also gathered data from personal ads, in which
advertisers indicated a preferred age or age range for their respondents,
and the pattern of average preferred age differences from the ads closely
matched the pattern of average age differences in the marriage data. As the
age of male advertisers increased, the average age difference between them
and their desired respondents also increased. However, although men in
their fifties and sixties did express a preference for much younger women,
on average, the ages they preferred still fell near the end of or beyond female
reproductive potential. So, on average, older males not only marry women
who are postreproductive or nearly so, but seek them as well.
This is not what we should expect given Kenrick and Keefe’s hypothesis.
Even if males choose mates by weighing both age similarity and reproductive
potential, when a potential mate has little or no reproductive
potential, age similarity should count for little or nothing in mate choice.
For, by Kenrick and Keefe’s account, age similarity factors into male
mate choice only because it facilitates extended cooperation in providing
parental care. But, if a postreproductive mate is chosen, there will be no
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offspring for whom to provide parental care. So a preference for age similarity
can facilitate parental cooperation only if it plays second fiddle to
the preference for reproductive potential. However, Kenrick and Keefe’s
marriage data and preference data appear to show that a preference for age
similarity among older males virtually trumps any preference for reproductive
potential.
A second problem is that the samples of marriages of males in their fifties
and sixties consist almost entirely of males who are remarrying, as Kenrick
and Keefe acknowledge. Evolutionary Psychologists argue that it is enlightening
to examine the choices that males of those age groups make when they are seeking new mates. But their mating decisions present only a partial picture of the mating decisions of males in those age groups.
Consider the fact that the National Marriage Project of 2000 found that
40 to 50 percent of all marriages end in divorce. Suppose we adopt the
extreme estimate that 50 percent of marriages end in divorce. Some of
these divorces are attributable to serial marriers (or serial divorcers, depending
on whether you’re a romantic or not), for whom two out of three, three
out of four, or even seven out of eight marriages end in divorce. So, even
if as many as 50 percent of all marriages end in divorce, it is not the case
that 50 percent of all those who marry end up getting divorced. At least
50 percent of all men who marry do not get divorced, hence never remarry,
so about half of all men in their fifties and sixties have decided to remain
married. Since this half will have married much earlier in life, by Kenrick
and Keefe’s own data, their wives will be relatively close to their own ages.
The divorce data, and independent data about the frequency of infidelity,
however, shows that married people frequently have the option of taking
up with a new mate. So, these males are making genuine choices to remain
married, since they always have the option of divorcing and looking for a
new mate. Remaining married is actually a continual choice of one’s spouse
over others. Thus, half the older male population is choosing to remain in
mateships with women who are no longer capable of bearing children. A
hypothesis about male mate preferences can’t be tested exclusively on the
males who choose to remarry after fifty. The choices of males who remain
in mateships with no reproductive potential have to be considered as well.
Third, Kenrick and Keefe’s analysis of the data suffers from a problem
that plagues Buss’s analysis as well. They base their analysis entirely on the
averages in their samples and ignore the variation. But, as Kenrick and Keefe
admit: “Individual subjects showed wide variation in their preferences,
however, and in their choice of marriage partners. There were older men
who sought, and others who married, women their own age.”14
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Kenrick and Keefe don’t report the variation in their data, but the
evolutionary psychologist Karl Grammer reconstructed the variation from
their personal-ad data. Grammer found the variation to be much higher
than one would expect if males prefer mates with high reproductive potential.
Consider a couple of examples. Among 53-year-old males, preferences
for mate age ranged from 35 to 57, and among 56-year-old males they
ranged from 46 to 52. The marriage data undoubtedly exhibit similar
ranges, although they aren’t reported by Kenrick and Keefe. This means,
however, that a significant number of males in their fifties and sixties both
prefer and choose postreproductive women as mates. And this doesn’t
conform to Kenrick and Keefe’s predictions.
Apart from these specific problems with Kenrick and Keefe’s hypothesis,
there is a general reason why it’s problematic to test hypotheses about mate
preferences against sample averages alone, as both Buss and Kenrick and
Keefe do. As we saw in chapter 1, variation is not only the fuel on which
selection burns, but is itself often produced and maintained by selection.
As a consequence, patterns of variation can be highly significant, because
they can indicate that different, possibly frequency-dependent, strategies
are being pursued. To put this another way, Evolutionary Psychologists
assume that each hypothesis about past selection entails a prediction about
a single adaptation that evolved in response to it. As a result, Evolutionary
Psychologists tend to focus only on the sample average to see whether
it conforms to the phenotypic value they derive from their hypothesis
about past selection. But hypotheses about past selection can entail the
coexistence of multiple adaptive phenotypes in a population. In such cases,
phenotypic values in a population may be bimodally (or trimodally) distributed.
Such distributions, however, are concealed when only sample
averages are calculated. So, rather than collapsing variation inside sample
averages, we should always ask whether there is a potential explanation of
the variation itself.
There is not sufficient data at this point to strongly confirm any hypotheses
about the precise nature and source of variation in male preferences
regarding age differences between themselves and their mates. But there is
sufficient data to suggest a possibility. Let’s review a few of the relevant
facts.
First, even if we focus only on the average age differences between males
and their mates, we find that older males, on average, both prefer and mate
with females who are very near or at the end of their reproductive careers.
So, if males are weighing both age similarity and reproductive potential
in choosing their mates, they are placing too great a weight on age
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similarity if they are still looking to reproduce. Second, when we consider
the variation in the data, rather than just the averages, we find that many
older males both prefer and choose postmenopausal females as mates.
Third, the older males in Kenrick and Keefe’s preference data and marriage
data are either on the market for mates or remarrying, respectively. This
group fails to represent that larger portion of older males who have chosen
to remain in mateships with postreproductive females.
When these facts are considered together, they seem to call into question
Evolutionary Psychology’s standard depiction of the mating life of the
human male. Evolutionary Psychologists typically focus only on the fact
that a female’s reproductive career is limited by menopause while a male
can, theoretically, produce offspring well into old age. This focus portrays
males throughout the life cycle as virile and sexually heroic.
Although it is, indeed, possible for most males to sire offspring even into
old age, the fact is that precious few males do sire offspring in old age,
even in hunter-gatherer populations. A fertility study of the !Kung of the
Kalahari Desert showed that male fertility peaks at thirty, declines slightly
to the age of forty, then declines rapidly. Although 25 percent of all individuals
born survive to age sixty, fifty-year-old males have only about a 3
percent chance of siring an offspring, and by age fifty-five male fertility
drops to zero. In addition, a British fertility study of 8,515 couples found
that males over thirty-five were half as likely as males under twenty-five
to impregnate their partners within twelve months, even after the study
controlled for their partners’ age and health. Moreover, male sex drive
peaks in the twenties, then declines continually throughout the rest of life.
Accompanying this decline in sex drive is a reduction in the size of the
testes, a reduction in the volume and force of ejaculation, and a significant
reduction in the number of motile sperm in an ejaculate. The above
facts hardly paint a picture of a well-oiled sex machine designed to impregnate
females even on a deathbed. There is a very real degradation in male
sexual function beginning in middle age and continuing throughout the
latter part of the life cycle. Might this change be accompanied by a shift
in male reproductive effort?
Reproductive effort refers to the allocation of physiological resources
among the component demands of survival and reproduction. It is
expended throughout the life cycle and includes factors such as growth.
For our purposes, we can consider just the strategic allocation of resources
between mating effort and parenting effort. Mating effort, of course, is effort
expended to mate. An organism that mates indiscriminately and invests
nothing in the spawn of its matings allocates all of its reproductive effort
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to mating effort. In contrast, an organism that mates only until offspring
are produced, then forgoes any further mating in order to invest itself fully
in caring for its offspring, allocates part of its reproductive effort to mating
effort and the majority to parenting effort.
Parenting effort, recall, is essential to reproductive success. For, if one’s
children fail to reproduce, one has hit a genetic dead end just as surely as
if one failed to have children. Reproductive success requires providing care
and resources for one’s children in an effort to ensure that they in turn
reproduce. But it’s just as important that one’s grandchildren and greatgrandchildren reproduce as well, for precisely the same reasons. So, wherever selection favors parental investment, it should also, and for the same
reasons, favor some investment in one’s remoter descendants in an effort
to help ensure their survival and reproduction. Of course, life is limited,
and in humans the opportunity to invest in descendants is typically
limited to grandchildren. Since the lifespan is long enough to overlap with
the lives of one’s grandchildren, however, there is an opportunity to allocate
some reproductive effort to grandparenting effort—to caring or providing
resources for, or aiding one’s children in caring or providing resources
for, one’s grandchildren.
The anthropologist Kristen Hawkes and her colleagues have found that,
through caring and providing resources for their daughters and their
daughters’ children, grandmothers can promote their reproductive success
more than if they were to have more offspring themselves. By providing
care and resources to her daughter and her daughter’s children, a grandmother
enables her daughter to resume childbearing more quickly than
she would if she had to care for herself and her children on her own. And
the number of additional children this enables the daughter to produce
exceeds the number of children the grandmother would be able to produce
and successfully care for were she to continue having offspring of her own.
In other words, even if postmenopausal women could still have children,
they would nonetheless increase their expected genetic contribution to
future generations more through grandparenting effort than through continued
mating effort.
No one has studied grandfathering to a fraction of the extent that
Hawkes and her colleagues have studied grandmothering. But the psychologists
Harald Euler and Barbara Weitzel did a study of grandparental
investment in Germany. Euler and Weitzel asked adult subjects to indicate
on a seven-point scale, ranging from 1 (not at all) to 7 (very much), how
much each of their grandparents had provided care for them up to the age
of seven years. Euler and Weitzel were primarily interested in whether
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paternity uncertainty affected the degree of grandparental investment.
They reasoned that the father’s parents should invest less than the mother’s
parents because of the possibility of mistaken paternity. They also reasoned
that grandfathers should invest less than grandmothers because of their
own possibly mistaken paternity of their putative children. As a consequence,
they claimed, paternal grandfathers should invest the least of all
due to the possibility of two counts of mistaken paternity. Euler and
Weitzel did find that maternal grandmothers invested more than maternal
grandfathers, that paternal grandmothers invested more than paternal
grandfathers, and that the maternal grandfather invested more than the
paternal grandmother. These patterns seem to support their hypothesis
that paternity uncertainty affects grandparental investment.
What is most interesting for our purposes, however, is their finding that
grandfathers were rated as significantly investing in their grandchildren.
The average rating of investment by maternal grandmothers was 5.09,
whereas the average rating of investment by maternal grandfathers was
4.51. Similarly, the average rating of investment by paternal grandmothers
was 4.20, whereas the average rating of investment by paternal grandfathers
was 3.80. Clearly, grandmothers were more investing than grandfathers.
However, the difference between grandmothers’ averages and
grandfathers’ averages is not very large. And this is not simply an artifact
of grandfathers’ being pressed into service by nagging grandmothers,
for the average investment rating of widowed grandfathers was still fairly
high. Widowed maternal grandfathers got an average rating of 4.17,
and widowed paternal grandfathers got an average rating of 3.89. Thus,
although grandmothers are clearly more investing than grandfathers, there
is still evidence of significant grandparenting effort on the part of older
males, especially on the part of fathers of daughters with children.
This at least raises the possibility that, like grandmothers, though to a
lesser extent, grandfathers can enhance their reproductive success through
grandparenting effort. This could be the case if the rate of return on grandparenting effort exceeded the rate of return on continued mating effort.
This seems possible given the decline in sexual function in older males and
the fact that most older males are simply not as attractive as younger men
to young women (so they cannot compete as successfully for matings with
young fertile women). So, for most older males, continued mating effort
would likely not pay. Further, since the rate of return on joint grandparenting
effort would be greater than that on single grandparenting effort,
it might pay older males to remain in mateships with their postmenopausal
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Full Text: 1998 - current 
