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Why Sex Matters- A Darwinian Look at Human Behavior

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内容提示: productive competition. In some fig wasps, for example, there aretwo kinds of males, winged and wingless. They follow very differ-ent strategies and have access to different populations of females;wingless males mate with their sisters before the sisters emerge fromthe fig wasp, while winged males are larger and emerge to seek ot...

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productive competition. In some fig wasps, for example, there aretwo kinds of males, winged and wingless. They follow very differ-ent strategies and have access to different populations of females;wingless males mate with their sisters before the sisters emerge fromthe fig wasp, while winged males are larger and emerge to seek otherfemales and fight other males for them.49The two kinds of males arenot really in the same “population” of competitors. In fact, in anysuch species with clear “alternate male strategies,” a reproductivestratification exists that makes it inappropriate to combine individu-als from different groups or strata.Stratification can be permanent, as in the above examples; or on-togenetic, as, for example, in species such as bullfrogs in which malesmove through age or size categories in which they have considerablydifferent means and variance in their success. In species in whichthere are true “conditional” strategies (all males may switch amongstrategies),50stratification is not an issue except as it interacts withsurvivorship; but when genotype or age influence possible strate-gies, stratification can have a powerful influence. In humans, cul-tural stratification (e.g., heritable class) can have the same reproduc-tive impact as permanent stratification.51One important ramification of sexual selection theory is predict-ing how hard males should “strive.” For this reason, it is importantto incorporate any stratification in analyses, for it influences successof striving.52How hard one should strive and how many risks oneshould take depend greatly on the costs and benefits of striving, andthese depend on whether payoffs are constrained by one’s stratum.It has often been observed that intensity of male fights depends onhow closely matched the males are in size and other similar attrib-utes of power.53In bluegill sunfish, and in Ruffs, a European lek-breeding bird,54there are clearly two sorts of males who behave verydifferently. In sunfish, large males pursuing a territorial strategyexert more effort and take more risks than small males, who becomefemale mimics. Both the mean success and the variation in successare greater for large males than for small ones, and any attempt tounderstand the intensity of sexual selection would fail if these maleswere lumped in the same analyses.To begin thinking about such systems effectively, it might be use-ful to partition the variance. For striving behavior by an individualin any stratum, what are the costs? What are the opportunities forgain—from striving, and from being in a particular stratum? This S E X A N D S T A T U S A M O N G T H E A P E S73question of “What produces variation in success?” is not simply amale question; for example, in rhesus macaques, female status has amatrilineally inherited component,55suggesting that female repro-ductive success associated with status can also sometimes be parti-tioned into an inherited rank component and a behavioral strivingcomponent.Our ability to study behavioral dimorphisms in the context of sex-ual selection would be more precise if we first partitioned variancesin male and female reproductive successes into a nonbehavioralcomponent, including morphological characters and inherited re-sources, and a second component that would predict behavior.56In-formally, biologists Steve Frank and James Crow have suggested asimple method for quantifying the opportunity for reproductive gainthrough striving and risky competitive behavior, in which competi-tive success has both a stratum-related component independent ofcompetitive behavior, and a variable component in which successdepends on the intensity of striving.57Their method suggests that:1. When opportunities for gain through striving differ among classesor strata, different behavioral patterns are expected: high-variancestrata will contain the most competitive and risk-taking individuals.2. Within-class variance is most important in strata that are on aver-age most successful (i.e., when Ri3. Among species in which status explains a similar proportion ofvariance, if two species differ in the amount of total variance ex-plained by heritable rank, then the two species are expected to dif-fer correspondingly in the levels of aggression over status.2is high in the model).Using this model, an example can be placed in the wider context ofpartitioning variance into behavioral and nonbehavioral compo-nents. Some interesting insights follow from the general model.Among human societies, variance in male reproductive success candiffer greatly.58The extent to which resources (status, wealth, etc.)are inherited also varies widely. For a society in which the variancein resource control among males is high and resources or status areheritable (e.g., strong patrilines), resource control can create stratifi-cation, and thus may well influence the utility of striving andachievement—and risk taking.Heritability of resources should be inversely related to striving be-havior. When heritability of resources is high, then the opportunityfor gain within strata is likely to be low and competitive behavior 74C H A P T E R F O U Rmuted. Data on striving behavior are far more difficult to obtainthan data showing that resources influence reproductive success, butsome patterns are clear. The more polygynous the society (the higherthe potential rewards), the more sons are taught to strive using sev-eral measures—but only in nonstratified societies, in which a man’sstriving can make a difference in his ability to marry (chapter 7).When a man’s reproductive success is largely set by his heritable re-sources or social/class position, and unlikely to be changed muchby potentially expensive and dangerous striving, parents are un-likely to teach the value of striving.Women’s Gains and Losses in PolygynySuccessfully polygynous men are always reproductively better offthan their nonpolygynous competitors; that’s why it is worth all thecost and all the risk. But the situation is more complicated forwomen. Above, it appeared that when serious pathogens madesome men poor choices, women might prefer polygyny. And some-times polygynous marriage with a high-status man appears to bepreferred by women or their families, even when there are no ap-parent reproductive benefits.59Women often suffer costs in polygynous systems: in a number ofsocieties, second and subsequent polygynous wives have lower fer-tility than monogamous wives, or than first wives in polygynoushouseholds.60Children are likely to survive less well in polygynoushouseholds, and a major cause of divorce in polygynous societies isconflict among co-wives.61A variety of proximate factors undoubtedly interact: for example,men may be older when they marry their “later” wives; women whoare not considered desirable are likely both to marry late (and thushave low reproductive value) and to be a later wife. Nonetheless, thenet result is that within a polygynous society, a woman’s (or her fam-ily’s) choice between an already married man and a not-yet-marriedman may be complicated.The Ecology of Monogamy and PolyandryWomen seldom fully share men’s reproductive interests.Males will strive for polygyny when resources are sufficient; when S E X A N D S T A T U S A M O N G T H E A P E S75females can be an independent unit with their offspring, polygynypredominates. We expect to see monogamy and polyandry when re-sources are more limited—in harsh and unproductive habitats,when men may do better reproductively by helping to raise a childwith true parental investment, rather than continuing their matingeffort.62We expect social groups in such environments to be small,and to have relatively little variation in the resources controlled byindividuals.63At a crude level, this pattern holds: in the Standard Cross-CulturalSample, highly polygynous societies are found in areas of the worldwith significantly higher plant productivity (a measure of environ-mental richness) than others; polyandrous societies are found inareas of significantly lower plant productivity; and there is no dif-ference in the plant productivity of “monogamous” and mildlypolygynous societies. Here, more than ever, it is important to distin-guish between the anthropological and the ecological definitions of“monogamy.”64In this case, when the definitions of marriage sys-tems of the societies in the Standard Cross-Cultural Sample comefrom ethnographies, the term “monogamous” means monogamous-to-mildly polygynous, and thus it is not surprising that “monoga-mous” societies show no difference from mildly polygynous ones.Similarly, reading the ethnographic descriptions of societies deemed“monogamous” leads one to conclude that even when a few menmanage (through skill in hunting, or getting novel sources of in-come) to be polygynous when most men remain monogamous, it isin habitats with a poor resource base—insufficient for many men tomanage to gain more than a single wife.Polyandry is extremely rare; of the 186 societies in the StandardCross-Cultural Sample, only three are reported as polyandrous. Al-most all polyandrous systems are fraternal: co-husbands are broth-ers. Of course, the interests of the two brothers differ; and the impacton fitness will also differ by birth order, sex, and opportunity costs(other available options).65Polyandry seems to occur under two cir-cumstances, both related to the conservation and concentration of(rare) resources. Among the Lepcha of northern India, for example,brothers marry the same woman.66The land is extremely poor, andit apparently takes the work of two men to support one woman andher children. Prince Peter of Greece argued that in Tibet both re-sources and familial considerations lay at the heart of Tibetanpolyandry: 76C H A P T E R F O U RTaking more than one wife for each son of a house would oblige themto partition the property, something which simply could not be donein a difficult environment such as that of Tibet. . . . Polyandry, then, isthe ideal solution to the problem, for if the wife were tempted in theabsence of her husband to have sexual relations with someone else, atleast by having them with his brother, her offspring will always be ofthe same family blood.67While Tibetan polyandry is common among wealthier families,among the Kandyans of Sri Lanka polyandry apparently occursamong poor families. Yet here, too, polyandry appears to be anarrangement by which two brothers join their land and maintain acommon family, minimizing the number of potential heirs and rais-ing living standards.68Polyandry thus can result from brother-brother coalitions in orderto combat resource scarcity or from attempts to control the distribu-tion of a resource like land, which is immobile and loses its valuewhen too much divided; it is “a rare but adaptive system for pre-serving family estates, and hence reliably supporting lineal descen-dants, across the generations.”69 5.Sex, Resources, Appearance,and Mate ChoiceI’ve been called handsome, adventurous, athletic, humble, honest, intelligent, and messy. Seeking woman, 27–34, with most of thosetraits.—Ann Arbor Observer, personal ad, April 1998M EN AND WOMEN are not all that different in size ormale and female, and think they were different species (as has hap-pened among some bird species). In general, male humans, regard-less of current marriage system, are slightly larger than females, con-sistent with our evolutionary history of mild polygyny. This isbecause much, perhaps most, male-male competition in humans isnot a matter of size, but of other traits: wealth, political savvy, andso on—traits that help in complex social competition more thansheer size.As we saw in the last chapter, females in many other species sim-ply choose their mates, and while the criteria may vary, the choiceprocess looks relatively straightforward. Depending on the ecologyof parental care, females choose different traits. They might seek“good genes” in species with no male parental care; this can involvesomething as straightforward as expensive displays (see below), orsomething as subtle as choosing an individual whose geneticmakeup is different, so offspring will be heterozygous. Femalesmight, as in some grasshoppers, choose males with the best foragingabilities (and in response, males with poor foraging ability try toforce copulations). Or, as in some cockroaches, females may be ableto use a male’s status and pheromone cues to discriminate againstappearance; no naive biologist would take two human specimens, 78C H A P T E R F I V Emales reared in poor environments. For some species, resource con-trol is so central that females appear simply to choose the resourceand get whatever male controls it, as do elephant seal females(choosing a beach on which to give birth), and Red-winged Black-bird females (choosing a rich marsh in which to provision and reartheir young).1In contrast, the prospective bride and groom in many human so-cieties may have little to say in choosing their mates. Or the bride’sand groom’s preferences may count, but in an informal way that isdifficult to document. Because marriage and mating in humans in-volves others besides a male and a female, a whole set of conflicts ofinterests may exist. Not only might the man and woman seek quitedifferent qualities in a potential mate, so might their families (seebelow).What Men and Women WantResource control is clearly important to women, or their fam-ilies, as we saw in the last chapter. Freud asked: what do womenwant? Resources, as Shakespeare knew,Dumb jewels in their silent kindMore than quick words do move a woman’s mind.(Two Gentlemen of Verona)Anita Loos was pithier, if not as poetic: “Kissing your hand maymake you feel very, very good, but a diamond and sapphire braceletlasts forever.” In our evolutionary past, women whose mates pro-vided resources for them and their children did better than others.Even today, women choosing mates are interested in men’s re-source control. The anthropologist Daniel Pérusse found that amongFrench Canadian men and women, men (but not women) of highersocial status had more sexual partners, suggesting that status is im-portant for men in the mating game. He also found that women’s(but not men’s) number of partners decreased linearly with age, sug-gesting that women’s reproductive potential is important for them.And the psychologist David Buss, asking questions in thirty-sevencultures around the world, found that while some particulars aboutmate preference vary across these cultures, there are some consistent S E X , R E S O U R C E S , A N D M A T E C H O I C E79preferences that are obviously related to evolved sex differences inmate quality. Women rank men’s ability to get resources high, andmen rank women’s youth and health high; both sexes rank socialabilities like “sense of humor” high as well.2We don’t usually think (after we graduate from high school, any-way) much about physical cues in mate choice, but certain physicalcues may matter in the mating game. Anthropologists Doug Jonesand Kim Hill find that men prefer neotenic (childlike) faces inwomen in a number of cultures. Both sexes prefer symmetry in bothface and body, reflecting health. I have argued, as have others, thatphysical sex differences beyond size reflect our polygynous back-ground—that breasts, hips, and buttocks have served as sexual sig-nals when females compete for the attention and investment ofpowerful, parental, resource-investing males.3When males investparentally, as in humans, males as well as females may profit repro-ductively from exercising choice in mates.4If both sexes can exercisesome choice, and if men and women have been reproductively suc-cessful through different strategies, they are likely to look for quitedifferent things in their mates.Of course, ideas about beauty or desirable traits in a mate willsurely be influenced by cultural norms. Even in relatively simplespecies like guppies, females not only choose (costly) male signalsand displays that reflect good condition, no parasites, high energy,and so forth, but young females copy the choices of older females.5Here, there is an obvious possible selective logic. But what of hu-mans? Certainly we manipulate all sorts of signals: hair, eye, andskin color, body shape; what possible reproductive value could, forexample, blue hair and nose rings reflect? We can break preferencesdown into signals that reflect health (shiny hair, clear skin) or youth(no wrinkles or sags) and current reproductive stage (waist-hip ratio,color of nipples); signals that suggest other reproductively impor-tant attributes like wealth; signals that reflect social awareness (styl-ishness, which may be purely culturally defined); signals of belong-ing to a certain group. Cross-culturally among traditional societies,the things people describe as attractive in the other sex turns up allof these categories. But just as in David Buss’s studies of mate pref-erences and Doug Jones’s and Kim Hill’s cross-cultural study of fa-cial attractiveness in contemporary societies, selectively relevanttraits consistently rank high.Signals of women’s youth and health, directly related to selective 80C H A P T E R F I V Eadvantage, are universally described as desirable: clear, unwrinkledskin, firm breasts, lustrous hair for women. In men, women favorstrength, energy, and vigor. Some traits reflect an interplay of cul-tural practices and selection; for example, among the Iban peopleshort hair is undesirable: they cut a sick person’s hair very short, soshort hair in this case reflects recent illness. Some of these “interplay”traits may reflect directly favored traits; in a number of societies, aman’s tattoos reflect his status, and, of course, undergoing the tat-tooing further reflects pain tolerance and fortitude. But we shouldbe cautious; I don’t think we have good data to tell us just what allthe functions are. Finally, some preferences may simply be “purelycultural”: in some societies, relative hairlessness is a sign of beautyin a woman; in others, robust and luxurious hair is desired.6If weexamine the ethnographies, comments people in traditional societiesmake are largely related to direct measures of fitness rather than“purely cultural” or conditional culture-interaction traits.7An important widespread physical preference by men—one thatwould not occur to most of us but which is intriguing in light of thisargument—is for a particular relationship between a woman’s waistand hip size. Across all sorts of cultures with quite different specificideas about beauty, both men and women see as most attractive a fe-male waist-hip ratio of about 7/10 or 8/10. This is true whether thepreference is for rather generous, Reubenesque proportions, or forslender Julia Roberts builds.8Why? Women of reproductive age will,unless they are pregnant, tend to put fat on their hips, breasts, andbuttocks. Older women (of lower reproductive value) and pregnantwomen (not currently fecundable) thicken at the waist, giving ahigher waist-hip ratio. The relative size of waist versus hips givesimportant reproductive cues. A relatively narrow waist means “I’mfemale, I’m young, and I’m not pregnant.” The waist-hip ratio re-flects many complex relationships, but they all boil down to: Is shefertile? Is she fecundable?Hips, breasts, and buttocks are physical signals that communicateage, no prior births, and even, in the case of buttocks, one’s ability tometabolize scarce food efficiently. At least some of these physical sig-nals can be deceptive, even without deliberate manipulation. Con-sider how we put fat on our bodies. Little children tend to have faton their faces, fingers, and toes, presumably for protection againsttemperature extremes. All other age and sex categories distribute fat S E X , R E S O U R C E S , A N D M A T E C H O I C E81relatively evenly over the body—with one exception. Reproductive-aged women, unlike all other age-sex categories, deposit fat prefer-entially on the breasts, hips, and buttocks, contributing to that ap-proximately 7/10 waist-hip ratio men prefer. If a woman’s hips arebroad because she stores fat there, rather than because her pelvicstructure is wide, a man gets potentially inaccurate or confusing in-formation (wide pelvic girdles provide a headstart on easy birth oflarge-headed infants; fat doesn’t). It isn’t that fat is “bad,” but it de-notes energy reserves rather than a structurally wide pelvis. Simi-larly, if a woman’s breasts are large not because they comprise mam-mary tissue for milk production but because fat is stored there, amale gets information not about a woman’s lactational capability,but about her stored energy. This seems an irrelevant issue today,when food supplements and medical attention are readily available,but it may well have been an issue in mate choice in our evolution-ary history. Even today, insufficient mammary tissue means awoman will have difficulty nursing effectively.9In extreme, food-limited environments, obvious fatty deposits onwomen’s buttocks signal ability to gain sufficient nutrition on a lim-ited diet—a subtle reflection of maternal quality, important in amale-parental species. Thus, it is no surprise that extreme, harsh en-vironments are the context for both steatopygy (the condition inwhich fat is obviously concentrated on the buttocks; fig. 5.1) and acultural preference for extremely fat women. Darwin gave a second-hand report of one example of steatopygy as a sexual preferencetrait. Among the !Kung (then called Hottentot), Darwin’s informantreported, a truly sexy woman was one who was unable to rise fromlevel ground because of the weight of fat on her buttocks. Fat on thebuttocks is probably a “true” signal of ability to store fat on any par-ticular diet. In contrast, fat deposits on the breasts and hips are likelyto be confusing and even deceptive signals; at the least, such fat islikely to be confused with mammary tissue and wide pelvises, traitscontributing to two very different aspects of maternal quality.10In Western societies today, a man’s mate choice is likely to focuson a woman’s health, her reproductive value (which means heryouth), and her current reproductive status (fertile, not pregnant).11In many societies, the ideal is a healthy young virgin. In an interest-ing twist, a man’s preference might depend on whether he was seek-ing a short-term mate (in which case, we would expect him to 82C H A P T E R F I V EFigure 5.1. Steatopygy, the preferential deposition of fat on the buttocks, is as-sociated with harsh and unproductive environments. It probably represents anhonest signal of nutritional competence: “Even in this harsh environment I cannot only maintain myself, but store fat.” (Photo courtesy of the Denver Museumof Natural History.) Image Not Available S E X , R E S O U R C E S , A N D M A T E C H O I C E83prefer a woman at peak ability to conceive, perhaps in her twenties)or a long-term mate (in which case we would expect him to prefer awoman with peak reproductive value, about age seventeen).Certainly cultural and historical factors strongly influence thesepreferences, but some preferences—healthy, young, not pregnant—are virtually universal. In Thailand, for example, men, regardless ofwhether they are rich or working class, living in the city or country-side, tend to prefer young virgins (though virginity is not so profounda preference as in some societies); they insist that while they them-selves may have extramarital intercourse, their wives may not—theirwives are to make a good home, stay there, and be faithful.12Womentend to accept that men will be sexually active outside marriage, andtheir first concern is that any such activity not divert financial re-sources from the home and children; a good husband, whatever elsehe does, provides for his family faithfully. An interesting attitude shiftis occurring, related to the ecology of hiv transmission. Most womenstill prefer, when their husband has other women, that he visit com-mercial sex workers: the cost is modest, and the transaction is com-plete with the payment. (Traditionally, wives had objected less tocommercial sex workers than to minor wives.) But as hiv and knowl-edge about it have become prevalent, some women are beginning toprefer that their husband have a steady mistress, or even a minorwife: for although these women represent a greater threat to a wife’sresources, they represent a smaller disease risk.Beauty, Resources, and Mate ChoiceEven simple physical differences between the sexes reflectthat what is valuable in a wife is likely to differ from what is valu-able in a husband; differences and preferences are relatively consis-tent across quite different societies. Put simply, in our evolutionaryhistory, it seems likely that a woman’s value was usually her repro-ductive value, and a man’s value was his resource value. Cross-culturally today, while everyone values such traits as a sense of humorin both sexes, women seek signals of resource control in potentialmates while men seek signals of youth, health, and “beauty.”13Aswe saw, assessments of beauty vary across cultures, but typicallythey reflect health and youth (and a low waist-hip ratio). 84C H A P T E R F I V EOccasionally, at least for periods of time, very wealthy subgroupsmay value something that reflects helplessness, but in general “paleand wan” does not become and remain a widespread preference. Ex-ceptions appear when men and women have conflicts of interest andmen hold enough power to resolve those differences in their ownfavor. Consider women with bound feet in Mandarin China; here fa-thers and families favored a condition that reduced women’s fitnessunder ordinary selection—except that, because supporting an es-sentially helpless wife who has bound feet reflects a man’s wealth,suitors favored it, and fathers helped enforce it. Female circumci-sion, as practiced in parts of Africa, probably also represents a male-female conflict of interest: the practice clearly does not increasewomen’s general health or fecundity, but so long as men demand itand refuse marriage to uncircumcised women (in a closed society),the practice will continue. Exceptions like these not only reflect im-portant cultural variation but suggest the strength of the general cor-relation.14Awoman’s or her family’s resources are not irrelevant in marriagechoices. In some societies, men with few resources may explicitlytrade off reproductive value for resource value (see below)—pickingan older, reproductively less valuable woman who controls, in herown right, some resources; perhaps today’s pattern of famous ac-tresses “of a certain age” marrying younger men is relevant here.Subtle biases across societies, and fluctuations over time, have typi-cally given an advantage to richer families in marrying. When mostwomen worked in fields in western Europe, the standard of beautywas a pale complexion (which only the daughters of the rich couldmaintain); when we all began to work indoors, the Caucasian stan-dard of beauty became a winter tan, suggesting that one could afforda trip to warm climates.15Signals of Desirability and Their ManipulationStatus signals have a cost and will be maintained only if theybenefit the bearer. Some signals make actual confrontation less likely,saving calories and avoiding risk. Other signals serve as sexual at-tractants and are the source of much physical dimorphism. In non-humans, the sex competing for mates is the sex that gives such sig- S E X , R E S O U R C E S , A N D M A T E C H O I C E85nals. Males in polygynous species are usually big, colorful, andlikely to have weapons, because those are the males who win statusand are chosen by females (see above). Females in the few polyan-drous species are usually a little bigger, and sometimes a bit morebrightly colored, than males. Because much of the selection on dis-plays is preference by the choosing sex rather than relative survivalenhancement, sexual selection on expensive—and possibly danger-ous—displays can “run away.”16Thus, males in polygynous species are likely to grow antlers, orlarge horns, or bright feathers, or long decorative tail feathers—allcostly, and sometimes risky, displays that may do nothing more thanadvertise a male’s ability to take these risks: the “Handicap Princi-ple.” The message is: I am so fit that I can support this expensivehandicap, which would kill a lesser individual.17And when femalesprefer these costly displays, they work. For example, female Euro-pean swallows prefer to mate with longer-tailed males: these malesmore often get mates, and get them sooner, than other males. Thesuccess of longer-tailed males is high—but these tails carry a cost interms of survivorship; long-tailed males die sooner.18Most nonhuman examples, including those given at the beginningof this chapter, principally involve male physical (energetic) re-sources, even when, as in Bower Birds (for whom the criteria are thenumber and color of decorations on the bower), the display is notsimply a physical part of the displayer’s body.19In contrast, humansinvent, augment, and change signals; and females do a great dealof signaling. Bras make our breasts look large and/or young, girdlescan imitate an ideal waist-hip ratio, shoulder pads mimic goodphysical condition, makeup reflects light and hides wrinkles, cheekand lip color make us look healthy and sexually interested. Our ma-nipulations have sometimes been intrusive: in the nineteenth cen-tury, for example, some women underwent surgery to remove theirfloating ribs in order to have a small waist; today we have faceliftsand liposuction. These manipulations imitate signals of youth andhealth.Cross-culturally, cultural augmentation of sexual signals or orna-ments is virtually universal, favored for the same reasons selectionfavors physical ornaments and displays in other species. Rememberthe old adage, “If you’ve got it, flaunt it.” Males and females profitby signaling or flaunting different attributes. Humans are actuallyrather paradoxical with regard to sexual selection and sexual dis- 86C H A P T E R F I V Eplays: most scholars agree that human evolutionary history, like thatof most other primates, is polygynous: 83 percent of societies forwhich we have information are polygynous.20All of these patternssuggest that in humans, males should be the “ornamented” sex, yetmost people talk about women’s adornments. But ornaments can bewhat either sex advertises.Because of our polygynous history, men’s and women’s culturalaugmentation of sexual signals should give information about dif-ferent characteristics. Men are likely to signal wealth and power sta-tus, and members of cultural subgroups with limited real resourcesseem likely to concentrate those resources in highly visible signals.Sociological studies of wealth and status signals among contempo-rary poor groups, for example, find the “ghetto Cadillac” phenome-non common.21Because humans show male parental investment, awoman’s reproductive value becomes important; thus, womenshould signal reproductive value, things that reflect youth andhealth. Today, billion-dollar industries exist to do just that: makeupand cosmetic surgery, for example, are designed to signal youth,health, and sexual interest—the products and processes are aimedat making the skin tauter and less wrinkled. Though a few men in-dulge themselves this way, most clients are women, who getfacelifts, and undergo liposuction to obtain an attractive waist-hipratio. In the nineteenth century, women put belladonna in their eyes,dilating their pupils, ordinarily a strong signal of sexual interest.Women use rouge to make the cheeks rosy, indicating they have ei-ther been exercising or are sexually aroused, and lipstick to mimicthe dark, blood-engorged state of the lips during sexual excitement.The specifics change across time and societies, but the desired resultdoes not.Women, like females of other species, can signal interest and avail-ability behaviorally. Patterns of eye contact in flirting appear to bevirtually universal and invariant in widely differing societies.22Thefacts that women frequently wear signals of “unavailability” (e.g.,wedding rings, styles of clothing or hair worn only by marriedwomen), and that in some cultures they undergo treatment that maydecrease their general health and vigor (foot binding, clitori...

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