Is there a link between paternity concern and female genital cutting in West Africa?

Here we explore the relationship between female genital cutting (FGC), sexual behaviour, and marriage opportunities in five West African countries. Using large demographic datasets (n 72,438 women, 12,704 men, 10,695 couples) we explore key (but untested) assumptions of an evolutionary proposal that FGC persists because it provides evolutionary fitness benefits for men by reducing non-paternity rates. We identify and test three assumptions implicit in this proposal. We test whether cut women have reduced extra-pair sex before or within marriage; whether FGC is associated with a younger age at marriage as an indication of partner preference; and whether individual and group-level indicators of paternity concern are associated with a stronger preference for marriage to women with FGC.Our results show that FGC status does not affect the odds of women engaging in several indicators of premarital sex, however women with FGC have significantly lower odds of having more than one lifetime sexual partner. We also show that women with FGC get married at a younger age which supports the argument that FGC status influences women's marriage opportunities, even when it does not restrict sexual activity. Finally, we find that in population groups where reported sexual activity and perceived risk of women's extra-pair sex is high, men have higher odds of marrying a first wife with FGC. Together, these results indicate that paternity certainty may be one of several factors contributing to the persistence of FGC in this sample, and that group-level sexual norms are key to maintaining the practice of FGC through the marriage market.     

Age at menarche, schooling, and sexual debut in northern Malawi

Background

Age at sexual debut is a key behavioural indicator used in HIV behavioural surveillance. Early age at menarche may precipitate early sex through perceived readiness for sex, or through school drop-out, but this is rarely studied. We investigated trends and circumstances of sexual debut in relation to schooling and age at menarche.

Methods and Findings

A cross-sectional sexual behaviour survey was conducted on all individuals age 15–59 within a demographic surveillance site in Karonga District, Malawi. Time trends were assessed using birth cohorts. Survival analysis was used to estimate the median age at menarche, sexual debut and first marriage. The 25^th centile was used to define “early” sex, and analyses of risk factors for early sex were restricted to those who had reached that age, and were done using logistic regression. Of the 8232 women and 7338 men resident in the area, 88% and 78%, respectively, were seen, and, 94% and 92% of these were interviewed. The median reported age at first sex was 17.5 for women and 18.8 for men. For women, ages at menarche, sexual debut and first marriage did not differ by birth cohort. For men, age at sexual debut and first marriage decreased slightly in later birth cohorts. For both men and women increased schooling was associated with later sexual debut and a longer delay between sexual debut and first marriage, but the associations were stronger for women. Earlier age at menarche was strongly associated with earlier sexual debut and marriage and lower schooling levels. In women early sexual debut (<16 years) was less likely in those with menarche at age 14–15 (odds ratio (OR) 0.31, 95%CI 0.26–0.36), and ≥16 (OR 0.04, 95%CI 0.02–0.05) compared to those with menarche at <14. The proportion of women who completed primary school was 46% in those with menarche at <14, 60% in those with menarche at 14–15 and 70% in those with menarche at ≥16. The association between age at menarche and schooling was partly explained by age at sexual debut. The association between age at menarche and early sex was not altered by adjusting for schooling.

Conclusions

Women with early menarche start sex and marry early, leading to school drop-out. It is important to find ways to support those who reach menarche early to access the same opportunities as other young women.     

Handsome Wants as Handsome Does: Physical Attractiveness and Gender Differences in Revealed Sexual Preferences

In this article I evaluate the effect of physical attractiveness on young adults' sexual and romantic outcomes to reveal gender differences in acted preferences. Using data from the National Longitudinal Study of Adolescent Health (Add Health), a probability sample of young adults (n?=?14,276), I investigate gender differences in desired sexual partner accumulation, relationship status, and timing of sexual intercourse. I find gender differences in sexual and romantic strategies consistent with those predicted by the double standard of sexuality and evolutionary theory. Specifically, compared to men, women pursue more committed relationships, fewer sexual partners, and delayed sexual intercourse.     

Psychosexual Development of Women with Congenital Adrenal Hyperplasia

Women with congenital adrenal hyperplasia (CAH) (N= 31) and their unaffected sisters or female cousins (N= 15) participated in a study of psychosexual development. All participants were ≥18 years of age (mean age, 25 years; range, 18–40). Comparisons were also made between the CAH women with the salt-wasting (SW) form of the disorder and those with simple virilization (SV). A psychosexual assessment protocol examined six variables: (1) sex assignment at birth (probands only); (2) recalled sex-typed behavior during childhood; (3) gender identity and gender role identification in adulthood; (4) relationship status; (5) sexual orientation in fantasy; and (6) sexual orientation in behavior. Salt-wasting status and sex assignment at birth were also ascertained for the CAH women who either refused to participate in the study (N= 10) or could not be traced (N= 13). Compared to the controls, the women with CAH recalled more cross-gender role behavior and less comfort with their sense of “femininity” during childhood. The two groups did not differ in degree of gender dysphoria in adulthood, although the probands showed more cross-gender role identification. Three of the nonparticipant probands were living, as adults, in the male social role (2 reared from birth as boys and 1 who changed from the female to the male social role during adolescence). The CAH women and the controls did not differ in relationship status (married/cohabiting vs. single). The CAH women had lower rates of exclusive heterosexual fantasy and fewer sexual experiences with men than the controls; however, the CAH women did not have more sexual experiences with women than the controls. Comparisons between the SW and SV revealed several differences: the SW were less likely to be assigned to the female sex at birth, recalled more cross-gender role behavior during childhood, were less likely to be married or cohabiting, and had lower rates of sexual experiences with men. The results were discussed in relation to the effects of prenatal androgens on psychosexual differentiation.     

Mate choice in modern societies

Most research on mate choice in modern societies is based on data that may or may not reflect actual mating behavior (e.g., stated preferences, personal advertisements). In the present study, real-life matings were reported by a large representative sample of men and women (N = 1,133). These data were used to test an evolutionary model in which mate choice is hypothesized to depend on resources potentially contributed to reproduction by each sex. Consistent with the model, it was found that (a) men (but not women) of higher social status acquire more mating partners, suggesting that male status is an important criterion in female choice; (b) women’s (but not men’s) number of partners decreases linearly with age, suggesting that female reproductive potential is an important criterion in male choice; and (c) women (but not men) display a significant relationship between marital dissolution and promiscuity, suggesting that female sexual exclusivity is an important criterion in male choice. These results are discussed in relation to understanding mate choice mechanisms from behavioral data. Daniel Pérusse is an assistant professor in the Department of Anthropology at the Université de Montréal. His research interests include the evolutionary biology of human social and reproductive behavior, sexual selection theory, and biocultural evolution. His current research bears on human socialization processes and psychosocial development from an evolutionary and behavior-genetic perspective. Recent publications include “Cultural and Reproductive Success in Industrial Societies: Testing the Relationship at the Proximate and Ultimate Levels” (Behavioral and Brain Sciences 16(2):267–322, 1993) and “Human Parental Behavior: Evidence for Genetic Influence and Potential Implications for Gene-Culture Theory” with M. C. Neale, A. Heath, and L. J. Eaves (Behavior Genetics, in press).     

Height and reproductive success

In Western societies, height is positively correlated with reproductive success (RS) for men but negatively correlated with RS for women. These relationships have been attributed to sexual selection: women prefer tall men, and men prefer short women. It is this success in the marriage market which leads to higher RS for tall men and short women. We have already shown that the relationship between height and RS for women is quite different in a non-Western context. In a subsistence farming community in rural Gambia, height is positively correlated with reproductive success for women, largely owing to the higher survival of the children of tall women. Here, the relationship between height and reproductive success is analyzed for men in the same community. For these Gambian men, there is no significant relationship between height and the number of children they produce, although tall men do contract more marriages than shorter men. We conclude that environmental context needs to be taken into account when analyzing human reproductive behavior.     

Dressed for sex: red as a female sexual signal in humans

Background

In many non-human primate species, a display of red by a female serves as a sexual signal to attract male conspecifics. Red is associated with sex and romance in humans, and women convey their sexual interest to men through a variety of verbal, postural, and behavioral means. In the present research, we investigate whether female red ornamentation in non-human primates has a human analog, whereby women use a behavioral display of red to signal their sexual interest to men.

Methodology/Principal Findings

Three studies tested the hypothesis that women use red clothing to communicate sexual interest to men in profile pictures on dating websites. In Study 1, women who imagined being interested in casual sex were more likely to display red (but not other colors) on their anticipated web profile picture. In Study 2, women who indicated interest in casual sex were more likely to prominently display red (but not other colors) on their actual web profile picture. In Study 3, women on a website dedicated to facilitating casual sexual relationships were more likely to prominently exhibit red (but not other colors) than women on a website dedicated to facilitating marital relationships.

Conclusions/Significance

These results establish a provocative parallel between women and non-human female primates in red signal coloration in the mating game. This research shows, for the first time, a functional use of color in women''s sexual self-presentation, and highlights the need to extend research on color beyond physics, physiology, and preference to psychological functioning.     

Secular trends in human sex ratios

Secular change in sex ratios is examined in relation to experience in the family. Two theoretical perspectives are outlined: Guttentag and Secord’s (1983) adaptation of social exchange theory, and sexual selection theory. Because of large-scale change in number of births and typical age differentials between men and women at marriage, low sex ratios at couple formation ages existed in the U.S. between 1965 and the early 1980s. The currently high sex ratios, however, will persist until the end of the century. High sex ratios appear to be associated with lower divorce rates, male commitment to careers that promise economic rewards, male willingness to engage in child care, higher fertility, and higher rates of sexual violence. Sexual selection theory calls attention to intrasexual competition in the numerically larger sex.     

The factors influencing transactional sex among young men and women in 12 sub‐Saharan African countries

Abstract

Transactional sex may put young women and young men in sub‐Saharan Africa at increased risk of contracting sexually transmitted infections (STIs), including HIV/AIDS. This behavior may also put young women at higher risk of pregnancy and childbearing. Policymakers and program managers need to know what factors put youth at increased risk. We investigated this issue using logistic regression analyses of data from male and female modules of Demographic and Health Surveys from 12 sub‐Saharar African countries. We found that young men and young women are at greater risk of engaging in transactional sex than are older people. Unmarried young women and young men were significantly more likely to engage in transactional sex than married youth. Based on these results, our conclusions were that programs geared toward reducing the incidence of transactional sex or protecting men and women already in transactional sexual relationships should be aimed at both young women and young men. Due to our finding that unmarried young women and young men are more vulnerable to experiencing transactional sex, programs to prevent transactional sex should be specifically directed to this subgroup of young people.     

Female sexual advertisement reflects resource availability in twentieth-century UK society

Evolutionary theory suggests that men and women differ in the characteristics valued in potential mates. In humans, males show a preference for physical attractiveness, whereas females seek cues that relate to resources and future earning potential. If women pursue marriage as an economic strategy, female sexual advertisement should increase during periods of poor economic conditions when the number of high-quality male partners becomes a limited resource. To test this prediction, measures of skin display and clothing tightness were taken for clothes portrayed in UK Vogue magazine from 1916 to 1999. These estimates of sexual advertisement were analyzed in relation to an index of economic prosperity (GDP), while controlling for general increases in economic conditions and sexual display over the course of the past century. The results indicate that female sexual display increases as economic conditions decline, with the level of breast display and the tightness of clothing at the waist and hips the key factors underlying this increase. Breast size and symmetry and female body form are secondary sexual characteristics that play an important role in sexual attractiveness. Since advertisement of these features increases as levels of competition for high-quality partners increases, females appear to use marriage as an economic strategy. Patterns of female fashion appear to be underpinned by evolutionary considerations relating resource availability to female reproductive success. Russell Hill, B.Sc., M.Phil., Ph.D. is an Addison Wheeler Research Fellow at the University of Durham. His main research interests are in the evolution of mammalian social systems and his current projects span evolutionary anthropology, conservation biology, and theoretical modeling. Sophie Donovan B.Sc., M.Sc. has recently completed an M.Sc. in Speech and Language Sciences at University College London, focusing on the development of a deaf child’s vocabulary through conversations in the classroom. She is currently practicing as a Speech and Language Therapist. Nicola Koyama, B.Sc., Ph.D. is a Senior Lecturer at Liverpool John Moores University. Her principal research interests lie in the evolution of social systems in human and non-human primates, in particular the regulation of social relationships, mate preferences, and conflict management.     

Control tactics and partner violence in heterosexual relationships

This study investigated sex-specific predictors of violent and nonviolent mate guarding used by men (n=399) and women (n=951) in heterosexual relationships, using both self-reports and reports on partners. We found, contrary to some previous evolutionary assumptions, that men and women showed similar degrees of controlling behavior, and that this predicted physical aggression to partners in both sexes. We also predicted from evolutionarily based studies that men's and women's control and aggression would vary as a function of female fecundity and mate value (relative to peer group and to partner). Fecundity was associated with men's and women's controlling behavior, but not their physical aggression: relationships where the woman was fecund showed higher rates of control. According to partners' reports, men and women who had lower mate values showed more controlling behavior and (to a lesser extent) more physical aggression. There was no support for the prediction that higher mate-value partners would be guarded more than lower mate-value ones. The following limitations are discussed: the sample and method of data collection, and the lack of information on the women's hormonal status.     

Gender differences in sexual jealousy: Adaptionist or social learning explanation?

It has often been speculated, and some evidence suggests, that men and women differ in the elicitation of jealousy: Men appear to be more likely than women to become upset over threats to sexual exclusivity; whereas women are more likely than men to react negatively to potential loss of partner time and attention. Both adaptionist and traditional social learning theories have been used to explain these apparent gender differences. In the present article we outline both explanations and review the relevant psychological literature on gender differences in the elicitation of jealousy. We propose that the difference in men's and women's psychological mechanisms for elicitation of jealousy is best characterized (at least in this culture) as a greater sensitivity among men to cues indicative of possible sexual infidelity rather than greater emotional upset in response to the occurence of extradyadic sex on the part of one's mate. We also provide data testing a traditional social learning explanation for the elicitation of jealousy. Results of a survey administered to college students (N = 223) demonstrate the subtle nature of gender differences in the elicitation of jealousy within this culture. Men and women were most likely to differ (in the hypothesized directions) when items pertained to concern over a partner's potential extradyadic sex rather than to reactions to sexual infidelity that is suspected to have already occurred. Although men reported placing more value on sexual activity within dating relationships and women reported placing more value on emotional intimacy, these ratings of relationship rewards did not explain the gender differences in reported jealousy. Results failed to support a traditional social learning explanation of jealousy and are discussed with regard to evolutionary theory and directions for future research.     

The marriage squeeze in Colombia, 1973–2005: The role of excess male death

Abstract

Colombia has been characterized by extreme levels of civil violence throughout the latter part of the twentieth century, and the burden of excess mortality attributable to this violence has been borne primarily by young men. Populations with a large violent death burden are likely to experience consequences in terms of (1) marriage markets, (2) the dynamics of family formation and dissolution, and (3) patterns of parental investment in offspring. Using data from national censuses and household surveys, we calculate a measure of the marital sex ratio in order to explore the impact of differential male mortality on marriage markets in Colombia. Overall, Colombia is characterized by a female biased sex ratio at all ages. This relative excess of women is particularly pronounced in certain departments of the Central and Pacific regions which have been especially affected by civil violence. We suggest that the low sex ratios which characterized Colombia are partially responsible for the increasingly high frequency of consensual unions and, potentially, female‐biased rural‐urban migration.     

Prevalence of Consensual Male–Male Sex and Sexual Violence,and Associations with HIV in South Africa: A Population-Based Cross-Sectional Study

Background

In sub-Saharan Africa the population prevalence of men who have sex with men (MSM) is unknown, as is the population prevalence of male-on-male sexual violence, and whether male-on-male sexual violence may relate to HIV risk. This paper describes lifetime prevalence of consensual male–male sexual behavior and male-on-male sexual violence (victimization and perpetration) in two South African provinces, socio-demographic factors associated with these experiences, and associations with HIV serostatus.

Methods and Findings

In a cross-sectional study conducted in 2008, men aged 18–49 y from randomly selected households in the Eastern Cape and KwaZulu-Natal provinces provided anonymous survey data and dried blood spots for HIV serostatus assessment. Interviews were completed in 1,737 of 2,298 (75.6%) of enumerated and eligible households. From these households, 1,705 men (97.1%) provided data on lifetime history of same-sex experiences, and 1,220 (70.2%) also provided dried blood spots for HIV testing. 5.4% (n = 92) of participants reported a lifetime history of any consensual sexual activity with another man; 9.6% (n = 164) reported any sexual victimization by a man, and 3.0% (n = 51) reported perpetrating sexual violence against another man. 85.0% (n = 79) of men with a history of consensual sex with men reported having a current female partner, and 27.7% (n = 26) reported having a current male partner. Of the latter, 80.6% (n = 21/26) also reported having a female partner. Men reporting a history of consensual male–male sexual behavior are more likely to have been a victim of male-on-male sexual violence (adjusted odds ratio [aOR] = 7.24; 95% CI 4.26–12.3), and to have perpetrated sexual violence against another man (aOR = 3.10; 95% CI 1.22–7.90). Men reporting consensual oral/anal sex with a man were more likely to be HIV+ than men with no such history (aOR = 3.11; 95% CI 1.24–7.80). Men who had raped a man were more likely to be HIV+ than non-perpetrators (aOR = 3.58; 95% CI 1.17–10.9).

Conclusions

In this sample, one in 20 men (5.4%) reported lifetime consensual sexual contact with a man, while about one in ten (9.6%) reported experience of male-on-male sexual violence victimization. Men who reported having had sex with men were more likely to be HIV+, as were men who reported perpetrating sexual violence towards other men. Whilst there was no direct measure of male–female concurrency (having overlapping sexual relationships with men and women), the data suggest that this may have been common. These findings suggest that HIV prevention messages regarding male–male sex in South Africa should be mainstreamed with prevention messages for the general population, and sexual health interventions and HIV prevention interventions for South African men should explicitly address male-on-male sexual violence. Please see later in the article for the Editors'' Summary     

Beautiful parents have more daughters: a further implication of the generalized Trivers-Willard hypothesis (gTWH)

The generalized Trivers-Willard hypothesis (gTWH) [Kanazawa, S., 2005. Big and tall parents have more sons: further generalizations of the Trivers-Willard hypothesis. J. Theor. Biol. 235, 583-590) proposes that parents who possess any heritable trait which increases the male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio. One heritable trait which increases the reproductive success of daughters much more than that of sons is physical attractiveness. I therefore predict that physically attractive parents have a lower-than-expected offspring sex ratio (more daughters). Further, if beautiful parents have more daughters and physical attractiveness is heritable, then, over evolutionary history, women should gradually become more attractive than men. The analysis of the National Longitudinal Study of Adolescent Health (Add Health) confirm both of these hypotheses. Very attractive individuals are 26% less likely to have a son, and women are significantly more physically attractive than men in the representative American sample.     

HIV status and age at first marriage among women in Cameroon

Recent research has highlighted the risk of HIV infection for married teenage women compared with their unmarried counterparts (Clark, 2004). This study assesses whether a relationship exists, for women who have completed their adolescence (age 20-29 years), between HIV status with age at first marriage and the length of time between first sex and first marriage. Multivariate analysis utilizing the nationally representative 2004 Cameroon Demographic and Health Survey shows that late-marrying women and those with a longer period of pre-marital sex have the highest risk of HIV. Although women in urban areas overall marry later than their rural counterparts, the positive relationship between age at marriage and HIV risk is stronger in rural areas. The higher wealth status and greater number of lifetime sexual partners of late-marrying women contribute to their higher HIV risk. Given that the age at first marriage and the gap between first marriage and first sex have increased in recent years, focusing preventive efforts on late-marrying women will be of much importance in reducing HIV prevalence among females.     

Determinants of sex allocation in a gynodioecious wild strawberry: implications for the evolution of dioecy and sexual dimorphism

One evolutionary pathway from plants with combined male and female functions (hermaphroditism) to those with separate sexes (dioecy) involves females coexisting with hermaphrodites (gynodioecy). The research presented here explores sex allocation in Fragaria virginiana (a gynodioecious wild strawberry), within the context of theory on the gynodioecy–dioecy transition. By growing clonally replicated plants in the greenhouse and surveying six populations in situ, I evaluated the effects of plant size, genotype, sexual identity, population of origin and female frequency on sex allocation. I found significant positive effects of plant size on most sex allocation traits studied. In addition to strong sex-specific allocation patterns, I found significant broad-sense heritabilities for all traits, suggesting that plants could respond to selection. Moreover, there was a negative genetic correlation between pollen production and fruit set per flower within hermaphrodites, lending support to a basic assumption of sex allocation theory. On the other hand, several sex allocation traits, namely pollen and ovules per flower in hermaphrodites, were positively genetically correlated, suggesting that they may act to constrain the evolution of sexual dimorphism. Populations differed in the frequency of females, and females were more prevalent on sites with lower soil moisture and where hermaphrodites were least likely to produce fruit, suggesting that females’ seed fitness relative to that of hermaphrodites may be strongly environment-dependent in this species.     

The reality and evolutionary significance of human psychological sex differences

The aims of this article are: (i) to provide a quantitative overview of sex differences in human psychological attributes; and (ii) to consider evidence for their possible evolutionary origins. Sex differences were identified from a systematic literature search of meta‐analyses and large‐sample studies. These were organized in terms of evolutionary significance as follows: (i) characteristics arising from inter‐male competition (within‐sex aggression; impulsiveness and sensation‐seeking; fearfulness; visuospatial and object‐location memory; object‐centred orientations); (ii) those concerning social relations that are likely to have arisen from women's adaptations for small‐group interactions and men's for larger co‐operative groups (person‐centred orientation and social skills; language; depression and anxiety); (iii) those arising from female choice (sexuality; mate choice; sexual conflict). There were sex differences in all categories, whose magnitudes ranged from (i) small (object location memory; negative emotions), to (ii) medium (mental rotation; anxiety disorders; impulsivity; sex drive; interest in casual sex), to (iii) large (social interests and abilities; sociosexuality); and (iv) very large (escalated aggression; systemizing; sexual violence). Evolutionary explanations were evaluated according to whether: (i) similar differences occur in other mammals; (ii) there is cross‐cultural consistency; (iii) the origin was early in life or at puberty; (iv) there was evidence for hormonal influences; and (v), where possible, whether there was evidence for evolutionarily derived design features. The evidence was positive for most features in most categories, suggesting evolutionary origins for a broad range of sex differences. Attributes for which there was no sex difference are also noted. Within‐sex variations are discussed as limitations to the emphasis on sex differences.     

The Morning after the Night Before

Benefits to females of short-term mating have recently been identified, and it has been suggested that women have evolved adaptations for this strategy. One piece of evidence supporting such a female adaptation would be that women find the experience of a one-night stand as affectively positive as men. Individuals (N = 1,743) who had experienced a one-night stand were asked to rate aspects of their “morning after” feelings (six positive and six negative). Women were significantly more negative and less positive than men. Although women did not especially view these relationships as a prelude to long-term relationships, they felt greater regret than men about having been “used.” Extra-pair copulations were rated more negatively, but not less positively, than singles’ experiences. There was no interaction between gender and mated status on positivity or negativity. Although, in terms of subsequent affective response, women do not seem well adapted to casual sexual encounters, it may be important to distinguish impelling sexual motivation preceding intercourse from later evaluations of the event. Menstrual cycle changes may also be important in altering the strength and target of sexual motivation.     

How does marriage affect length of life? Analysis of a French historical dataset from an evolutionary perspective

There are broadly two explanations for why human longevity appears to be extended by marriage. First, there is the social explanation, whereby the companionship, division of labour and the economic support that marriage offers is thought to extend life. Second, there is a selective explanation, whereby those individuals with high potential longevity are more attractive to the opposite sex and therefore more likely to get married. Here we analyse the “TRA” dataset from 19th century France, using an evolutionary approach to address the question of why marriage is linked to longevity, focussing particularly on sex differences. The dataset is based on death and marriage records from all of France between 1798 and 1901 and includes information on age at death, marriage and wealth for individuals whose surnames began with the letters TRA. We find that marriage is positively associated with longevity, particularly for men. In part, this is related to the higher rate of deaths for single males during marriageable age, as compared to a higher rate of deaths for females during marriage. There is a positive association between wealth (at death) and longevity for individuals who were single or married at death, with a stronger effect for singles. Analysis of the effect of spousal age gap on duration of survival after first marriage indicates that men who were married to younger women lived longer, whereas the longevity of women was not associated with the spousal age gap. We put forward an evolutionary perspective on marriage and longevity, hypothesizing that there is an important role for sexual selection in the association between marriage and longevity, with women selecting on characteristics associated with longevity, whilst men select on characteristics associated with reproductive potential.