Willingness to engage in casual sex

Sexually dimorphic mate selection strategies were examined in 200 university students reporting their willingness to engage in casual sexual encounters with hypothetical individuals of the opposite sex. Using a questionnaire format, the possibility of forming a long-term relationship was manipulated, while risk of disease, pregnancy, and detection was eliminated across all conditions. In addition, potential partners varied in level of attractiveness, and in personality and behavioral characteristics. As expected, men reported a greater anticipated willingness to engage in sexual intercourse across all conditions compared with women. The possibility of forming a long-term relationship elevated women’s, but not men’s, willingness for sexual intercourse. While a potential partner’s attractiveness had a significant positive overall effect on responses, reducing their relative attractiveness had a greater negative impact on men’s responses. Reference to the parental qualities of a potential partner significantly increased women’s, but not men’s, anticipated willingness for sexual intercourse. Describing a hypothetical partner as non-aggressive (safe) marginally increased women’s willingness (p<.09) and did not affect men’s responses. The wording of items relevant to this condition may have resulted in the potential partner sounding "wimpy" rather than nonaggressive, and this may have reduced the expected effect of this manipulation. The possibility that women may trade off personality and behavioral characteristics with attractiveness to a greater degree than men when assessing potential mates is considered.     

Hormonal influences on women's extra-pair sexual interests: The moderating impact of partner attractiveness

Based on the idea that women are especially attracted to ancestral markers of male genetic quality when conceptive in their cycle, scholars have conjectured that increases in women's extra-pair sexual interests during the conceptive phase of the cycle are moderated by their primary partners' sexual attractiveness. Multiple studies have examined this prediction, with largely supportive but mixed results. The current study is the first to examine whether hormonal influences—thought to mediate cycle shifts—on women's extra-pair sexual interests are moderated by partner attractiveness. 213 naturally cycling, romantically involved women (181 attending multiple sessions) participated in up to four sessions over about a month. Estrogen and progesterone levels were measured multiple times across women's cycles. Male partner attractiveness moderated associations between progesterone levels (though not estrogen levels) and women's extra-pair sexual interests. A negative association between progesterone levels and extra-pair sexual interests, a composite measure, was observed for women with relatively unattractive partners. For women with relatively attractive partners, no clear association emerged. The interaction between progesterone and partner attractiveness was robust for women's interest in extra-pair sex, a component preregistered as exploratory. The interaction effect was also significant for the absolute intensity of women's extra-pair attraction (a component item referenced in the preregistration) but was non-significant for the frequency of women's extra-pair attraction relative to typical days (a composite component specified in the preregistration). These findings inform theoretical understandings of women's sexuality.     

Conditional expression of women's desires and men's mate guarding across the ovulatory cycle

Thirty-eight normally cycling women provided daily reports of sexual interests and feelings for 35 days. Near ovulation, both pair-bonded and single women reported feeling more physically attractive and having greater interest in attending social gatherings where they might meet men. Pair-bonded women who were near ovulation reported greater extra-pair flirtation and greater mate guarding by their primary partner. As predicted, however, these effects were exhibited primarily by women who perceived their partners to be low on hypothesized good genes indicators (low in sexual attractiveness relative to investment attractiveness). Ovulation-contingent increases in partner mate guarding were also moderated by female physical attractiveness; midcycle increases in mate guarding were experienced primarily by less attractive women, whereas more attractive women experienced relatively high levels of mate guarding throughout their cycle. These findings demonstrate ovulation-contingent shifts in desires and behaviors that are sensitive to varying fitness payoffs, and they provide support for the good genes hypothesis of human female extra-pair mating. The daily assessment method provides an important supplement to existing studies using scheduled laboratory visits as the purpose of the study (examining cycle-related variation) is not known by participants.     

Strategic Sexual Signals: Women's Display versus Avoidance of the Color Red Depends on the Attractiveness of an Anticipated Interaction Partner

The color red has special meaning in mating-relevant contexts. Wearing red can enhance perceptions of women’s attractiveness and desirability as a potential romantic partner. Building on recent findings, the present study examined whether women’s (N = 74) choice to display the color red is influenced by the attractiveness of an expected opposite-sex interaction partner. Results indicated that female participants who expected to interact with an attractive man displayed red (on clothing, accessories, and/or makeup) more often than a baseline consisting of women in a natural environment with no induced expectation. In contrast, when women expected to interact with an unattractive man, they eschewed red, displaying it less often than in the baseline condition. Findings are discussed with respect to evolutionary and cultural perspectives on mate evaluation and selection.     

Are human female preferences for symmetrical male faces enhanced when conception is likely?

Human females show a preference for the scent of symmetrical male bodies and appearance of masculine male faces only when conception is likely. These traits are thought to be signs of male quality. We examined whether females show an enhanced visual preference for another putative sign of mate quality, facial symmetry, when conception is likely. Twenty-nine females not taking oral contraceptives (nonpill users) rated the attractiveness of male faces varying in symmetry level (low, normal, high and perfect) for a short-term sexual partner at two phases of the menstrual cycle (low and high conception risk). They also rated the attractiveness of male faces for a long-term sexual partner and female faces for general attractiveness. We also tested a control group of 27 pill users. An overall preference for symmetry was found in all participants. However, the hypothesized enhanced cyclic preference for symmetrical male faces in nonpill users was not found. Nor was there an effect of relationship context. These results may be more consistent with a direct benefit or sensory bias model for preference evolution than with an indirect genetic benefit model. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Male sexual attractiveness predicts differential ovulatory shifts in female extra-pair attraction and male mate retention

Because ancestral women faced trade-offs in choosing mates, they may have evolved to pursue a dual-mating strategy in which they secured investment through one partner and obtained good genes through others. The dual-mating theory predicts that women will display greater interest in extra-pair sex near ovulation, especially if they are mated to a primary male partner who is low in sexual attractiveness. Forty-three normally ovulating women rated their partner's sexual attractiveness and separately reported their own desires and their partner's mate retention behaviors at high and low fertility (confirmed using luteinizing hormone tests). In the high-fertility session relative to the low, women who assessed their partners as being lower in sexual attractiveness reported greater extra-pair desires and more expressed love and attention from their male partners. Women's desire for their own partners did not differ significantly between high and low-fertility sessions.     

Ovulatory Shifts in Women��s Attractions to Primary Partners and Other Men: Further Evidence of the Importance of Primary Partner Sexual Attractiveness

Previous research has documented shifts in women’s attractions to their romantic partner and to men other than their partner across the ovulation cycle, contingent on the degree to which her partner displays hypothesized indicators of high-fitness genes. The current study set out to replicate and extend this finding. Forty-one couples in which the woman was naturally cycling participated. Female partners reported their feelings of in-pair attraction and extra-pair attraction on two occasions, once on a low-fertility day of the cycle and once on a high-fertility day of the cycle just prior to ovulation. Ovulation was confirmed using luteinizing hormone tests. We collected two measures of male partner sexual attractiveness. First, the women in the study rated their partner’s sexual attractiveness. Second, we photographed the partners and had the photos independently rated for attractiveness. Shifts in women’s in-pair attractions across the cycle were significantly moderated by women’s ratings of partner sexual attractiveness, such that the less sexually attractive women rated their partner, the less in-pair attraction they reported at high fertility compared with low fertility (partial r = .37, p _dir = .01). Shifts in women’s extra-pair attractions across the cycle were significantly moderated by third-party ratings of partner attractiveness, such that the less attractive the partner was, the more extra-pair attraction women reported at high relative to low fertility (partial r = −.33, p _dir = .03). In line with previous findings, we found support for the hypothesis that the degree to which a woman’s romantic partner displays indicators of high-fitness genes affects women’s attractions to their own partner and other men at high fertility.     

Parental Care and Sexual Selection in the Bluethroat, Luscinia s. svecica: A Field-experimental Test of the Differential Allocation Hypothesis

One of the processes by which secondary sexual characters in monogamous species with biparental care may evolve is described by the differential allocation hypothesis. It predicts that mates adjust their parental effort to the attractiveness of their partner. The more attractive sex is expected to withhold parental effort while the less attractive sex is expected to compensate for the partner's decreased effort thus preventing a reduction in breeding success. Asymmetry in attractiveness then imposes costs on the less attractive sex which are thought to be exceeded by the indirect (i.e. genetic) benefits which result from sharing offspring with a highly attractive partner. This idea was tested with bluethroats, Luscinia s. svecica , a bird species in which the male is characterized by a complex multiple-coloured plumage ornament on the throat and chest. We manipulated male attractiveness by attaching conspicuous coloured leg bands which matched the blue and chestnut components of the male ornament. Control males were banded with green and yellow bands. The results from another field experiment on mate-guarding behaviour demonstrate the effectiveness of this treatment. As measures of parental effort we determined: 1. clutch size; 2. feeding rates by video recording feeding activity of both parents; and 3. brood mass up to day 9. The results do not support the differential allocation hypothesis. The experimental and control groups differed in none of the above variables. On the basis of these results we discuss the ecological factors which may influence the differential allocation process in monogamous birds.     

Face and voice attractiveness judgments change during adolescence

Attractiveness judgments are thought to underpin adaptive mate choice decisions. We investigated how these judgments change during adolescence when mate choice is becoming relevant. Adolescents aged 11–15 evaluated faces and voices manipulated along dimensions that affect adults' judgments of attractiveness and that are thought to cue mate value. Facial stimuli consisted of pairs of faces that were more or less average, more or less feminine, or more or less symmetric. The adolescents selected the more average, symmetric, and feminine faces as more attractive more often than chance, but judgments of some facial traits differed significantly with rater age and sex, indicating a role of development in judgments of facial cues. Vocal stimuli consisted of pairs of voices manipulated to raise or lower perceived pitch. The older but not younger girls selected the lower-pitched male voices as more attractive at rates above chance, while the younger but not older boys selected the higher-pitched female voices as more attractive. Controlling for rater age, increased pubertal development was associated with increased selection of lower-pitched boys' voices by girls and decreased selection of feminized male faces by boys. Our results are the first to demonstrate that adolescents show somewhat similar attractiveness judgments to adults in age-matched stimuli and that age, sex, and pubertal development have measurable effects on adolescents' attractiveness judgments. They suggest that attractiveness judgments in humans, at least for some traits, are facultatively calibrated to the individual's life stage, only reaching adult values upon sexual maturity when mate choice decisions become relevant.     

Fertility in the cycle predicts women's interest in sexual opportunism

Research over the past decade has documented clear, robust changes in women's sexual preferences and interests across the ovarian cycle. When fertile, women are particularly attracted to a number of masculine male features (e.g., masculine faces, voices, scents and bodies) and other traits, and especially when they evaluate men's “sexiness” rather than their attractiveness as long-term partners. The current research extended this line of research by examining changes in women's self-reported sexual interests across the cycle. We asked 68 normally ovulating women in committed romantic relationships to fill out questionnaires about their sexual preferences and interests (at that time, not in general) twice across their cycles: once when fertile and once during the luteal phase. Relative to during the luteal phase, fertile women expressed (a) greater emphasis on the physical attractiveness of a partner; (b) greater arousal at the sight or thought of attractive male bodily features; (c) greater willingness to engage in and interest in sex with attractive men, even ones who they do not know well (interest in sexual opportunism). These findings importantly extend our understanding of women's fertile-phase sexuality.     

Negative frequency-dependent preferences and variation in male facial hair

Negative frequency-dependent sexual selection maintains striking polymorphisms in secondary sexual traits in several animal species. Here, we test whether frequency of beardedness modulates perceived attractiveness of men''s facial hair, a secondary sexual trait subject to considerable cultural variation. We first showed participants a suite of faces, within which we manipulated the frequency of beard thicknesses and then measured preferences for four standard levels of beardedness. Women and men judged heavy stubble and full beards more attractive when presented in treatments where beards were rare than when they were common, with intermediate preferences when intermediate frequencies of beardedness were presented. Likewise, clean-shaven faces were least attractive when clean-shaven faces were most common and more attractive when rare. This pattern in preferences is consistent with negative frequency-dependent selection.     

Social preferences based on sexual attractiveness: a female strategy to reduce male sexual attention

Male sexual harassment of females is common across sexually reproducing species and can result in fitness costs to females. We hypothesized that females can reduce unwanted male attention by constructing a social niche where their female associates are more sexually attractive than themselves, thus influencing the decision-making of males to their advantage. We tested this hypothesis in the Trinidadian guppy (Poecilia reticulata), a species with high levels of male sexual harassment. First, we confirmed that non-receptive females were harassed less when they were paired with a more sexually attractive (receptive) female than with another non-receptive female. We then found that, indeed, females exploit this as a strategy to reduce sexual harassment; non-receptive females actively preferred to associate with receptive over non-receptive females. Importantly, when given access only to chemosensory cues, non-receptive females still showed this preference, suggesting that they use information from chemical cues to assess the sexual attractiveness of potential female partners. Receptive females in contrast showed no such preferences. Our results demonstrate that females can decrease male harassment by associating with females that are more sexually attractive than themselves and that they perform active partner choices based on this relative attractiveness. We propose that this strategy is likely to represent an important pathway by which females can construct social niches that influence the decision-making of others to their advantage; in this case, to reduce the sexual harassment they experience.     

Physical attractiveness influences reproductive success of modern men

Theory suggests that reproductive success is positively associated with an individual’s genetic quality. However, the association between physical attractiveness and reproductive success (i.e., number of offspring) in modern humans remains less clear. Here we examined associations between men’s reproductive success and physical attractiveness from retrospective data obtained from married, divorced, and single samples of Slovakian men. As predicted, facially more attractive and taller men were more likely to engage in marriage. In turn, married men had higher reproductive success than single men. Even when men’s marital status was considered, facially more attractive men had higher reproductive success than their less attractive counterparts. This supports the importance of physical attractiveness in sexual selection in modern humans.     

Male cockroaches prefer a high carbohydrate diet that makes them more attractive to females: implications for the study of condition dependence

Sexual selection is a major force driving the evolution of elaborate male sexual traits. Handicap models of sexual selection predict that male sexual traits should covary positively with condition, making them reliable indicators of male quality. However, most studies have either manipulated condition through varying diet quantity and/or caloric content without knowledge of specific nutrient effects or have correlated proxies of condition with sexual trait expression. We used nutritional geometry to quantify protein and carbohydrate intake by male cockroaches, Nauphoeta cinerea, and related this to sex pheromone expression, attractiveness, and dominance status. We found that carbohydrate, but not protein, intake is related to male sex pheromone expression and attractiveness but not dominance status. Additionally, we related two condition proxies (weight gain and lipid reserves) to protein and carbohydrate acquisition. Weight gain increased with the intake of both nutrients, whereas lipid reserves only increased with carbohydrate intake. Importantly, lipid accumulation was not as responsive to carbohydrate intake as attractiveness and thus was a less-accurate condition proxy. Moreover, males preferentially consumed high carbohydrate diets with little regard for protein content suggesting that they actively increase their carbohydrate intake thereby maximizing their reproductive fitness by being attractive.     

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.     

Sexual experience interacts with steroid exposure to shape the partner preferences of rats

A three-phase experiment manipulated sexual experience and hormone exposure (perinatally and in adulthood) in female rats housed individually from weaning so as to limit peripubertal social and sexual experience. Noncontact partner preference for a male or estrous female rat was measured both before and after sexual experience, first while rats were under the influence of circulating testosterone propionate (TP) and later after priming them with ovarian hormones (estradiol benzoate and progesterone; EB & P). When implanted with TP capsules and tested while sexually naive, all groups of female rats preferred females to males without differing statistically. However, following three sexual experience sessions with estrous females, differences emerged between the masculinized and control groups in the magnitude of their female-directed preference, with masculinized females demonstrating a significantly greater preference for estrous females. Sexual experience with male rats under EB & P did not result in a significant shift in preference in any group. Histological assessment indicated that the volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) was increased by exposure to TP postnatally, and SDN-POA volume correlated positively with partner preference scores but only when rats were both sexually experienced and exposed to circulating TP in adulthood. These results suggest that sexual experience interacts with steroid exposure to shape partner preference.     

Maternal antibodies but not carotenoids in barn swallow eggs covary with embryo sex

Mothers influence their offspring phenotype by varying egg quality. Such maternal effects may be mediated by transmission of antibodies and antioxidants. Mothers should adjust allocation of maternal substances depending on embryonic sex because of differences in reproductive value, potentially dependent on paternal genetic effects as reflected by secondary sexual characters. We manipulated sexual attractiveness of male barn swallows (Hirundo rustica) and investigated maternal investment in eggs in relation to offspring sex. Mothers allocated more antibodies against a pathogen to eggs with a daughter than a son. However, concentration of antioxidants was independent of embryonic sex. Sex-dependent allocation was independent of paternal attractiveness. Thus, mothers adjusted allocation of substances to offspring in a complex manner, that may be part of a strategy of favouritism of daughters, which have larger mortality than sons. Such effects may have important consequences for secondary and tertiary sex ratios, but also for ontogeny of adult phenotype.     

The neurobiology of sexual partner preferences in rams

The question of what causes a male animal to seek out and choose a female as opposed to another male mating partner is unresolved and remains an issue of considerable debate. The most developed biologic theory is the perinatal organizational hypothesis, which states that perinatal hormone exposure mediates sexual differentiation of the brain. Numerous animal experiments have assessed the contribution of perinatal testosterone and/or estradiol exposure to the development of a male-typical mate preference, but almost all have used hormonally manipulated animals. In contrast, variations in sexual partner preferences occur spontaneously in domestic rams, with as many as 8% of the population exhibiting a preference for same-sex mating partners (male-oriented rams). Thus, the domestic ram is an excellent experimental model to study possible links between fetal neuroendocrine programming of neural mechanisms and adult sexual partner preferences. In this review, we present an overview of sexual differentiation in relation to sexual partner preferences. We then summarize results that test the relevance of the organizational hypothesis to expression of same-sex sexual partner preferences in rams. Finally, we demonstrate that the sexual differentiation of brain and behavior in sheep does not depend critically on aromatization of testosterone to estradiol.     

A multivariate approach to human mate preferences

Human mate choice is complicated, with various individual differences and contextual factors influencing preferences for numerous traits. However, focused studies on human mate choice often do not capture this multivariate complexity. Here, we consider multiple factors simultaneously to demonstrate the advantages of a multivariate approach to human mate preferences. Participants (N = 689) rated the attractiveness of opposite-sex online dating profiles that were independently manipulated on facial attractiveness, perceived facial masculinity/femininity, and intelligence. Participants were also randomly instructed to either consider short- or long-term relationships. Using fitness surfaces analyses, we assess the linear and nonlinear effects and interactions of the profiles' facial attractiveness, perceived facial masculinity/femininity, and perceived intelligence on participants' attractiveness ratings. Using hierarchical linear modeling, we were also able to consider the independent contribution of participants' individual differences on their revealed preferences for the manipulated traits. These individual differences included participants' age, socioeconomic status, education, disgust (moral, sexual, and pathogen), sociosexual orientation, personality variables, masculinity, and mate value. Together, our results illuminate various previously undetectable phenomena, including nonlinear preference functions and interactions with individual differences. More broadly, the study illustrates the value of considering both individual variation and population-level measures when addressing questions of sexual selection, and demonstrates the utility of multivariate approaches to complement focused studies.     

Trustworthy but not lust-worthy: context-specific effects of facial resemblance

If humans are sensitive to the costs and benefits of favouring kin in different circumstances, a strong prediction is that cues of relatedness will have a positive effect on prosocial feelings, but a negative effect on sexual attraction. Indeed, positive effects of facial resemblance (a potential cue of kinship) have been demonstrated in prosocial contexts. Alternatively, such effects may be owing to a general preference for familiar stimuli. Here, I show that subtly manipulated images of other-sex faces were judged as more trustworthy by the participants they were made to resemble than by control participants. In contrast, the effects of resemblance on attractiveness were significantly lower. In the context of a long-term relationship, where both prosocial regard and sexual appeal are important criteria, facial resemblance had no effect. In the context of a short-term relationship, where sexual appeal is the dominant criterion, facial resemblance decreased attractiveness. The results provide evidence against explanations implicating a general preference for familiar-looking stimuli and suggest instead that facial resemblance is a kinship cue to which humans modulate responses in a context-sensitive manner.