Cross-cultural variation in women's preferences for cues to sex- and stress-hormones in the male face.

Women in the UK prefer the faces of men with low levels of the stress hormone cortisol, and the relationship is moderated by the sex hormone testosterone. In a Latvian sample, however, women's preferences were not affected by cortisol, and the interaction with testosterone differed from that of the UK. To further explore cross-cultural variation in preferences for facial cues to sex- and stress-hormones, we tested the preferences of women from 13 countries for facial composites constructed to differ in combinations of the hormones. We found significant relationships between a measure of societal development (the United Nations human development index 2011) and preferences for cues to testosterone in the face, and the interaction between preferences for cues to testosterone and cortisol. We also found a significant relationship between preferences for cues to testosterone and a societal-level measure of parasite stress. We conclude that societal-level ecological factors influence the relative value of traits revealed by combinations of sex- and stress-hormones.

Kin recognition: evidence that humans can perceive both positive and negative relatedness.

The evolution of spite entails actors imposing costs on 'negative' relatives: those who are less likely than chance to share the actor's alleles and therefore more likely to bear rival alleles. Yet, despite a considerable body of research confirming that organisms can recognize positive relatives, little research has shown that organisms can recognize negative relatives. Here, we extend previous work on human phenotype matching by introducing a cue to negative relatedness: negative self-resembling faces, which differ from an average face in the opposite direction to the way an individual's own face differs from the average. Participants made trustworthiness and attractiveness judgements of pairs of opposite-sex positive and negative self-resembling faces. Analyses revealed opposing effects of positive and negative self-resembling faces on trustworthiness and attractiveness judgements. This is the first clear evidence that humans are sensitive to negative relatedness cues, and suggests the potential for the adaptive allocation of spiteful behaviour.

Transient pupil constrictions to faces are sensitive to orientation and species.

Previous studies have reported transient pupil constrictions to basic visual attributes (e.g., color and movement) that are processed along the ventral and the dorsal pathways. Specific cortical areas are activated more for faces than most other types of stimuli, raising the possibility that stimulus-specific transient pupil constrictions might also occur for faces. Such pupil responses may be sensitive to stimulus orientation and species since these parameters have been found to affect electrophysiological and behavioral responses to faces. Here we show transient pupil constrictions to upright human faces that are greater than those to scrambled versions, inverted versions, or macaque monkey faces. Similar to findings from electrophysiological studies, the inversion effect occurred for human faces but not macaque faces. Collectively, our findings show that transient pupil constrictions to faces are sensitive to the same parameters that have been found to influence electrophysiological and behavioral measures of face processing (i.e., orientation and species) and thus reveal a novel, objective, and non-invasive method for studying face perception.

Evidence for adaptive design in human gaze preference.

Many studies have investigated the physical cues that influence face preferences. By contrast, relatively few studies have investigated the effects of facial cues to the direction and valence of others' social interest (i.e. gaze direction and facial expressions) on face preferences. Here we found that participants demonstrated stronger preferences for direct gaze when judging the attractiveness of happy faces than that of disgusted faces, and that this effect of expression on the strength of attraction to direct gaze was particularly pronounced for judgements of opposite-sex faces (study 1). By contrast, no such opposite-sex bias in preferences for direct gaze was observed when participants judged the same faces for likeability (study 2). Collectively, these findings for a context-sensitive opposite-sex bias in preferences for perceiver-directed smiles, but not perceiver-directed disgust, suggest gaze preference functions, at least in part, to facilitate efficient allocation of mating effort, and evince adaptive design in the perceptual mechanisms that underpin face preferences.

Sex-contingent face aftereffects depend on perceptual category rather than structural encoding.

Many studies have used visual adaptation to investigate how recent experience with faces influences perception. While faces similar to those seen during adaptation phases are typically perceived as more 'normal' after adaptation, it is possible to induce aftereffects in one direction for one category (e.g. female) and simultaneously induce aftereffects in the opposite direction for another category (e.g. male). Such aftereffects could reflect 'category-contingent' adaptation of neurons selective for perceptual category (e.g. male or female) or 'structure-contingent' adaptation of lower-level neurons coding the physical characteristics of different face patterns. We compared these explanations by testing for simultaneous opposite after effects following adaptation to (a) two groups of faces from distinct sex categories (male and female) or (b) two groups of faces from the same sex category (female and hyper-female) where the structural differences between the female and hyper-female groups were mathematically identical to those between male and female groups. We were able to induce opposite aftereffects following adaptation between sex categories but not after adaptation within a sex category. These findings indicate the involvement of neurons coding perceptual category in sex-contingent face aftereffects and cannot be explained by neurons coding only the physical aspects of face patterns.

Raised salivary testosterone in women is associated with increased attraction to masculine faces.

Women's preferences for masculinity in men's faces, voices and behavioral displays change during the menstrual cycle and are strongest around ovulation. While previous findings suggest that change in progesterone level is an important hormonal mechanism for such variation, it is likely that changes in the levels of other hormones will also contribute to cyclic variation in masculinity preferences. Here we compared women's preferences for masculine faces at two points in the menstrual cycle where women differed in salivary testosterone, but not in salivary progesterone or estrogen. Preferences for masculinity were strongest when women's testosterone levels were relatively high. Our findings complement those from previous studies that show systematic variation in masculinity preferences during the menstrual cycle and suggest that change in testosterone level may play an important role in cyclic shifts in women's preferences for masculine traits.

Salience of emotional displays of danger and contagion in faces is enhanced when progesterone levels are raised.

Findings from previous studies of hormone-mediated behavior in women suggest that raised progesterone level increases the probability of behaviors that will reduce the likelihood of disruption to fetal development during pregnancy (e.g. increased avoidance of sources of contagion). Here, we tested women's (N=52) sensitivity to potential cues to nearby sources of contagion (disgusted facial expressions with averted gaze) and nearby physical threat (fearful facial expressions with averted gaze) at two points in the menstrual cycle differing in progesterone level. Women demonstrated a greater tendency to perceive fearful and disgusted expressions with averted gaze as more intense than those with direct gaze when their progesterone level was relatively high. By contrast, change in progesterone level was not associated with any change in perceptions of happy expressions with direct and averted gaze, indicating that our findings for disgusted and fearful expressions were not due to a general response bias. Collectively, our findings suggest women are more sensitive to facial cues signalling nearby contagion and physical threat when raised progesterone level prepares the body for pregnancy.

Menstrual cycle, trait estrogen level, and masculinity preferences in the human voice.

Men with low testosterone (feminine men) invest in relationships and offspring more than men with high testosterone (masculine men). Women's attraction to testosterone dependent traits (e.g. masculine face shape) is enhanced during the late-follicular, fertile phase of the menstrual cycle. Attractive, feminine women have stronger preferences for masculine men as possible long-term partners than less attractive, masculine women. We manipulated 2 testosterone related vocal traits (voice pitch and apparent vocal-tract length) in voices to test if women prefer masculinized men's voices to feminized men's voices; masculinity preferences are enhanced at the fertile (late-follicular) menstrual cycle phase; the amount that masculinity preferences shift cyclically relates to average estrone-3-glucuronide concentration (the primary urinary metabolite of estrone, E3G). We found women displayed general masculinity preferences for men's voices; masculinity preferences were greater in the fertile (late-follicular) phase of the cycle than the non-fertile (early-follicular and luteal) phase; and this effect was most pronounced for women with low average E3G concentration. As feminine women (i.e. those with high average E3G levels) are most able to obtain investment even from masculine men, these women may not need to change their mating preference or strategy during the menstrual cycle as much as masculine women.

Commitment to relationships and preferences for femininity and apparent health in faces are strongest on days of the menstrual cycle when progesterone level is high.

Previous studies of changes in women's behavior during the menstrual cycle have offered insight into the motivations underpinning women's preferences for social cues associated with possible direct benefits (e.g., investment, low risk of infection) and indirect benefits (e.g., offspring viability). Here we sought to extend this work by testing for systematic variation in women's preferences for male and female faces and in their attitudes to their romantic relationship during the menstrual cycle. In Study 1, we found partnered women's reported commitment to their romantic relationship and preferences for femininity in male and female faces were strongest on days of the menstrual cycle when progesterone levels are increased (and fertility is low). Happiness in relationships did not change across the cycle. In Study 2, we found that the effect of cycle phase on women's preference for feminine faces was independent of increased attraction to apparent health in faces during the luteal phase. Collectively, these findings are further evidence that women's preferences for social cues associated with possible direct benefits and commitment to relationships are strongest during conditions characterized by raised progesterone level, while attraction to men displaying cues associated with possible indirect benefits is strongest when women are most fertile.