Men’s oxidative stress, fluctuating asymmetry and physical attractiveness

Preference for Self-Resembling Male Faces in Gay Men in China

Self-resemblance refers to couples with similar characteristics, also known as homogamy or positive assortative mating. Previous studies have indicated that heterosexual men and women prefer partners with similar facial features. In this study, we examined whether Chinese gay men preferred self-resemblance to faces. The participants (N = 70) completed a personal information questionnaire and preference selection task involving 10 pairs of self-resembling/control male faces. Ten pairs of self-resembling/control male faces of each participant were also rated by another gay man. The results revealed that the proportion of the participants who chose self-resembling faces was significantly higher than that of the control faces. However, the preference for "self-resembling" and control faces by other-rating was not significantly different. These findings indicate homogeneity in facial preferences among gay men.

Birth size and the serum level of biological age markers in men

Previous studies showed that intrauterine growth restrictions, resulting in smaller body size at birth, are associated with altered development and the risk of age-related diseases in adult life. Thus, prenatal development may predict aging trajectories in humans. The study aimed to verify if body size at birth is related to biological age in adult men. The study sample consisted of 159 healthy, non-smoking men with a mean age of 35.24 (SD 3.44) years. Birth weight and length were taken from medical records. The ponderal index at birth was calculated. Biological age was evaluated based on serum levels of s-Klotho, hsCRP, DHEA/S, and oxidative stress markers. Pregnancy age at birth, lifestyle, weight, cortisol, and testosterone levels were controlled. The results showed no relationship between birth size and s-Klotho, DHEA/S level, inflammation, or oxidative stress. Also, men born as small-for-gestational-age (N = 49) and men born as appropriate-for-gestational-age (N = 110) did not differ in terms of biological age markers levels. The results were similar when controlled for pregnancy week at birth, chronological age, BMI, testosterone, or cortisol level. The results suggest that there is no relationship between intrauterine growth and biomarkers of aging in men aged 30-45 years from the affluent population.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Formal models for the study of the relationship between fluctuating asymmetry and fitness in humans

OBJECTIVES

To evaluate three of the main verbal models that have been proposed to explain the relationship between fluctuating asymmetry and fitness in humans: the "good genes," the "good development," and the "growth" hypotheses.

MATERIALS AND METHODS

A formal model was generated for each verbal model following three steps. First, based on the literature, a theoretical causal model and the theoretical object of inquiry were outlined. Second, an empirical causal model and the targets of inference were defined using observational data of facial asymmetries and life-history traits related to fitness. Third, generalized linear models and causal inference were used as the estimation strategy.

RESULTS

The results suggest that the theoretical and empirical assumptions of the "good genes" hypothesis should be reformulated. The results were compatible with most of the empirical assumptions of "the good development" hypothesis but suggest that further discussion of its theoretical assumptions is needed. The results were less informative about the "growth" hypothesis, both theoretically and empirically. There was a positive association between facial fluctuating asymmetry and the number of offspring that was not compatible with any of the empirical causal models evaluated.

CONCLUSIONS

Although the three hypotheses focus on different aspects of the link between asymmetry and fitness, their overlap opens the possibility of a unified theory on the subject. The results of this study make explicit which assumptions need to be updated and discussed, facilitating the advancement of this area of research. Overall, this study elucidates the potential benefit of using formal models for theory revision and development.

Developmental instability, fluctuating asymmetry, and human psychological science

Developmental instability (DI) is an individual's inability to produce a specific developmental outcome under a given set of conditions, generally thought to result from random perturbations experienced during development. Fluctuating asymmetry (FA) - asymmetry on bilateral features that, on average, are symmetrical (or asymmetry deviating from that arising from design) - has been used to measure DI. Dating to half a century ago, and accelerating in the past three decades, psychological researchers have examined associations between FA (typically measured on bodily or facial features) and a host of outcomes of interest, including psychological disorders, cognitive ability, attractiveness, and sexual behavior. A decade ago, a meta-analysis on findings from nearly 100 studies extracted several conclusions. On average, small but statistically reliable associations between FA and traits of interest exist. Though modest, these associations are expected to greatly underestimate the strength of associations with underlying DI. Despite the massive sample size across studies, we still lack a good handle on which traits are most strongly affected by DI. A major methodological implication of the meta-analysis is that most studies have been, individually, woefully underpowered to detect associations. Though offering some intriguing findings, much research is the past decade too has been underpowered; hence, the newer literature is also likely noisy. Several large-scale studies are exceptions. Future progress depends on additional large-scale studies and researchers' sensitivity to power issues. As well, theoretical assumptions and conceptualizations of DI and FA driving psychological research may need revision to explain empirical patterns.

Socioeconomic position, immune function, and its physiological markers

The development of costly traits such as immune function and secondary sexual traits is constrained by resource availability. The quality of developmental conditions and the availability of resources in ontogeny may therefore influence immune system functions and other biological traits. We analyzed causal pathways between family socioeconomic position, strength of immune response, and five physiological biomarkers in young Latvian men (n = 93) using structural equation modeling. Men from wealthier families had higher testosterone levels (r_s = 0.280), stronger immune response (r_s = 0.551), and higher facial attractiveness (r_s = 0.300). There were weak, non-significant correlations between family income, body fat percentage (r_s = -0.147), and fluctuating asymmetry (r_s = -0.159). Testosterone partially (33.8%) mediated the effect of family income on facial masculinity. Testosterone (positively) and adiposity (negatively) partially (4%) mediated the relationship between family income and immune function. Higher facial masculinity, higher facial symmetry, and lower adiposity were reliable and independent cues of better immune function (R² = 0.238) in a larger sample of young Latvian men (N = 146). Resource availability in ontogeny has an important role for the development of immune function and physical appearance, and it is a key parameter to be included in human eco-immunological research.

Oxidative stress as a hidden cost of attractiveness in postmenopausal women

Perceived facial attractiveness, a putative marker of high biological fitness, is costly to maintain throughout a lifetime and may cause higher oxidative stress (OS). We investigated the association between the facial features of 97 postmenopausal women and their levels of OS biomarkers 8-hydroxy-2′-deoxyguanosine (8-OHdG), superoxide dismutase (Cu-Zn SOD), and thiobarbituric acid reactive substances (TBARS). In study 1, 966 judges rated the composites (facial averages) of women with higher OS as more attractive, healthier, younger, and less symmetric. In study 2, Geometric Morphometric analysis did not reveal significant differences in facial morphology depending on OS levels. In study 3, measured facial averageness and symmetry were weakly negatively related to 8-OHdG levels. Maintaining higher perceived facial attractiveness may be costly due to increased oxidative damage in the postmenopausal period. These costs may remain hidden during the reproductive period of life due to the protective mechanisms of oxidative shielding and revealed only after menopause when shielding has ceased.

Testing an Adaptationist Theory of Trait Covariation: Relative Bargaining Power as a Common Calibrator of an Interpersonal Syndrome

This article provides the first test of an adaptationist ‘common calibration’ theory to explain the origins of trait covariation, which holds that (i) personality traits are often facultatively calibrated in response to cues that ancestrally predicted the reproductive payoffs of different trait levels and (ii) distinct traits that are calibrated on the basis of common input cues will exhibit consistent patterns of covariation. This theory is applied to explain the covariation within a ‘personality syndrome’ encompassing various interpersonal trait dimensions (e.g. extraversion, emotionality and attachment styles). Specifically, it is hypothesized that these traits are inter–correlated because each is calibrated in response to relative bargaining power (RBP)—a joint function of one's ability to benefit others and harm others. Path analyses from a correlational study compellingly supported this theoretical model: Objective and self–perceived measures of RBP–enhancing phenotypic features (physical attractiveness and physical strength) influenced an internal regulatory variable indexing RBP (i.e. self–perceived RBP), which in turn had robust effects on each of the focal personality traits. Moreover, in support of the theory's core postulate, controlling for self–perceived RBP greatly reduced the covariation within the interpersonal syndrome. These novel findings illustrate the promise of an evolutionary psychological approach to elucidating trait covariation. Copyright © 2013 John Wiley & Sons, Ltd.

Feeling Stressed and Ugly? Leave the City and Visit Nature! An Experiment on Self-and Other-Perceived Stress and Attractiveness Levels

Natural environments, compared to urban environments, usually lead to reduced stress and positive body appreciation. We assumed that walks through nature and urban environments affect self- and other-perceived stress and attractiveness levels. Therefore, we collected questionnaire data and took photographs of male participants' faces before and after they took walks. In a second step, female participants rated the photographs. As expected, participants felt more restored and attractive, and less stressed after they walked in nature compared to an urban environment. A significant interaction of environment (nature, urban) and time (pre, post) indicated that the men were rated by the women as being more stressed after the urban walk. Other-rated attractiveness levels, however, were similar for both walks and time points. In sum, we showed that the rather stressful experience of a short-term urban walk mirrors in the face of men and is detectable by women.

Facial appearance and metabolic health biomarkers in women

Facial appearance has been suggested to provide an honest cue of an individual’s biological condition. However, there is little direct evidence that facial attractiveness reflects actual health. Here we tested if facial appearance is related with metabolic health biomarkers. Face photographs of 161 healthy, young women (M_age = 28.59, SD_age = 2.34) were assessed in terms of perceived attractiveness and health. Metabolic health was evaluated based on levels of markers of lipid and glucose metabolism balance, liver functioning, and inflammation. BMI, testosterone (T), and estradiol (E2) levels were controlled. Facial attractiveness, but not health, was negatively related with lipid profile components detrimental to health (total cholesterol, LDL, triglycerides) but not with relatively protective for health HDL. When controlled for BMI, E2, and T, only the relationship between attractiveness and triglycerides remained significant. Facial appearance was unrelated with glucose metabolism, liver functioning, and inflammatory markers. The results suggest, that for healthy women of reproductive age, such measures as BMI and sex hormone levels may be better predictors of attractiveness, compared to measures of metabolic health. Markers of lipid, glucose homeostasis, liver functioning or low-grade inflammation may be rather indicators of future health, of lesser importance in mating context, thus only modestly reflected in facial appearance.

Drosophila melanogaster Response to Feeding with Neomycin-Based Medium Expressed in Fluctuating Asymmetry

The fruit fly Drosophila melanogaster is a model species used for a wide range of studies. Contamination of Drosophila cultures with bacterial infection is common and is readily eradicated by antibiotics. Neomycin antibiotics can cause stress to D. melanogaster's larvae and imagoes, which may affect the interpretation of the results of research using culture from neomycin-based medium. In the present study, fluctuating asymmetry (FA), one of the important bioindicators of stress, was measured. Larvae and imagoes of a wild-type D. melanogaster strain were exposed to various concentrations of neomycin. The size of anal papillae and selected wing veins were measured using scanning electron and light microscopy, respectively. Next, the FA was checked. The values obtained for larval anal papillae appeared to be concentration-dependant; the FA indices increased with the concentration of neomycin. The wing FA presented a large but variable correlation, depending on the measured vein. However, the mean length of veins was the highest for the control group, with neomycin-exposed groups showing lower values. The research showed that neomycin may cause sublethal stress in D. melanogaster, which manifests in increased FA indices. This suggests that neomycin can cause physiological and developmental stress in insects, which should be taken into account when interpreting the results of studies using these model organisms.

Perception of Human Age from Faces: Symmetric Versus Asymmetric Movement

Dynamic movements of a face affect human perception of a person’s identification, emotional expression, speech, and so on. Findings of studies related to age perception, however, have mainly been obtained from static features of texture such as wrinkles and spots on the skin. Our goal is to investigate the perception of human age related to dynamic information. Systematically manipulated bilateral symmetric and asymmetric facial movements were utilized as stimuli in the age perception experiment. All images were low-pass filtered so that the judgment would not depend on detailed texture information. In the experiment, viewers judged the age level (the first half (indicating 20–24 and 60–64) or the latter half (indicating 25–29 and 65–69) of two age groups: 20’s and 60’s. Results revealed that faces with symmetric dynamic movements of expression (from a neutral face to one pronouncing “i”) were not only judged at the level of chance, but were also perceived to be statistically significantly younger than faces with asymmetric dynamic movements. It was also found that types of asymmetry were also effective in age perception, which might be a reflection of laterization of facial processing in a human brain.

Testosterone, cortisol, and status-striving personality features: A review and empirical evaluation of the Dual Hormone hypothesis

Decades of research in behavioral endocrinology has implicated the gonadal hormone testosterone in the regulation of mating effort, often expressed in primates in the form of aggressive and/or status-striving behavior. Based on the idea that neuroendocrine axes influence each other, recent work among humans has proposed that links between testosterone and indices of status-striving are rendered conditional by the effects of glucocorticoids. The Dual Hormone hypothesis is one particular instance of this argument, predicting that cortisol blocks the effects of testosterone on dominance, aggression, and risk-taking in humans. Support for the Dual Hormone hypothesis is wide-ranging, but considerations of theoretical ambiguity, null findings, and low statistical power pose problems for interpreting the published literature. Here, we contribute to the development of the Dual Hormone hypothesis by (1) critically reviewing the extant literature-including p-curve analyses of published findings; and, (2) "opening the file drawer" and examining relationships between testosterone, cortisol, and status-striving personality features in seven previously published studies from our laboratories (total N = 718; median N per feature = 318) that examined unrelated predictions. Results from p-curve suggest that published studies have only 16% power to detect effects, while our own data show no robust interactions between testosterone and cortisol in predicting status-striving personality features. We discuss the implications of these results for the Dual Hormone hypothesis, limitations of our analyses, and the development of future research.

Behavioral immune system activity predicts downregulation of chronic basal inflammation

Here, we present a mechanistically grounded theory detailing a novel function of the behavioral immune system (BIS), the psychological system that prompts pathogen avoidance behaviors. We propose that BIS activity allows the body to downregulate basal inflammation, preventing resultant oxidative damage to DNA and promoting longevity. Study 1 investigated the relationship between a trait measure of pathogen avoidance motivation and in vitro and in vivo proinflammatory cytokine production. Study 2 examined the relationship between this same predictor and DNA damage often associated with prolonged inflammation. Results revealed that greater trait pathogen avoidance motivation predicts a) lower levels of spontaneous (but not stimulated) proinflammatory cytokine release by peripheral blood mononuclear cells (PBMCs), b) lower plasma levels of the proinflammatory cytokine interleukin-6 (IL-6), and c) lower levels of oxidative DNA damage. Thus, the BIS may promote health by protecting the body from the deleterious effects of inflammation and oxidative stress.

Body symmetry and reproductive hormone levels in women

Fluctuating asymmetry (FA), a morphological marker of developmental stability, may be related to an individual's biological condition, e.g., health or fertility. The aim of this study was to test if the level of a woman's FA was related to her fertility and reproductive potential as measured by reproductive hormone levels. Fifty-three healthy, non-pregnant, naturally cycling women (mean age = 23.42, SD = 1.85 years), participated in the study, conducted in Wrocław (Poland) in May 2015. Early-follicular phase serum levels of anti-Müllerian hormone (AMH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and estradiol (E2) were measured. FA was calculated based on anthropometric measures of six bilateral body traits, and the composite FA index was used in statistical analyses. No relationship was observed between FA and the levels of FSH, LH, and AMH (p > .05), controlled for potential confounders. However, the level of E2 was positively correlated with FA (p < .05). Thus, in young women, FA was not related to hormones levels related to ovarian reserve, but more symmetrical women had lower E2 levels. As FA is an index of developmental stability, environmental, and genetic stress, the results of the study confirm previous research suggesting that developmental conditions may be related to women's endogenous estrogen levels.

Own attractiveness and perceived relationship quality shape sensitivity in women's memory for other men on the attractiveness dimension

Although recent work suggests that opposite-sex facial attractiveness is less salient in memory when individuals are in a committed romantic relationship, romantic relationship quality can vary over time. In light of this, we tested whether activating concerns about romantic relationship quality strengthens memory for attractive faces. Partnered women were exposed briefly to faces manipulated in shape cues to attractiveness before either being asked to think about a moment of emotional closeness or distance in their current relationship. We measured sensitivity in memory for faces as the extent to which they recognized correct versions of studied faces over versions of the same person altered to look either more or less-attractive than their original (i.e., studied) version. Contrary to predictions, high relationship quality strengthened hit rate for faces regardless of the sex or attractiveness of the face. In general, women's memories were more sensitive to attractiveness in women, but were biased toward attractiveness in male faces, both when responding to unfamiliar faces and versions of familiar faces that were more attractive than the original male identity from the learning phase. However, findings varied according to self-rated attractiveness and a psychometric measure of the quality of their current relationship. Attractive women were more sensitive to attractiveness in men, while their less-attractive peers had a stronger bias to remember women as more-attractive and men as less-attractive than their original image respectively. Women in better-quality romantic relationships had stronger positive biases toward, and false memories for, attractive men. Our findings suggest a sophisticated pattern of sensitivity and bias in women's memory for facial cues to quality that varies systematically according to factors that may alter the costs of female mating competition ('market demand') and relationship maintenance.

Predictors of facial attractiveness and health in humans

Facial attractiveness has been suggested to provide signals of biological quality, particularly health, in humans. The attractive traits that have been implicated as signals of biological quality include sexual dimorphism, symmetry, averageness, adiposity, and carotenoid-based skin colour. In this study, we first provide a comprehensive examination of the traits that predict attractiveness. In men, attractiveness was predicted positively by masculinity, symmetry, averageness, and negatively by adiposity. In women, attractiveness was predicted positively by femininity and negatively by adiposity. Skin colour did not predict attractiveness in either sex, suggesting that, despite recent interest in the literature, colour may play limited role in determining attractiveness. Male perceived health was predicted positively by averageness, symmetry, and skin yellowness, and negatively by adiposity. Female perceived health was predicted by femininity. We then examined whether appearance predicted actual health using measures that have been theoretically linked to sexual selection, including immune function, oxidative stress, and semen quality. In women, there was little evidence that female appearance predicted health. In men, we found support for the phenotype-linked fertility hypothesis that male masculinity signalled semen quality. However, we also found a negative relationship between averageness and semen quality. Overall, these results indicate weak links between attractive facial traits and health.

The relationship between health and mating success in humans

Health has been claimed to play an important role in human sexual selection, especially in terms of mate choice. Our preferences for attractive individuals are said to represent evolved adaptations for finding high-quality, healthy mates. If this is true, then we expect health to predict mating success in humans. We tested this hypothesis using several important physiological indicators of health, including immune function, oxidative stress and semen quality, and self-reported measures of sexual behaviour that contribute to mating success. In contrast to our hypothesis, we did not find a relationship between the physiological measures of health and sexual behaviour. Our results provide little support for claims that health, at least the health measures we used, increases mating success in relatively healthy humans.

About Face

The physician’s general survey is an invaluable method for quickly assessing a patient’s overall health and demeanor. Recent studies in psychology have found that qualities of one’s face—including skin coloration and facial adiposity—are associated with one’s underlying health. Subtle variations in these features can indicate serious health problems but can be detected with the untrained eye. Here, we describe how these facial traits link to health and discuss how such cues could be incorporated into the physician’s general survey. In light of research on the barriers preventing physicians from implementing new approaches in their practice, we also consider how scans of facial cues to health could be rapidly and effortlessly integrated into a physician’s current general survey. Integrating assessments of facial cues to health into the general survey would be a simple, low-risk, and low-cost way to detect, monitor, and track possible health concerns, particularly in the burgeoning field of telemedicine.

Correlated Preferences for Male Facial Masculinity and Partner Traits in Gay and Bisexual Men in China

Previous studies have documented the correlation between preferences for male facial masculinity and perceived masculinity: women who rate their male partner as more masculine tend to prefer more masculine faces. Men's self-rated masculinity predicts their female partner's preference for masculinity. This study examined the association between other trait preferences and preference for male facial masculinity among 556 gay and bisexual men across multiple cities in China. Participants were asked to choose the three most important traits in a romantic partner from a list of 23 traits. Each participant was then asked to choose a preferred face in each of 10 pairs of male faces presented sequentially, with each pair consisting of a masculinized and feminized version of the same base face. The results indicated that preferences for health and status-related traits were correlated with preferences for male facial masculinity in gay and bisexual men in China; individuals who were more health- or status-oriented in their preferences for a romantic partner preferred more masculine male faces than individuals with lower levels of these orientations. The findings have implications for the correlated preferences for facial masculinity and health- and status-related traits and may be related to perceived health and dominance/aggression of masculine faces based on a sample of non-Western gay and bisexual men.

Sociosexual attitudes and dyadic sexual desire independently predict women's preferences for male vocal masculinity

Research suggests that the desire to behave sexually with a partner (dyadic sexual desire) may reflect desire for intimacy whereas solitary sexual desire may reflect pleasure seeking motivations more generally. Because direct reproductive success can only be increased with a sexual partner, we tested whether dyadic sexual desire was a better predictor of women's preferences for lower pitched men's voices (a marker of relatively high reproductive success) than was solitary sexual desire. In Study 1, women (N = 95) with higher dyadic sexual desire scores on the Sexual Desire Inventory-2 preferred masculinized male voices more than did women with lower dyadic sexual desire scores. We did not find a significant relationship between women's vocal masculinity preferences and their solitary sexual desire scores. In Study 2, we tested whether the relationship between voice preferences and dyadic sexual desire scores was related to differences in sociosexual orientation. Women (N = 80) with more positive attitudes towards uncommitted sex had stronger vocal masculinity preferences regardless of whether men's attractiveness was judged for short-term or long-term relationships. Independent of the effect of sociosexual attitudes, dyadic sexual desire positively predicted women's masculinity preferences when assessing men's attractiveness for short-term but not long-term relationships. These effects were independent of women's own relationship status and hormonal contraceptive use. Our results provide further evidence that women's mate preferences may independently reflect individual differences in both sexual desire and openness to short-term relationships, potentially with the ultimate function of maximizing the fitness benefits of women's mate choices.

Self-reported impulsivity, rather than sociosexuality, predicts women's preferences for masculine features in male faces

Previous research has suggested that an individual's sociosexual orientation (i.e., their willingness to engage in sexual behavior outside of long-term relationships) may influence the qualities they find attractive in a potential mate. Results, however, have not been consistent and, moreover, studies have tended to draw from specific social groups. Here, we tested the relationship between sociosexuality and female's preferences for masculinity in male faces, using a diverse population. We furthermore investigated impulsivity alongside sociosexuality, as this trait has been suggested as a "root" cause of variation in sexual behavior (Cross, 2010) and thus may better explain variation in mate choice. Results showed a significant association between increases in both sociosexuality and two subcomponents of impulsivity and greater preferences for masculine male features. Regression analysis suggested that a subcomponent of impulsivity, namely lack of planning, was the primary determinant of preferences. We discuss the implications these results have for our understanding of female attraction to masculine features.

Women's luteal-phase sexual proceptivity and the functions of extended sexuality

Women's sexuality, unlike that of most mammals, is not solely defined by sexual receptivity during the short window of fertility. Women demonstrate extended sexuality (in which they initiate and accept sexual advances outside of the fertile phase) more than any other mammalian female. In this light, surprisingly little research has addressed the functions of women's luteal-phase sexuality. On the basis of theory and comparative evidence, we propose that women's initiation of sex during nonfertile phases evolved in part to garner investment from male partners. If so, women should be particularly prone to initiate luteal-phase sex when the potential marginal gains are greatest. Results from a study of 50 heterosexual couples showed that women increasingly initiate sex in the luteal phase (but not the fertile phase) when they perceive their partners' investment to lag behind their own. These findings provide evidence for the distinct nature of women's extended sexuality and may contribute to a more comprehensive understanding of women's sexuality.

The influence of symmetry on children's judgments of facial attractiveness

In experiment 1, we examined developmental changes in the influence of symmetry on judgments of attractiveness by showing adults and children pairs of individual faces in which one face was transformed 75% toward perfect symmetry, while the other face was transformed by exaggerating its asymmetries by 75%. Adults and 9-year-olds, but not 5-year-olds, rated the more symmetric faces as more attractive than the less symmetric faces, although the effect was stronger in adults than 9-year-olds. The preference for symmetry was stronger for male than female faces, and stronger for adults' than children's faces. In experiment 2, comparisons of the symmetry of the original male and female faces revealed no measured differences but lower ratings by adults of symmetry in the male faces. Overall, the results suggest that the influence of symmetry on attractiveness judgments emerges after the age of 5 years, and matures after the age of 9 years. The stronger effects for adult viewers may reflect an increase in sensitivity to symmetry as experience with faces increases and/or as the visual system matures. As well, attractiveness may become more salient after puberty, so that honest signals of mate quality, such as symmetry, have a stronger effect for adult viewers, especially when judging adult faces.

Putting All Your Eggs in One Basket

Diversification of resources is a strategy found everywhere from the level of microorganisms to that of giant Wall Street investment firms. We examine the functional nature of diversification using life-history theory—a framework for understanding how organisms navigate resource-allocation trade-offs. This framework suggests that diversification may be adaptive or maladaptive depending on one’s life-history strategy and that these differences should be observed under conditions of threat. In three studies, we found that cues of mortality threat interact with one index of life-history strategy, childhood socioeconomic status (SES), to affect diversification. Among those from low-SES backgrounds, mortality threat increased preferences for diversification. However, among those from high-SES backgrounds, mortality threat had the opposite effect, inclining people to put all their eggs in one basket. The same interaction pattern emerged with a potential biomarker of life-history strategy, oxidative stress. These findings highlight when, and for whom, different diversification strategies can be advantageous.

When the economy falters, do people spend or save? Responses to resource scarcity depend on childhood environments

Just as modern economies undergo periods of boom and bust, human ancestors experienced cycles of abundance and famine. Is the adaptive response when resources become scarce to save for the future or to spend money on immediate gains? Drawing on life-history theory, we propose that people's responses to resource scarcity depend on the harshness of their early-life environment, as reflected by childhood socioeconomic status (SES). In the three experiments reported here, we tested how people from different childhood environments responded to resource scarcity. We found that people who grew up in lower-SES environments were more impulsive, took more risks, and approached temptations more quickly. Conversely, people who grew up in higher-SES environments were less impulsive, took fewer risks, and approached temptations more slowly. Responses similarly diverged according to people's oxidative-stress levels-a urinary biomarker of cumulative stress exposure. Overall, whereas tendencies associated with early-life environments were dormant in benign conditions, they emerged under conditions of economic uncertainty.

FACIAL ASYMMETRY IS NEGATIVELY RELATED TO CONDITION IN FEMALE MACAQUE MONKEYS

The face is an important visual trait in social communication across many species. In evolutionary terms there are large and obvious selective advantages in detecting healthy partners, both in terms of avoiding individuals with poor health to minimise contagion and in mating with individuals with high health to help ensure healthy offspring. Many models of sexual selection suggest that an individual's phenotype provides cues to their quality. Fluctuating asymmetry is a trait that is proposed to be an honest indicator of quality and previous studies have demonstrated that rhesus monkeys gaze longer at symmetric faces, suggesting preferences for such faces. The current study examined the relationship between measured facial symmetry and measures of health in a captive population of female rhesus macaque monkeys. We measured asymmetry from landmarks marked on front-on facial photographs and computed measures of health based on veterinary health and condition ratings, number of minor and major wounds sustained, and gain in weight over the first four years of life. Analysis revealed that facial asymmetry was negatively related to condition related health measures, with symmetric individuals being healthier than more asymmetric individuals. Facial asymmetry appears to be an honest indicator of health in rhesus macaques and asymmetry may then be used by conspecifics in mate-choice situations. More broadly, our data support the notion that faces are valuable sources of information in non-human primates and that sexual selection based on facial information is potentially important across the primate lineage.

Mate preferences and infectious disease: theoretical considerations and evidence in humans

Mate preferences may operate in part to mitigate the threats posed by infectious disease. In this paper, we outline various ways in which preferring healthy mates can offer direct benefits in terms of pathogen avoidance and indirect benefits in terms of heritable immunity to offspring, as well as the costs that may constrain mate preferences for health. We then pay special attention to empirical work on mate preferences in humans given the depth and breadth of research on human mating. We review this literature and comment on the degree to which human mate preferences may reflect preferences for health.