Evolution and Human Behavior

Published by Elsevier BV

Print ISSN: 1090-5138

Articles


The Ratio of 2nd to 4th Digit Length and Male Homosexuality
  • Article

October 2000

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1,291 Reads

SJ Robinson

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Sexual orientation may be influenced by prenatal levels of testosterone and oestrogen. There is evidence that the ratio of the length of 2nd and 4th digits (2D:4D) is negatively related to prenatal testosterone and positively to oestrogen. We report that (a) 2D:4D was lower in a sample of 88 homosexual men than in 88 sex- and age-matched controls recruited without regard to sexual orientation, (b) within the homosexual sample, there was a significant positive relationship between mean 2D:4D ratio and exclusive homosexuality, (c) overall, there was a decrease in 2D:4D from controls to homosexual men to bisexual men and (d) fraternal birth order, a positive predictor of male homosexuality, was not associated with 2D:4D in a sample of 240 Caucasian men recruited without regard to sexual orientation and 45 homosexual men.Further work is needed to confirm the relationships between 2D:4D and sexual orientation. However, these and other recent data tend to support an association between male homosexuality and high fetal testosterone. Very high testosterone levels may be associated with a sexual preference for both men and women.
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Fig. 1. Reported number of children regressed on 2D:4D ratio of the right hand of English men (n 117) and women (n 183).
Table 1 Results of multiple regression analyses of the English, Spanish, German, Hungarian, Polish, and Jamaican samples
Fig. 3. Offspring 2D:4D (expressed as deviation from offspring mean) regressed on father 2D:4D (expressed as deviation from father mean). The equation for the line of best fit is y 0.35x 0.0003.
Fig. 6. Reported number of children regressed on 2D:4D of Hungarian men and women (ethnic Hungarians (n 51) and Gypsies (n 45)).
Fig. 7. Reported numbers of children regressed on 2D:4D of Polish men (n 107) and women (n 103).

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The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success: Evidence for sexually antagonistic genes?
  • Article
  • Full-text available

June 2000

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1,758 Reads

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The ratio between the length of the 2nd and 4th digit (2D:4D) is sexually dimorphic, with mean male 2D:4D lower than mean female 2D:4D. It recently was suggested that 2D:4D is negatively correlated with prenatal testosterone and positively correlated with prenatal estrogen. It is argued that high prenatal testosterone and low estrogen (indicated by low 2D:4D) favors the male fetus and low prenatal testosterone and high estrogen (indicated by high 2D:4D) favors the female fetus. The patterns of expression of 2D:4D are interpreted in terms of sexually antagonistic genes.We report data on the following. (a) reproductive success and 2D:4D from England, Germany, Spain, Hungary (ethnic Hungarians and Gypsy subjects), Poland, and Jamaica (women only). Significant negative associations were found between 2D:4D in men and reproductive success in the English and Spanish samples and significant positive relationships between 2D:4D in women and reproductive success in the English, German, and Hungarian samples. The English sample also showed that married women had higher 2D:4D ratios than unmarried women, suggesting male choice for a correlate of high ratio in women, and that a female 2D:4D ratio greater than male 2D:4D predicted high reproductive success within couples. Comparison of 2D:4D ratios of 62 father:child pairs gave a significant positive relationship. This suggested that genes inherited from the father had some influence on the formation of the 2D:4D ratio. Waist:hip ratio in a sample of English and Jamaican women was negatively related to 2D:4D. (b) Sex and population differences in mean 2D:4D in samples from England, Germany, Spain, Hungary (including ethnic Hungarians and Gypsy subjects), Poland, Jamaica, Finland, and South Africa (a Zulu sample). Significant sex and population differences in mean 2D:4D were apparent.
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Fig. 1. Number of induced abortions per 1000 pregnancies in different age classes in Sweden 1994. Sample sizes above each bar refer to the total number of pregnancies (number of abortions plus live births).
Fig. 2. Number of induced abortions per 1000 pregnancies in relation to parity (number of previous children) in Sweden 1994. Sample sizes refer to the number of pregnancies in each category. 
Table 2 Total number of pregnancies (live births and abortions) for each parity and age group
Fig. 3. Number of induced abortions per 1000 pregnancies in relation to age group and parity in Sweden 1994. Sample sizes are found in Table 2.
Induced abortion ratio in modern Sweden falls with age, but rises again before menopause

January 2001

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81 Reads

A woman's reproductive value decreases over her reproductive life span and it is therefore predicted that the likelihood of termination of investment in a child decreases with increasing age. An eventual increase in termination ratio in the oldest age groups, as is often found in abortion statistics, could depend on older women on average having larger families rather than on age per se. We used data on abortions and births in Sweden during 1994 to investigate how abortion ratio is related to age and parity of women. We found that age-specific abortion ratio is U-shaped (i.e. that it is highest for the youngest and for the oldest age groups) in each parity class from zero to four children but that age-dependence breaks down in higher parity classes (5, >/=6). Thus, for each of the parity classes 0-4, the incidence of abortion decreases with age up to a point, but increases again as women approach menopause. This late increase in induced abortion ratio seems to depend on age per se. The data indicate that abortion ratio is an inverse function of fertility, and that investment in new reproduction gradually decreases as a woman approaches menopause. Assuming grandmothering as an important driving force in human life history evolution, such a pattern might indicate that the transition from behavioural investment in one's own children to one's grandchildren is a gradual process similar to the decline in ovarian function.

Table 1 "What are the worst arguments with your spouse in the past year, and throughout your marriage in other years?" (free list; n=21 husbands, 25 wives)
Fig. 2. Probability of wife abuse in the past year (wives' reports) by wife's age and reproductive value ( V x ). Wives' reports are used because this model 
Table 2 Spousal consistency in reporting marital arguments in the past year (n=21 couples)
Table 3 Prevalence of wife abuse during marital arguments (n=21 husbands, 25 wives)
consistency in reporting wife abuse in the past year (n=6 couples where either spouse reported wife abuse)
Infidelity, Jealousy, and Wife Abuse Among Tsimane Forager-Farmers: Testing Evolutionary Hypotheses of Marital Conflict

September 2012

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343 Reads

The role of men's jealousy over a wife's infidelity in precipitating marital conflict and wife abuse is well documented. The role of women's jealousy over a husband's infidelity has received little attention, which is puzzling given high potential costs to women of withdrawal of paternal investment. We address this gap by investigating marital conflict and wife abuse among Tsimane forager farmers of Bolivia. We test predictions derived from male jealousy and paternal disinvestment hypotheses, which consider threats and consequences of infidelity by women (male jealousy hypothesis) and men (paternal disinvestment hypothesis). The paternal disinvestment hypothesis proposes that wife abuse is employed by husbands to limit wives' mate retention effort and maintain men's opportunities to pursue extrapair sexual relationships. Interviews were conducted among husbands and wives in the same marriages using a combination of open-ended and structured items. Spouses agree that the most frequently reported type of marital argument is women's jealousy over a husband's infidelity (N=266 arguments). Roughly 60% of abusive events occurred during arguments over men's diversion of household resources (N=124 abusive events). In multivariate analyses, likelihood of wife abuse is greater in marriages where husbands have affairs, where wives are younger, and where spouses spend more time apart (N=60 husbands, 71 wives). While we find strong support for both male jealousy and paternal disinvestment hypotheses, it is men's infidelity, not women's, that precipitates most instances of marital conflict and wife abuse. We conclude that men's aggression towards their wives facilitates men's diversion of family resources for their selfish interests. Published by Elsevier Inc.

Adaptive Allocation of Attention: Effects of Sex and Sociosexuality on Visual Attention to Attractive Opposite-Sex Faces

October 2007

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163 Reads

We tested the hypothesis that, compared with sociosexually restricted individuals, those with an unrestricted approach to mating would selectively allocate visual attention to attractive opposite-sex others. We also tested for sex differences in this effect. Seventy-four participants completed the Sociosexual Orientation Inventory, and performed a computer-based task that assessed the speed with which they detected changes in attractive and unattractive male and female faces. Differences in reaction times served as indicators of selective attention. Results revealed a Sex X Sociosexuality interaction: Compared with sociosexually restricted men, unrestricted men selectively allocated attention to attractive opposite-sex others; no such effect emerged among women. This finding was specific to opposite-sex targets and did not occur in attention to same-sex others. These results contribute to a growing literature on the adaptive allocation of attention in social environments.

Detection of Propensity for Aggression Based on Facial Structure Irrespective of Face Race

March 2012

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214 Reads

The human face provides a wealth of information pertaining to the internal state and life-stage history of an individual. Facial width-to-height ratio is a size-independent sexually dimorphic trait, and estimates of aggression made by untrained adults judging own-race faces were positively associated with both facial width-to-height ratio and actual aggressive behavior. Given the significant adaptive value of accurately detecting aggressiveness based on facial appearance, we hypothesized that aggression estimates made by adults and 8-year-olds would be highly correlated with male facial width-to-height ratio even for a face category with which they had minimal experience-other-race faces. For each of the four race and age groups, estimates of aggression were positively correlated with facial width-to-height ratio irrespective of rating own-or other-race faces. Overall, the correlations between facial width-to-height ratio and ratings of aggression were stronger for adults than for children. Sensitivity to facial width-to-height ratio appears to be part of an evolved mechanism designed to detect threats in the external environment. This mechanism is likely broadly tuned and functions independently of experience.

Is friendship akin to kinship?

September 2007

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557 Reads

Although unrelated friends are genetically equivalent to strangers, several lines of reasoning suggest that close friendship may sometimes activate processes more relevant to kinship and that this may be especially true for women. We compared responses to strangers, friends, and kin in two studies designed to address distinct domains for which kinship is known to have functional significance: incest avoidance and nepotism. Study 1 examined emotional responses to imagined sexual contact with kin, friends, and strangers. Results revealed that women, compared to men, treated friends more like kin. Study 2 examined benevolent attributions to actual kin, friends, and strangers. Results revealed that women treated friends very much like kin, whereas men treated friends very much like strangers. The current findings support a domain-specific over a domain-general approach to understanding intimate relationships and raise a number of interesting questions about the modular structure of cognitive and affective processes involved in these relationships.

Leaving your wife and your brothers: When polyandrous marriages fall apart

February 2001

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672 Reads

Polyandry challenges both traditional sociocultural and evolutionary understandings of marriage. Its existence calls into question the importance of female sexual exclusivity and reproduction in marriage, two fundamental aspects of the affinal bond in most other marital forms. How does polyandrous marriage persist? When does it dissolve? This paper examines marriage patterns in the fraternally polyandrous Tibetan communities of northwestern Nepal, highlighting the most important factors associated with both the maintenance and dissolution of polyandrous unions. Behavioral ecological analyses of polyandry to date have focused on univariate analyses of the economic and fertility correlates of the two primary marital forms observed in nominally polyandrous communities, polyandry and monogamy. In this analysis, multivariate regression and survival analysis are used to extend our understanding of the factors that precipitate partitioning among the polyandrous marriages in the study. Wealth, long thought to be a major force shaping marital decisions in these communities, is modeled in both quantity and quality in this study. This represents an important departure from previous studies, as analyses show that it is not only the amount of wealth that influences the stability of polyandrous marriages, but also the type of wealth and extent to which it is diversified over economic spheres. Univariate analyses show that amount of wealth and the size of the brother/co-husband set are both associated with the probability that a polyandrous marriage will partition. Though these results support earlier findings, multivariate analyses show that controlling for other factors reduces their importance.

Fig. 1. Examples of a normal (asymmetric) image (left) and a morphed (symmetric) version (right).  
Fig. 2. (A) Plot of the average attractiveness ratings given by 30 female observers against the average fluctuating symmetry index of the normal (asymmetric) images (Pearson correlation r .31, p .17). (B) Plot of the average attractiveness ratings given by 30 male observers against the average fluctuating asymmetry index of the normal (asymmetric) images (Pearson correlation r .19, p .40).  
Is symmetry a visual cue to attractiveness in the human female body?

June 2000

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195 Reads

Small deviations from bilateral symmetry (a phenomenon called fluctuating asymmetry [FA]) are believed to arise due to an organism's inability to implement a developmental program when challenged by developmental stress. FA thus provides an index of an organism's exposure to adverse environmental effects and its ability to resist these effects. If one wishes to choose an individual with good health and fertility, FA could be used as an index of a potential partner's suitability. To explore whether this theory can be applied to human female bodies (excluding heads), we used a specially developed software package to create images with perfect symmetry. We then compared the relative attractiveness of the normal (asymmetric image) with the symmetric image. When male and female observers rated the images for attractiveness on a scale of 1 to 10, there was no significant difference in attractiveness between the symmetric and asymmetric images. However, in a two-alternative forced-choice experiment, the symmetric image was significantly more popular. The evidence suggests a role for symmetry in the perception of the attractiveness of the human female body.

Physical attractiveness and reproductive success in humans: Evidence from the late 20 century United States

September 2009

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658 Reads

Physical attractiveness has been associated with mating behavior, but its role in reproductive success of contemporary humans has received surprisingly little attention. In the Wisconsin Longitudinal Study (1244 women, 997 men born between 1937 and 1940) we examined whether attractiveness assessed from photographs taken at age ~18 predicted the number of biological children at age 53-56. In women, attractiveness predicted higher reproductive success in a nonlinear fashion, so that attractive (second highest quartile) women had 16% and very attractive (highest quartile) women 6% more children than their less attractive counterparts. In men, there was a threshold effect so that men in the lowest attractiveness quartile had 13% fewer children than others who did not differ from each other in the average number of children. These associations were partly but not completely accounted for by attractive participants' increased marriage probability. A linear regression analysis indicated relatively weak directional selection gradient for attractiveness (β=0.06 in women, β=0.07 in men). These findings indicate that physical attractiveness may be associated with reproductive success in humans living in industrialized settings.

The psychology of social chess and the evolution of attribution mechanisms: Explaining the fundamental attribution error

February 2001

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978 Reads

Theory of mind is the field devoted to understanding how organisms discern the mental states of others. Because mental states are not directly observable, they can only be inferred from observable features of the actor (such as behavior) and the situational context that the actor is in. Social psychologists, who study theory of mind processes under the rubric of attribution research, have shown that people often make a logical error of inference: The "fundamental attribution error" (FAE) is the tendency to assume that an actor's behavior and mental state correspond to a degree that is logically unwarranted by the situation. The social environment in which theory of mind capacities evolved may have influenced attributional processing in ways that could explain the error. In particular, the error could be caused by a psyche that is designed (1) to consider only those noncorresponding mental states (such as deception) that could have fitness consequences to the mind reader; (2) to bias inferences in a way that reduces the costs of erroneous inferences; or (3) to bias inferences in a way that yields reputational benefits. The existing literature is reviewed in light of these hypotheses.

Do facial averagness and symmetry signal health?

February 2001

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343 Reads

We investigated whether the attractive facial traits of averageness and symmetry signal health, examining two aspects of signalling: whether these traits are perceived as healthy, and whether they provide accurate health information. In Study 1, we used morphing techniques to alter the averageness and symmetry of individual faces. Increases in both traits increased perceived health, and perceived health correlated negatively with rated distinctiveness (a converse measure of averageness) and positively with rated symmetry of the images. In Study 2, we examined whether these traits signal real, as well as perceived, health, in a sample of individuals for whom health scores, based on detailed medical records, were available. Perceived health correlated negatively with distinctiveness and asymmetry, replicating Study 1. Facial distinctiveness ratings of 17-year-olds were associated with poor childhood health in males, and poor current and adolescent health in females, although the last association was only marginally significant. Facial asymmetry of 17-year-olds was not associated with actual health. We discuss the implications of these results for a good genes account of facial preferences.

Table 2 (a) Correlations of the center of attention variable with 13 measures
The Evolution of Prestige: Freely Conferred Deference as a Mechanism for Enhancing the Benefits of Cultural Transmission

June 2001

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1,689 Reads

This paper advances an "information goods" theory that explains prestige processes as an emergent product of psychological adaptations that evolved to improve the quality of information acquired via cultural transmission. Natural selection favored social learners who could evaluate potential models and copy the most successful among them. In order to improve the fidelity and comprehensiveness of such ranked-biased copying, social learners further evolved dispositions to sycophantically ingratiate themselves with their chosen models, so as to gain close proximity to, and prolonged interaction with, these models. Once common, these dispositions created, at the group level, distributions of deference that new entrants may adaptively exploit to decide who to begin copying. This generated a preference for models who seem generally "popular." Building on social exchange theories, we argue that a wider range of phenomena associated with prestige processes can more plausibly be explained by this simple theory than by others, and we test its predictions with data from throughout the social sciences. In addition, we distinguish carefully between dominance (force or force threat) and prestige (freely conferred deference).

Maternal trade-off in treating high-risk children

June 2001

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111 Reads

Low birthweight and the infant's health status are expected to strongly influence the child's reproductive value and, thus, the maternal decisions on the amount and timing of investment. A total of 590 Hungarian primiparous mothers giving birth in the late 1980s were recruited for the longitudinal study. Mothers of high-risk infants shortened the duration of breast-feeding and interbirth intervals, compared to those with an infant of higher survival prospects. The most powerful predictor of the length of the lactation period was the infant's weight at birth, whereas birth spacing was significantly influenced by the health status of the older child. Socioeconomic status had a positive effect on maternal care as well, but it did not change the basic pattern of diminishing maternal care as a function of the infants' low reproductive value. The combination of the above factors resulted in a cumulative effect on maternal investment of mothers with handicapped children of various degrees of risk. An attempt has been made to exclude alternative explanations and to discuss the proximate mechanisms of discriminative parental solicitude.

The Social Cognition of Social Foraging: Partner Selection by Underlying Valuation

November 2012

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54 Reads

Humans and other animals have a variety of psychological abilities tailored to the demands of asocial foraging, that is, foraging without coordination or competition with other conspecifics. Human foraging, however, also includes a unique element, the creation of resource pooling systems. In this type of social foraging, individuals contribute when they have excess resources and receive provisioning when in need. Is this behavior produced by the same psychology as asocial foraging? If so, foraging partners should be judged by the same criteria used to judge asocial patches of resources: the net energetic benefits they provide. The logic of resource pooling speaks against this. Maintaining such a system requires the ability to judge others not on their short-term returns, but on the psychological variables that guide their behavior over the long-term. We test this idea in a series of five studies using an implicit measure of categorization. Results showed that (1) others are judged by the costs they incur (a variable not relevant to asocial foraging) whereas (2) others are not judged by the benefits they provide when benefits provided are unrevealing of underlying psychological variables (despite this variable being relevant to asocial foraging). These results are suggestive of a complex psychology designed for both social and asocial foraging.

Sibling solidarity in a polygamous community in the USA

April 2000

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174 Reads

This pilot study explores the degree of solidarity felt between full and half siblings who are raised in a Mormon Fundamentalist polygamous community. The community under study is unique in that, at the level of official culture, it actively promotes full and half sibling solidarity through an ethos that strives to downplay genetic differences in favor of a harmonious family living together in one household. This community is an ideal cultural setting in which to examine the suitability of inclusive fitness theory for understanding the factors that promote family cohesion, sibling solidarity, and rivalry. Our main question becomes: is the degree of sibling solidarity a manifestation of genetic closeness or a natural byproduct of emotional closeness that arises from being raised together? We found evidence for more solidarity between full siblings than between half siblings. Our data suggest that, despite the force of religious ideals, and notwithstanding the continued close physical proximity of half siblings in the polygamous family, there is a pronounced clustering of feeling and affection in the polygamous family that is consistent with inclusive fitness theory.

Intrasexual Competition and Eating Restriction in Heterosexual and Homosexual Individuals

September 2010

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234 Reads

Restrictive eating attitudes and behaviors have been hypothesized to be related to processes of intrasexual competition. According to this perspective, within-sex competition for status serves the adaptive purpose of attracting mates. As such, status competition salience may lead to concerns of mating desirability. For heterosexual women and gay men, such concerns revolve around appearing youthful and thus, thinner. Following this logic, we examined how exposure to high-status and competitive (but not thin or highly attractive) same-sex individuals would influence body image and eating attitudes in heterosexual and in gay/lesbian individuals. Results indicated that for heterosexuals, intrasexual competition cues led to greater body image dissatisfaction and more restrictive eating attitudes for women, but not for men. In contrast, for homosexual individuals, intrasexual competition cues led to worse body image and eating attitudes for gay men, but not for lesbian women. These findings support the idea that the ultimate explanation for eating disorders is related to intrasexual competition.

Social exchange and reciprocity: Confusion or a heuristic?

December 2000

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608 Reads

We propose that a "social exchange heuristic" is as important as the cheater detection mechanism for attaining mutual cooperation in social exchange. The social exchange heuristic prompts people to perceive a mixed-motive situation, such as the Prisoner's Dilemma (PD), as an Assurance Game (AG) situation in which cooperation is a personally better choice than defection insofar as the partner is cooperating as well. We demonstrate the operation of the social exchange heuristic through a comparison of the ordinary one-shot, simultaneous PD with the one-shot, sequential PD. Participants in the current experiments, involving a total of 261 volunteers, committed a logical error in the direction of favoring mutual cooperation as the situation involved more serious consequences. This result strongly suggests the operation of a domain specific "bias" that encourages pursuit of mutual cooperation in social exchange.

Evidence of Traditional Knowledge Loss among a Contemporary Indigenous Society

July 2013

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1,215 Reads

As biological and linguistic diversity, the world's cultural diversity is on decline. However, to date there are no estimates of the rate at which the specific cultural traits of a group disappear, mainly because we lack empirical data to assess how the cultural traits of a given population change over time. Here we estimate changes in cultural traits associated to the traditional knowledge of wild plant uses among an Amazonian indigenous society. We collected data among 1151 Tsimane' Amerindians at two periods of time. Results show that between 2000 and 2009, Tsimane' adults experienced a net decrease in the report of plant uses ranging from 9% (for the female subsample) to 26% (for the subsample of people living close to towns), equivalent to a 1 to 3 % per year. Results from a Monte Carlo simulation show that the observed changes were not the result of randomness. Changes were more acute for men than for women and for informants living in villages close to market towns than for informants settled in remote villages. The Tsimane' could be abandoning their traditional knowledge as they perceive that this form of knowledge do not equip them well to deal with the new socio-economic and cultural conditions they face nowadays.

Turtle Hunting and Tombstone Opening: Public Generosity as Costly Signaling

August 2000

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502 Reads

Costly signaling theory (CST) offers an explanation of generosity and collective action that contrasts sharply with explanations based on conditional reciprocity. This makes it particularly relevant to situations involving widespread unconditional provisioning of collective goods. We provide a preliminary application of CST to ethnographic data on turtle hunting and public feasting among the Meriam of Torres Strait, Australia. Turtle hunting appears to meet the key conditions specified in CST: it is (1) an honest signal of underlying abilities such as strength, risk-taking, skill, and leadership; (2) costly in ways not subject to reciprocation; (3) an effective means of broadcasting signals, since the collective good (a feast) attracts a large audience; and (4) seems to provide benefits to signalers (turtle hunters) as well as recipients (audience). We conclude with some suggestions as to the broader implications of this research, and the costly signaling paradigm in general, for understanding collective action and generosity in human social groups.

Fig. 2. Vignette 2, Study 1 (vandalism): perceived seriousness of crime and perceived association value of the criminal as mediators of a specific criminal's past behavior. Unstandardized regression coefficients (b): all variables are coded between 0 and 1.  
To punish or repair? Evolutionary psychology and lay intuitions about modern criminal justice

November 2012

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547 Reads

We propose that intuitions about modern mass-level criminal justice emerge from evolved mechanisms designed to operate in ancestral small-scale societies. By hypothesis, individuals confronted with a crime compute two distinct psychological magnitudes: one that reflects the crime's seriousness and another that reflects the criminal's long-term value as an associate. These magnitudes are computed based on different sets of cues and are fed into motivational mechanisms regulating different aspects of sanctioning. The seriousness variable regulates how much to react (e.g., how severely we want to punish); the variable indexing the criminal's association value regulates the more fundamental decision of how to react (i.e., whether we want to punish or repair). Using experimental designs embedded in surveys, we validate this theory across several types of crime and two countries. The evidence augments past research and suggests that the human mind contains dedicated psychological mechanisms for restoring social relationships following acts of exploitation.

Does paternal uncertainty explain grandparental solicitude? A cross-cultural study in Greece and Germany

April 2000

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106 Reads

Recent research on kin investment as a reproductive strategy is based on the idea that differences in grandparental caregiving directly reflect degrees of differential grandpaternal versus grandmaternal certainty. In a cross-cultural study in Greece and Germany, 544 subjects (318 Greeks, 208 Germans, 18 of other origins) were asked for an assessment of their grandparents' (GPs') caregiving. In Germany and urban Greece (modern Western societies), the maternal GPs were rated as more intensive caregivers than the paternal GPs, but this was not the case in rural Greece, where paternal GPs provided more care. However, in all groups, grandmothers were more caring than grandfathers. Thus, contrary to previous theory and research, these two effects must be clearly distinguished, and may be explained by (1) more intense female caregiving in humans (as in other viviparous mammals) and (2) a socially engendered favoring of maternal relatives in Western industrial societies as opposed to the favoring of paternal GPs seen in the patrilateral culture of rural Greece.

The Force of Selection on the Human Life Cycle

September 2009

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87 Reads

In this paper, I present evidence for a robust and quite general force of selection on the human life cycle. The force of selection acts in remarkably invariant ways on human life histories, despite a great abundance of demographic diversity. Human life histories are highly structured, with mortality and fertility changing substantially through the life cycle. This structure necessitates the use of structured population models to understand human life history evolution. Using such structured models, I find that the vital rates to which fitness is most sensitive are pre-reproductive survival probabilities, particularly the survival of children ages 0-4. The fact that the preponderance of selection falls on transitions related to recruitment combined with the late age at first reproduction characteristic of the human life cycle, creates a fitness bottleneck out of recruitment. Because of this, antagonistic pleiotropy with any trait that detracts from the constituent transitions to recruitment is expected. I explore the predictors of variation in the force of selection on early survival. High fertility increases the selective premium placed on early survivorship, while high life expectancy at birth decreases it.

Psychopathy and Developmental Instability

April 2001

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308 Reads

Psychopaths are manipulative, impulsive, and callous individuals with long histories of antisocial behavior. Two models have guided the study of psychopathy. One suggests that psychopathy is a psychopathology, i.e., the outcome of defective or perturbed development. A second suggests that psychopathy is a life-history strategy of social defection and aggression that was reproductively viable in the environment of evolutionary adaptedness (EEA). These two models make different predictions with regard to the presence of signs of perturbations or instability in the development of psychopaths. In Study 1, we obtained data on prenatal, perinatal, and neonatal signs of developmental perturbations from the clinical files of 643 nonpsychopathic and 157 psychopathic male offenders. In Study 2, we measured fluctuating asymmetry (FA, a concurrent sign of past developmental perturbations) in 15 psychopathic male offenders, 25 nonpsychopathic male offenders, and 31 male nonoffenders. Psychopathic offenders scored lower than nonpsychopathic offenders on obstetrical problems and FA; both psychopathic and nonpsychopathic offenders scored higher than nonoffenders on FA. The five offenders from Study 2 meeting the most stringent criteria for psychopathy were similar to nonoffenders with regard to FA and had the lowest asymmetry scores among offenders. These results provide no support for psychopathological models of psychopathy and partial support for life-history strategy models.

Second to Fourth Digit Ratio and Male Ability in Sport: Implications for Sexual Selection in Humans

February 2001

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591 Reads

Fetal and adult testosterone may be important in establishing and maintaining sex-dependent abilities associated with male physical competitiveness. There is evidence that the ratio of the length of the 2nd and 4th digits (2D:4D) is a negative correlate of prenatal and adult testosterone. We use ability in sports, and particularly ability in football, as a proxy for male physical competitiveness. Compared to males with high 2D:4D ratio, men with low ratio reported higher attainment in a range of sports and had higher mental rotation scores (a measure of visual-spatial ability). Professional football players had lower 2D:4D ratios than controls. Football players in 1st team squads had lower 2D:4D than reserves or youth team players. Men who had represented their country had lower ratios than those who had not, and there was a significant (one-tailed) negative association between 2D:4D and number of international appearances after the effect of country was removed. We suggest that prenatal and adult testosterone promotes the development and maintenance of traits which are useful in sports and athletics disciplines and in male:male fighting.

The jus primae noctis as a male power display A review of historic sources with evolutionary interpretation

April 2000

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204 Reads

The jus primae noctis was, in the European late medieval context, a widespread popular belief in an ancient privilege of the lord of the manor to share the wedding bed with his peasants' brides. Symbolic gestures, reflecting this belief, were developed by the lords and used as humiliating signs of superiority over the dependent peasants in the fifteenth century, a time of diminishing status differences. Actual intercourse in the exercise of the alleged right is difficult to prove, and there is no hard evidence to suggest that it ever actually happened. However, the symbolic gestures can be best interpreted as a male power display, with a basis in the psychology of coercive social dominance, male competition, and male desire for sexual variety. Several non-European cultures have accounts of a similar custom related to a young girl's first sexual intercourse: ritual defloration by chiefs, priests, or strangers. This non-European custom differs from the jus primae noctis in its proximate details, but seems from an ultimate point of view, to be in conformity with the European evidence. In this article the origin, development, and relationship of both customs are discussed and interpreted in light of recent evolutionary studies of primate behavior and sexual psychology.

Why Not Donate Sperm? A Study of Potential Donors

October 1998

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235 Reads

In a between-group study, men were much more likely to profess willingness to donate sperm if it would be used solely for research purposes (67% yes) than if it would be used for reproduction through the process of donor insemination (24%). Offered several reasons why one might be willing to be a sperm donor, men nominated “money” and “helping out a couple in need” most often and “the chance to produce children” least often, whereas “the knowledge that it might produce children that I may never meet” was the most popular of several proposed rationales for not donating. These results are discussed in light of the specificity of male sexual psychologies and the possible costs associated with sperm donation, within the more general framework of identifying the circumstances in which men report the willingness to donate sperm.

Table 1 Descriptive statistics for independent and dependent variables. 
Table 4 Regional differences in personality scores (n = 281). Analyses are multiple linear regressions controlling for age, sex, age ⁎ sex, age ⁎ age, schooling and Spanish fluency. Baseline is near town.
between selected reproductive success measures and personality for men and women, using mixed effects regression models.
-heritability estimates (h 2 ) are result of quantitative genetic models conducted with Sequential Oligonucleotide Linkage Analysis Routines software (SOLAR). Models take as input pedigrees for individuals in our sample and include sex, age, age⁎age, age⁎sex, and village of residence as covariates.
The Evolutionary Fitness of Personality Traits in a Small-Scale Subsistence Society

January 2014

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358 Reads

The maintenance of personality variation remains an unexplained puzzle in evolutionary biology. Despite evidence among non-humans that personality variation affects fitness, few data exist to assess the personality-fitness relationship in humans. Among Tsimane forager-horticulturalists (n = 632), we test whether personality traits (assessed using a 43-item Big Five Inventory administered orally in native language) predict fertility, offspring survivorship, age of first reproduction, and other fitness correlates (extramarital affairs, conflicts, social visitation, food production, and several health measures). Among men, several personality factors associate with higher fertility, more time spent producing food and social visitation. Among women, the relationship between personality and fitness varies across regions of Tsimane territory. The only case of an intermediate personality level associated with highest fitness was found for Industriousness in men. We find that personality factors positively associated with fitness do not associate with greater health costs, although greater Extraversion and Openness may lead to more conflicts among men. Factor heritability ranges from 60% for Prosociality and Extraversion to 8% for Neuroticism. We interpret our results in light of evolutionary models that explain maintenance of personality variation, including incomplete directional selection, mutation-selection balance, condition-dependent reaction norms and fluctuating selection based on sex or spatial variability in selection pressures.

Fig. 1. Two sample items from the mental rotation test. The task is to designate which two of the four figures on the right depict the figure on the left in alternative positions.
Fig. 2. Schematic layout of the wayfinding route.
Table 2 Partial correlations, controlling for sex, between the four arrow placements
Table 3 Mean performances (SEM) on cognitive tests by sex Males Females F Test
Table 4 Correlations between wayfinding and cognitive tests by sex and for the total sample
Evolved mechanisms underlying wayfinding: Further studies on the hunter-gatherer theory of spatial sex differences

June 2000

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5,275 Reads

Based on Silverman and Eals' hunter-gatherer theory of the origin of sex-specific spatial attributes, the present research sought to identify the evolved mechanisms involved in hunting that contribute to the dimorphism. The focus of these studies was the relationship between three-dimensional mental rotations, the spatial test showing the largest and most reliable sex difference favoring males, and wayfinding in the woods. Space constancy was presumed to be the evolved mechanism fundamental to both of these abilities. Measures of wayfinding were derived by leading subjects individually on a circuitous route through a wooded area, during which they were stopped at prescribed places and required to set an arrow pointing in the direction the walk began. As well, subjects were eventually required to lead the experimenters back to the starting point by the most direct route. In support of the hypotheses, males excelled on the various measures of wayfinding, and wayfinding was significantly related across sexes to mental rotations scores but not to nonrotational spatial abilities or general intelligence.

Age-independent increases in male salivary testosterone during among Tsimane forager-farmers

September 2013

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283 Reads

Testosterone plays an important role in mediating male reproductive trade-offs in many vertebrate species, augmenting muscle and influencing behavior necessary for male-male competition and mating-effort. Among humans, testosterone may also play a key role in facilitating male provisioning of offspring as muscular and neuromuscular performance are deeply influenced by acute changes in testosterone. This study examines acute changes in salivary testosterone among 63 Tsimane men ranging in age from 16-80 (mean 38.2) years during one-hour bouts of tree-chopping while clearing horticultural plots. The Tsimane forager-horticulturalists living in the Bolivian Amazon experience high energy expenditure associated with food production, have high levels of parasites and pathogens, and display significantly lower baseline salivary testosterone than age-matched US males. Mixed-effects models controlling for BMI and time of specimen collection reveal increased salivary testosterone (p<0.001) equivalent to a 48.6% rise, after one hour of tree chopping. Age had no effect on baseline (p=0.656) or change in testosterone (p=0.530); self-reported illness did not modify testosterone change (p=0.488). A comparison of these results to the relative change in testosterone during a competitive soccer tournament in the same population reveals larger relative changes in testosterone following resource production (tree chopping), compared to competition (soccer). These findings highlight the importance of moving beyond a unidimensional focus on changes in testosterone and male-male aggression to investigate the importance of testosterone-behavior interactions across additional male fitness-related activities. Acutely increased testosterone during muscularly intensive horticultural food production may facilitate male productivity and provisioning.

Does women's greater fear of snakes and spiders originate in infancy?

November 2009

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1,268 Reads

Previous studies with adult humans and non-human animals revealed more rapid fear learning for spiders and snakes than for mushrooms and flowers. The current experiments tested whether 11-month-olds show a similar effect in learning associative pairings between facial emotions and fear-relevant and fear-irrelevant stimuli. Consistent with the greater incidence of snake and spider phobias in women, results show that female but not male infants learn rapidly to associate negative facial emotions with fear-relevant stimuli. No difference was found between the sexes for fear-irrelevant stimuli. The results are discussed in relation to fear learning, phobias, and a specialized evolved fear mechanism in humans.

Food-Sharing Networks in Lamalera, Indonesia: Status, Sharing, and Signaling

July 2012

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90 Reads

Costly signaling has been proposed as a possible mechanism to explain food sharing in foraging populations. This sharing-as-signaling hypothesis predicts an association between sharing and status. Using exponential random graph modeling (ERGM), this prediction is tested on a social network of between-household food-sharing relationships in the fishing and sea-hunting village of Lamalera, Indonesia. Previous analyses (Nolin 2010) have shown that most sharing in Lamalera is consistent with reciprocal altruism. The question addressed here is whether any additional variation may be explained as sharing-as-signaling by high-status households. The results show that high-status households both give and receive more than other households, a pattern more consistent with reciprocal altruism than costly signaling. However, once the propensity to reciprocate and household productivity are controlled, households of men holding leadership positions show greater odds of unreciprocated giving when compared to households of non-leaders. This pattern of excessive giving by leaders is consistent with the sharing-as-signaling hypothesis. Wealthy households show the opposite pattern, giving less and receiving more than other households. These households may reciprocate in a currency other than food or their wealth may attract favor-seeking behavior from others. Overall, status covariates explain little variation in the sharing network as a whole, and much of the sharing observed by high-status households is best explained by the same factors that explain sharing by other households. This pattern suggests that multiple mechanisms may operate simultaneously to promote sharing in Lamalera and that signaling may motivate some sharing by some individuals even within sharing regimes primarily maintained by other mechanisms.

Early separation and sibling incest: A test of the revised Westermarck theory

June 2000

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591 Reads

This was a follow-up study of earlier reported findings by the present investigators suggesting, albeit equivocally, that separation during early childhood inhibited later sexual acts of a potentially procreative nature between siblings but did not deter other sexual activity. The present study surveyed 170 subjects, mostly in the Toronto area, by telephone and mail. Respondents reporting potentially procreative, postchildhood sexual acts (attempted or completed genital intercourse) with siblings were compared with those reporting sexual relationships excluding procreative acts, and a third sample reporting no postchildhood sibling sexual behavior. Consonant with expectations from the earlier study, prolonged separation during early childhood was associated with procreative postchildhood sexual activity but not with other postchildhood sexual activity. Contrary to predictions, however, both sexual activity groups reported significantly more nudity and physical contact with siblings during childhood than subjects reporting no sexual activity. The findings are discussed in terms of a revised version of the Westermarck hypothesis, which is consistent with a domain-specific approach to evolved incest avoidance mechanisms.

Table 2 Estimated coefficients and test for proportionality of the standard Cox proportional hazards model for IBIs in final data selection Model 1: Cox Proportional Hazards n=1124; R 2 = 0.129; Wald test=168.6 on 10 df (pb.001)
Table 3 Multiplicative and additive terms in the Cox-Aalen model
Table 4 Tests for constant effects and test for non-significant effects of the Aalen additive hazard model Model III: Aalen additive hazards 
The presence of a paternal grandmother lengthens interbirth interval following the birth of a granddaughter in Krummhörn (18th and 19th centuries)

September 2011

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259 Reads

Because only daughters inherit the paternal X-chromosome, an asymmetry in adaptive investment decisions has been suggested for certain patrilineal kin. Namely, paternal grandmothers (PGMs) may favor a granddaughter over a grandson, because (within the limits of paternity uncertainty) the former definitely carries one of their X-chromosomes, while the latter definitely does not. Here, we test the hypothesis that the PGMs' sex-specific favoritism influences reproductive scheduling. Using family-reconstitution data, we analyzed interbirth intervals (IBIs) in the historical population from the Krummhörn (Ostfriesland, Germany). In order to account for potentially timevarying effects on IBIs we applied (and combined) both the additive hazards regression of Aalen and the Cox proportional hazards model. We found that the presence of the PGM but not that of the maternal grandmother (MGM), correlates with the IBI following the birth of a grandchild as a function of the grandchild's sex. Specifically, in the presence of a PGM, the IBIs following the birth of a granddaughter are longer than in her absence. However, contrary to predictions from theoretical life history framework, model estimates for a PGM's effect on a mother's IBI did not significantly vary over time This study supports the hypothesis that PGM behavior differs according to her grandchild's sex. Further research should now explore the biological mechanism underlying this phenomenon.

Provisional evidence that the arginine vasopressin 1a receptor gene is associated with musical memory

September 2007

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165 Reads

In a preliminary pilot study, 82 university students were administered an extensive battery of musical and phonological memory tasks; their scores were examined for an association with promoter repeats in the arginine vasopressin 1a receptor and serotonin transporter genes. We previously showed that these genes were associated with another music-related phenotype, creative dance. Highly significant Gene×Gene epistatic interactions were observed between promoter region polymorphisms and musical as well as phonological memory using family-based and population-based tests. Given the prominent role of vasopressin in social behavior, the preliminary association found in our study between musical memory and vasopressin could serve to support evolutionary accounts postulating a social adaptive role in music, such as mother–infant communication, sexual selection, group cohesion, and even early protolanguage.

Table 1 Number of children desired by sample and by sex
Table 1 (continued ) Men Women
Fig. 2. Zero-order correlation between the number of children desired and preferred spousal age difference for women, r = 0.14, N = 37, p > 0.10. 
Number of children desired and preferred spousal age difference: Context-specific mate preference patterns across 37 cultures

October 2000

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221 Reads

Men universally express a preference for youth in a long-term mate, presumably an evolved desire originating from the close and recurrent statistical association between a woman's age and her residual reproductive value (future reproductive potential). As a consequence, we hypothesized a positive correlation for men (but not women) between the number of children desired and preferred spousal age difference — a context-specific shift in mate preference depending on whether the man is pursuing a “quality” or “quantity” reproductive strategy. We tested this hypothesis with data provided by 9809 participants from 37 cultures located in six continents and five islands. Between-culture analyses confirmed the hypothesis, even after statistically controlling for preferred age at first marriage, current age of participant, and current marital status. Discussion notes limitations and focuses on other possible context-sensitive shifts in mate preferences.

Male Facial Attractiveness: Evidence for Hormone-Mediated Adaptive Design

July 2001

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6,360 Reads

Experimenters examining male facial attractiveness have concluded that the attractive male face is (1) an average male face, (2) a masculinized male face, or (3) a feminized male face. Others have proposed that symmetry, hormone markers, and the menstrual phase of the observer are important variables that influence male attractiveness. This study was designed to resolve these issues by examining the facial preferences of 42 female volunteers at two different phases of their menstrual cycle. Preferences were measured using a 40-s QuickTime movie (1200 frames) that was designed to systematically modify a facial image from an extreme male to an extreme female configuration. The results indicate that females exhibit (1) a preference for a male face on the masculine side of average, (2) a shift toward a more masculine male face preference during the high-risk phase of their menstrual cycle, and (3) no shift in other facial preferences. An examination of individual differences revealed that women who scored low on a “masculinity” test (1) showed a larger menstrual shift, (2) had lower self-esteem, and (3) differed in their choice of male faces for dominance and short-term mates. The results are interpreted as support for a hormonal theory of facial attractiveness whereby perceived beauty depends on an interaction between displayed hormone markers and the hormonal state of the viewer.

Sociobiology and the Naming of Adopted and Natural Children

January 1991

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120 Reads

A study of ninety-six adoptive families and one hundred and four nonadoptive families revealed that adopted children were significantly more likely to be named after a parent or relative than natural children were. When natural children were namesakes of relatives, they were more likely to be named after a patrilineal than a matrilineal relative, and boys were more likely to be namesaked than girls; neither was true of adopted children. The results are discussed in light of evolutionary theory, with naming seen as a form of parental input inversely related to the certainty of kinship between parent and child.

Anticipated verbal feedback induces altruistic behavior

July 2007

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122 Reads

A distinctive feature of humans compared to other species is the high rate of cooperation with non-kin. One explanation is that humans are motivated by concerns for social esteem. In this paper we experimentally investigate the impact of anticipated verbal feedback on altruistic behavior. We study pairwise interactions in which one subject, the “divider”, decides how to split a sum of money between herself and a recipient. Thereafter, the recipient can send an unrestricted anonymous message to the divider. The subjects’ relationship is anonymous and one-shot to rule out any reputation effects. Compared to a control treatment without feedback messages, donations increase substantially when recipients can communicate. With verbal feedback, the fraction of zero donations decreases from about 40% to about 20%, and there is a corresponding increase in the fraction of equal splits from about 30% to about 50%. Recipients who receive no money almost always express disapproval of the divider, sometimes strongly and in foul language. Following an equal split, almost all recipients praise the divider. The results suggest that anticipated verbal rewards and punishments play a role in promoting altruistic behavior among humans.

Fig. 1. Consistency of the findings across the three groups of men ( n =5 per group) is illustrated by the three OLS regression slopes of men's WAIS scores and ratings of appeal as a long-term mate. Each group of men was rated by 68 women. Note that the size of a point reflects the number of observations. 
Fig. 2. Comparison of perceived (women's ratings) and objectively measured verbal (WAIS scores) intelligence as predictors of a man's appeal as a long- and short-term mate (LTM, STM respectively). The y -axis indicates the amount of variance each predictor accounts for in a man's appeal as a mate. 
Intelligence and mate choice: Intelligent men are always appealing

January 2009

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6,775 Reads

What role does a man's intelligence play in women's mate preferences? Selecting a more intelligent mate often provides women with better access to resources and parental investment for offspring. But this preference may also provide indirect genetic benefits in the form of having offspring who are in better physical condition, regardless of parental provisioning. Intelligence then may serve as both a cue of a mate's provisioning abilities and his overall heritable phenotypic quality. In the current study, we examined the role of a man's intelligence in women's long- and short-term mate preferences. We used a rigorous psychometric measure (men's WAIS scores) to assess intelligence (the first study to our knowledge), in addition to women's subjective ratings to predict mate appeal. We also examined the related trait of creativity, using women's ratings as a first step, to assess whether creativity could predict mate appeal, above and beyond intelligence. Finally, we examined whether preferences for intelligent and creative short-term mates shifted according to a woman's conception risk. Multilevel modeling was used to identify predictors of mate appeal. Study participants (204 women) assessed the long- and short-term mate appeal of videos of 15 men with known measures of intelligence performing verbal and physical tasks. Findings indicate that both intelligence and creativity independently predicted mate appeal across mating contexts, but no conception-risk effects were detected. We discuss implications of these findings for the role of intelligence and creativity in women's mate choices.

Commitment bias: mistaken partner selection or ancient wisdom?

January 2010

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137 Reads

Evidence across the social and behavioral sciences points to psychological mechanisms that facilitate the formation and maintenance of interpersonal commitment. In addition, evolutionary simulation studies suggest that a tendency for increased, seemingly irrational commitment is an important trait of successful exchange strategies. However, empirical research that tests corresponding psychological mechanisms is still largely lacking. Here an experimental test is proposed for one such mechanism, termed the commitment bias, which is hypothesized to increase people's commitment to existing partners beyond instrumental reasons. To exclude one alternative explanation, the commitment bias is distinguished from uncertainty reduction. Results from a cross-culturally replicated laboratory experiment (USA, China, and the Netherlands) provide support for the argument but also point to the importance of culture as an alternative or mediating factor.

Student athletes claim to have more sexual partners than other students

January 2004

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2,058 Reads

Physical competition is widespread in human societies. Because performance in competitive sports can signal phenotypic quality and fighting ability, high level performance, especially on the part of men, is likely to be attractive to the opposite sex. We investigated the relationship between involvement in competitive sport and self-reported numbers of sexual partners. Both male and female students who compete in sports reported significantly higher numbers of partners than other students, and within the athletes, higher levels of performance predicted more partners. Among men, body mass index (BMI) and educational level also had significant effects. We discuss possible implications for the evolution of competitive sport, ritual fighting behavior, and the persistence of left-handedness.

Facial attractiveness predicts longevity

September 2003

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1,444 Reads

In the current investigation, 20 undergraduate students rated 50 high school yearbook photographs from the 1920s on two measures, attractiveness and perceived health. These measures were then correlated with each other and with the photographed subjects' longevity. Facial attractiveness was found to predict future longevity, but perceived health did not. The results are discussed in terms of sexual selection theory.

Differences in Time Use for Mating and Nepotistic Effort as a Function of Male Attractiveness in Rural Belize

January 1999

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13 Reads

This paper explores whether physical attractiveness was a determinant of reproductive strategy in a sample of men living in rural Belize. A theoretical argument is presented to explain why differences in male physical attractiveness should lead to differences in strategy as evidenced by time-use, and why these differences should be especially apparent in nonindustrialized societies. Retrospective data were collected on men’s time use during their last day off from work. The results were that more facially attractive men spent more time in mating effort and less time in nepotistic effort than less facially attractive men. Another component of physical attractiveness, fluctuating asymmetry, was not successful in predicting differences in time use. The results suggest that facially attractive men spend their leisure time seeking sexual access rather than spending it with kin, because their potential fitness returns are higher for this activity, whereas less attractive men receive higher returns to time spent with kin. This could be due directly to fitness returns to nepotism received by less attractive men, or because family involvement displays potential parental investment skills that are attractive to women. This may help build a reputation for reliability; in other words, time spent in nepotistic effort could be an alternative mating tactic that appeals to women’s desire for a responsible paternally investing mate.

The effect of nonphysical traits on the perception of physical attractiveness: Three naturalistic studies

March 2004

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2,465 Reads

From an evolutionary perspective, beauty is regarded as an assessment of fitness value. The fitness value of a social partner can be influenced by both physical and nonphysical traits. It follows that the perceived beauty of a social partner can be influenced by nonphysical traits such as liking, respect, familiarity, and contribution to shared goals in addition to physical traits such as youth, waist-to-hip ratio, and bilateral symmetry. We present three studies involving the evaluation of known social partners showing that judgments of physical attractiveness are strongly influenced by nonphysical factors. Females are more strongly influenced by nonphysical factors than males and there are large individual differences within each sex. In general, research on physical attractiveness based on the evaluation of purely physical traits of strangers might miss some of the most important factors influencing the perception of physical attractiveness among known associates.

On genetic variation in menarche and age at first sexual intercourse: A critique of the Belsky–Draper hypothesis

September 2002

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40 Reads

The association of age of menarche, nonvirginity status, and age of first sexual intercourse was investigated in teenage, female twins (mean age, 17 years) in the National Longitudinal Study of Adolescent Health (Add Health). The sample sizes were all relatively small, so complex biometric models were not fit to twin covariance matrices; rather, rough estimators of genetic effects were used. Accordingly, all three characteristics were influenced by genetic variation, with higher heritabilities on nonvirginity status and age of menarche than on age of first sex. In MZ twins, the phenotypic (i.e., on individuals) correlation between menarcheal age and age of first sexual intercourse was .27. The association of menarcheal age in identical (MZ) Twin 1 and sexual onset age in MZ Twin 2, and vice versa, was .25. The genetic correlation between them, rg, was roughly estimated to be .72. These findings weaken a conditional adaptation interpretation of this association as proposed by Belsky and Draper, suggesting instead that heritable individual differences may give rise to this association.

You can judge a book by its cover: Evidence that cheaters may look different from cooperators

July 2003

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525 Reads

Cosmides and Tooby argue that humans possess a domain-specific cheater detection module, which allows them to keep track of who has honored and who has violated social contracts. Consistent with this logic, others demonstrate that humans better recognize faces of known cheaters than those of known cooperators. We show, in Experiments 1–3, that humans better recognize faces of cheaters than those of cooperators when they do not know who are cheaters and cooperators. Experiment 4 demonstrates, however, that humans think they recognize cheaters' faces even when they have not seen them before. The results of these experiments suggest that cheaters might look different from cooperators, possibly due to beliefs and personality traits that make them less ideal exchange partners, and the human mind might be capable of picking up on subtle visual cues that cheaters' faces give off.

Conflict in childhood and reproductive development

March 1997

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122 Reads

Belsky, Steinberg, and Draper (1991) predicted that early childhood stress or conflict in the family environment would be associated with late childhood behavioral symptoms, early puberty, and early, less discriminate reproductive behavior. This report includes a review of previous research and the results from a cross-sectional self-report survey of childhood family life and adolescent development in 380 secondary school students aged 16 to 19 from southern Italy. In women, more stress in quality of family life throughout childhood (birth to age 11), more parental marital unhappiness throughout childhood (birth to age 11), more conflict with mother throughout childhood (birth to age 11), more rejection from father throughout childhood (birth to age 11), less emotional closeness to mother throughout childhood (birth to age 11), and more behavioral independence from mother or father in late childhood (age 8 to 11) were associated with earlier menarche. Earlier menarche was associated with earlier age at dating men and older age of first sexual intercourse partner relative to own age at first intercourse. In men, more parental marital conflict in early childhood (birth to age 7), less emotional closeness to father throughout childhood (birth to age 11), and more aggressiveness, unruliness, and externalizing symptoms (aggressiveness/unruliness) in late childhood (age 8 to 11) were associated with earlier spermarche. Earlier spermarche was associated with earlier age at dating women, more girlfriends, more likelihood of having had intercourse, and more intercourse partners. These results are considered together with alternative interpretations.

Identifying personality from the static, nonexpressive face in humans and chimpanzees: Evidence of a shared system for signaling personality

May 2011

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186 Reads

Many aspects of personality are honestly signaled on the human face, as shown by accurate identification of personality traits from static images of unknown faces with neutral expressions. Here, we examined the evolutionary history of this signal system. In four studies, we found that untrained human observers reliably discriminated characteristics related to extraversion solely from nonexpressive facial images of chimpanzees (Pan troglodytes). In chimpanzees, as in humans, there is therefore information in the static, nonexpressive face that signals aspects of an individual's personality. We suggest that this performance is best explained by shared personality structure and signaling in the two species.

The influence of postreliance detection on the deceptive efficacy of dishonest signals of intent: Understanding facial clues to deceit as the outcome of signaling tradeoffs

March 2002

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226 Reads

Evolutionary communication theory posits that signalers and receivers are in a coevolutionary arms race. Receivers attempt to predict the behavior of signalers, and signalers attempt to manipulate the behavior of receivers (often through the use of dishonest signals of intent). This has led to the perception that deceitful signalers prefer perfectly deceptive signals. However, it is often easy for receivers to determine that a signal of intent was dishonest after relying on it to their detriment. Even the best deceivers may then acquire a reputation for being dishonest. For instance, in Prisoner's Dilemma (PD)-like social situations, predictable defectors make better social partners than unpredictable defectors. When opportunities to engage in social interaction depend on one's reputation for predictability, those who are better at concealing their defecting intentions may suffer the most from the reputations they acquire. Deceivers then face a tradeoff between the short-term benefits of successful deception and the long-term costs to their reputations. A mathematical model is developed and it is shown that the tradeoff often favors signalers who produce imperfectly deceptive signals over perfectly honest or perfectly deceptive ones. Implications for understanding human facial expressions and sociopathy are drawn.

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