A couple being watched by a lone man and a lone woman

Mate-choice copying is a sex-general heuristic among humans

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– by Robert Bowers

There is a large ground bird from the American plains whose mating system could not be more convenient for studying sexual selection. Over a conveniently short mating season, male sage grouse gather in a conveniently compact space, within a few miles of each other, and there they all perform their best mating displays. The hens of this species browse the performances at their leisure and choose a mate. This event attracts not only competing cocks and broody hens, but also biologists keen on showing how the striking sexual dimorphisms expressed in this species relate to female choice and intersexual sexual selection. Unsurprisingly, mating variance among male sage grouse is large, and optimism was high that with enough clipboards, the exaggerated waddles and dances and girth of these males would all have a clear explanation. But the mating variance was too great. Even with all the elaborate male characters of these birds, there were still insufficient bases to explain the extent that some hens chose the most successful males. In apparent exasperation, there is a remark in the middle of a 1960’s doctoral thesis that the females might be copying each others’ choices.

The subsequent area within sexual selection research, initially termed “female copying”, grew in several ways. Theoreticians showed its relation to reproductive variance and sexual selection. Experimentalists demonstrated it empirically in laboratory demonstrations. And phylogenetically, it grew in the breadth of species that were shown to be affected. However, even with all these advances, it remained within the theoretical context of female choice, and continued to be studied as if it were an exclusively female phenomenon. Even among humans, a species in which both sexes choose, a large majority of “mate-choice copying” (what the phenomenon is now called) studies focused exclusively on females choosing males.

Among humans, however, the earliest documented recognition of mate-choice copying (in a dating advice column from 1928, predating the sage grouse observation by decades), was actually about men copying the choices of other men. Indeed, studies that have included both men and women have most often found that both sexes are affected by the choices of others. Despite this, it remains common to find mate-choice copying presented as a specifically female phenomenon. Typically, this sentiment is accompanied with reference to Parental Investment Theory, the popular view that the sex that suffers greater costs in reproduction will have greater use for any means of choosing better mates. The received view is that women should engage in social mate choice heuristics more than men.

But mate choice is complicated, and harnessing information about mates from attending to others’ choices is no less so. When one considers mate-choice copying more closely, several features of social mate-choice information (as well as of human parental care) muddy the theoretical waters that motivate looking for a sex difference. Mate-choice information is readily accessible, relevant to both sexes, indicative of characters not visible on the surface, and fallible. Copying others’ choices heightens competition. Paternal care among humans is long and extensive. All of these features push expectations in various directions about what kind of sex difference should or should not express in how people attend to others’ choices when choosing a mate.

Not long ago, we decided to address this question directly. One problem in assessing sex difference claims from previous data was that male and female participants are typically presented with different stimuli. Male mate choice is assessed with images or videos of women, and female mate choice is assessed with images or videos of men. And one point that has been clear from the earliest human mate-choice copying studies is that the specific stimuli matter; the strength of the effect depends heavily on such features as the relative attractiveness of the stimuli employed.

To address whether the sexes differ in the extent that they rely on social mate-choice heuristics, we removed all sex-specific stimuli so that we could present the very same stimuli to male and female participants. Profiles furthermore included only accurate information about real daters making sincere choices in the context of a speed-dating event.

Our results did not support the kind of sex difference expected by the received view. We did find evidence of human mate-choice copying, but both male and female participants changed their ratings in the direction of others’ choices, and to comparable extents.

This matters because mate-choice copying is both a cause and a consequence of sexual selection. As a consequence, it being sex-general implies that there is substantial pressure on both sexes to be selective. As a cause, the sex-generality of mate-choice copying is expected to have the effect of increasing reproductive variance, and so of intensifying selection on sex-specific characters in both sexes.

These results add human mate-choice copying to the growing list of phenomena that do not abide expectations of the still dominant set of views related to Parental Investment Theory, Sexual Strategies Theory, and Bateman’s Principle. These views satisfy aesthetically but are increasingly found to falter empirically, not necessarily because they are wrong, but because mate choice is complicated, and because human mate choice and parenting are not so one-sided as in the comparatively extreme mating systems amid which these ideas were developed. The good news is that with attention to the specific features of the structure of the problem mate seekers face, there are fine grounds for refining understanding of sexual selection in the context of how copying others’ choices allows mate seekers to harness social information about mate quality.

Read the original article: Bowers, R.I., Pinar, V., Sariyildiz, S.S., & Parlak, D. (2025) Is mate-choice copying a female phenomenon? Evolution & Human Behavior, 46(1), 106653.

Do long-term relationships help explain human sexual dimorphism in upper-body strength?

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– by Caroline B. Smith & Edward H. Hagen

Humans are sexually dimorphic, meaning there tend to be differences between men and women on traits like height and weight. In our study we focused on the sexual dimorphism in muscle mass: men have more muscle mass than women. Why? The standard story is that higher male strength leads to more sexual partners, leading to sexual selection on male upper body strength. Previous research found that higher male muscle mass was positively associated with more sexual partners and lower age at first sex, supporting the sexual selection hypothesis. That study also found that higher strength comes at a cost, specifically decreased immune function and higher energy needs.

However, past research did not investigate women. The sexual selection hypothesis predicts no association between women’s strength and mating success, because partner numbers are not thought to be related to women’s fitness in the same way they are for men’s. We tested the prediction from the sexual selection hypothesis that the association of upper-body strength, proxied by grip strength, with mating success is significantly positive for males and significantly less so for females. We also tested for tradeoffs of upper-body strength with immune function and nutritional intake.

In our study, we used two large independent representative samples of US adults, collected by the National Health and Nutrition Examination Survey (NHANES) in 2011-2012 (N=2853) and 2013-2014 (N=3244). In studies of human behavior, it is important that hypotheses are clearly specified prior to statistical analysis to prevent “fishing” for statistically significant results. To do this, we first conducted an exploratory pilot study using the data from 2011-2012. We used these data to refine our models and predictions, and then tested them in data from 2013-2014 which we had not observed.

We used four proxies of mating success as our model outcome variables: Number of lifetime sexual partners, number of sexual partners in last year, age at first sex, and being currently partnered (True/False). We used grip strength as our main explanatory variable as a proxy for upper-body strength. We also controlled for five groups of possible compounds including anthropometrics (e.g. height and weight), physical activity (e.g., vigorous recreation), socioeconomic factors (e.g., education, ethnicity), hormones (i.e., testosterone), and health factors (e.g., chronic illness, depression). We tested for tradeoffs of strength with immune function (white blood cell count) and energy and protein intake.

We found mixed evidence for the sexual selection explanation overall. In models of current partnered status, we found a significant interaction of strength and sex such that stronger men, but not stronger women, were more likely to be partnered, consistent with sexual selection. But no such significant interaction emerged in models of lifetime sexual partners—stronger men and stronger women both have more lifetime sexual partners contrary to the sexual selection hypothesis. We found no consistent association between strength and either age at first sex or number of past-year sexual partners.

We found positive associations of strength with protein and energy intake for both sexes. We found no significant association of strength with immune function, perhaps because we controlled for height, which was negatively associated, suggesting a tradeoff.

The stronger association of grip strength with partnered status for men than for women is consistent with a role for sexual selection in sexually dimorphic upper body strength, perhaps via female choice for long-term relationships with more productive mates.

It is not clear why women’s strength would be positively associated with sexual partner numbers. It could be that there was selection for more formidable men to prefer more partner variety, and stronger women have a similar preference as a byproduct of selection on men. It could be that there is assortative mating on strength: if stronger men are motivated to switch partners more frequently, their (stronger) mates would also likely have more mates. It might be that stronger women require less male investment, or can take more physical risks, and so instead benefit from greater partner numbers through, e.g. genetic bet hedging, forging relationships with multiple males, ability to conduct a more extensive search for a high-quality long-term mate, or through avoiding or leaving costly long-term partnerships. It might be the case that there are some sex-specific confounds that we failed to control for, or that our models are otherwise misspecified. There also might be reverse causation, e.g., women who are interested in greater partner variety keep in better shape. The sexual selection hypothesis also might be wrong. Or these results might simply be noise.

There were some limitations to our study which are important to note. The data we analyzed for our study are cross-sectional data, meaning we cannot infer causal relationships between variables. Participants self-reported their sexual behavior, including their numbers of sexual partners, and might have either accidentally or purposefully reported their behavior inaccurately. This study used data from the US, where widespread access to contraceptives uncouples reproductive success from mating behavior for both men and women. In addition, we removed 68 participants with 100 or more lifetime sexual partners on the grounds that these might involve evolutionarily novel patterns of behavior that would not conform to predictions of the sexual selection hypothesis.

In conclusion, the sexual selection hypothesis was clearly supported for the positive association of male strength with the probability of being partnered, but not for lifetime number of sexual partners which had a positive relationship with strength for both men and women. We found no consistent association between grip strength and age at first sex or number of past-year sexual partners.

We argue that in human evolution, men likely increased their fitness, not by obtaining additional sexual partners, but mostly by investing in long-term relationships and offspring, and reaping the efficiency benefits of a sexual division of labor. Our results suggest that, for human males, high upper body strength helps obtain a long-term partner through female choice, perhaps due its advantages in provisioning resources through big game hunting and providing protection, and in male-male physical competition.

Read the full article here: Smith, C.B., & Hagen, E.H. (2025). Strength, mating success, and immune and nutritional costs in a population sample of US women and men: A registered report. Evolution & Human Behavior, 46(1), 106647.

A woman having an ultrasound

Looks just like daddy: Investigating perceptions of fetal resemblance

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– By Carlota Batres

When it comes to children, women can be certain that a child is theirs, but men cannot. In other words, there is always some chance that another man fertilized the woman’s egg. The estimate of children who have genetic fathers different from their alleged fathers is between 1%-3%.

One of the sources of information that men use to determine the likelihood of being the genetic father is perceptions of the child’s physical similarity. Research has found that fathers who perceive that their children resemble them invest more in those children, for example by spending more time with them and by having more supportive/engaged parenting.

In our study, we aimed to examine whether mothers having an ultrasound would claim that the fetus looks more like the father. Even with such limited visual information, we predicted that mothers would be more likely to say that the fetus resembled the father in an effort to assure paternity.

In addition to perceptions of the child’s resemblance, men also use information about their partner’s faithfulness and trustworthiness to determine how likely they are to be the biological father. For instance, one study found that men’s perceptions of their partner’s fidelity predicted their parental investment. Marriage is one way in which couples pledged faithfulness to each other. Indeed, while infidelity takes place in married couples, the incidence of cheating is much lower than it is in unmarried couples. Therefore, for our study, we also aimed to examine differences between parents who were married and parents who were not. We predicted that among unmarried parents, mothers would be even more likely to say that the fetus resembled the father.

Participants were recruited at an obstetrics and gynecology clinic, mostly during their routine 20-week pregnancy ultrasound. If they decided to participate, they were given a questionnaire and then called in for their ultrasound. Once the fetus was visible in the 2D imaging, if the biological mother and the biological father were in the ultrasound room (as per the participants’ questionnaire responses), the ultrasound technician would ask them “Who do you each think the baby looks like?” and would record their answers.

We found that fathers were just as likely to say the fetus resembled themselves (49%) as they were to say that the fetus resembled the mother (51%). However, mothers were significantly more likely to say that the fetus resembled the father than to say that the fetus resembled themselves. In fact, 26% said that the fetus resembled themselves and 74% said that the fetus resembled the father. Moreover, mothers who were not married were even more likely to say that the fetus resembled the father than married mothers. More specifically, 93% of unmarried women said that the fetus resembled the father.

Evidence from our study supports the hypothesis that mothers should be more likely to say that the fetus resembles the father. If the fetuses actually did resemble the fathers, the fathers should also be more likely to say that the fetuses look like them. However, we did not find this to be the case. Rather, we only saw this effect with mothers, suggesting that the unconscious bias in mothers’ observations about similarity to the fathers does not reflect actual resemblance. This is supported by other research which has found that when unrelated participants were asked to match photographs of parents to newborns, they were more accurate in matching the mothers than the fathers.

In conclusion, our study provides evidence that even with extremely limited visual information, mothers have a bias toward claiming paternal similarity. This bias is likely an adaptation that helps the mothers secure investment in their unborn children since fathers who believe that their children resemble them invest more in those children.

Read the original article: Batres, C., Mullen, A., Krofl, S., & Trainor, L. (2025). Investigating perceptions of fetal resemblance. Evolution and Human Behavior, 46(2), 106670.

The Balancing Act of Life: How Having Children Historically Affected Lifespan in Rural Spain

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– By Ángel Luis González Esteban & Francisco Marco-Gracia

For centuries, scientists have pondered a fundamental question: is there a biological trade-off between reproduction and longevity? In simpler terms, does having more children mean a shorter lifespan for mothers? This is a core idea in evolutionary theory, suggesting that bodies have limited resources, and investing heavily in having offspring might leave less for maintaining the mother’s health and extending her life.

While this idea has been supported by studies on animals, the picture in humans has been less clear, with many studies yielding conflicting results. This is often because it’s challenging to disentangle evolutionary factors from the complex mix of genetics, environment, and societal influences that shape human lives.

To shed more light on this, we delved into the rich historical records of rural Spain, specifically parish registers from 17 villages, spanning a remarkable five centuries, from 1536 to 1965. This allowed us to examine the lives of over 5,000 women, meticulously tracking their fertility and longevity, and even accounting for the experiences of their parents. This long-term perspective and the ability to consider intergenerational patterns are what make our study particularly valuable.

What Did the Data Reveal?

Our analysis revealed some fascinating insights into the historical relationship between motherhood and lifespan in this rural Spanish population:

– More Children, Shorter Lives: Overall, we found a significant negative impact of the number of children a woman had on her longevity. On average, each additional child was associated with a reduction in the mother’s lifespan. Having a large family (8 or more children, which was above average for any period in our study) was also clearly linked to reduced longevity. This suggests that the biological costs of pregnancy, childbirth, and potentially the demands of raising a large family did take a toll on women’s bodies over the long term.

– The Impact of Surviving Children: Interestingly, the number of children who survived past the age of five had an even stronger negative association with the mother’s lifespan. This highlights that the physical demands of childcare and breastfeeding likely played a significant role in the wear and tear on mothers, beyond just the act of pregnancy itself. Even considering that mothers with more children might have received more support, the negative effects on their longevity seemed to outweigh any potential benefits.

– The Timing of Motherhood Mattered: Having a first child at a very young age (before 25, and especially before 20) was associated with shorter lifespans. This could be because early motherhood, especially in a time of limited resources, placed a greater strain on still-developing bodies and was often linked to longer reproductive periods. Conversely, having the last child after the age of 40 was linked to increased longevity. This might seem counterintuitive, but it likely reflects that women who were healthy enough to conceive and carry a pregnancy to term at a later age were already predisposed to longer lives.

– The Pace of Reproduction: We also looked at birth intervals and the total length of a woman’s reproductive life. While the average time between births didn’t show a strong link to lifespan on its own, women with a long reproductive span (over 15 years) combined with consistently short birth intervals (what we termed “natural fertility”) lived significantly shorter lives, on average about 1.5 years less than others. This suggests that insufficient recovery time between pregnancies could accelerate aging.

– The Shift with Modernity: When we compared the pre-industrial period to the time after the onset of fertility control in the late 19th century, some patterns changed. For example, the number of surviving children became a more significant factor in the later period, likely due to lower child mortality rates meaning mothers spent more years actively raising their offspring. Notably, the negative impact of having children very early or very late seemed to lessen after the fertility transition, possibly due to changes in family planning and improved healthcare. The reduction in the overall reproductive period in more recent times appears to have been a key factor in improving women’s longevity.

– Nature vs. Nurture: We also considered whether these patterns could simply be due to inherited traits – perhaps long-lived women came from long-lived families and had fewer children due to genetic or culturally transmitted behaviors. However, even after accounting for the longevity and fertility of the women’s parents, our core findings remained consistent. This strengthens the idea that there was indeed a trade-off between fertility and longevity driven by the biological costs of reproduction.

Implications for Today

While our study focuses on historical populations with vastly different living conditions and healthcare access than today, it provides a valuable long-term perspective on the fundamental biological trade-offs that women have faced. The dramatic increase in women’s lifespan in modern times is undoubtedly linked to factors like improved nutrition, healthcare, and, crucially, the ability to control fertility, allowing for greater biological recovery between pregnancies and a shorter overall reproductive window. Our research underscores the profound impact that reproductive decisions and environmental context have on women’s health and longevity across generations.

We hope this glimpse into our research has been engaging. We believe that understanding the historical interplay between fertility and longevity can offer valuable insights into human biology and the remarkable changes that have shaped our lives over the centuries.

Read the original article: Marco-Gracia, F.J., & González Esteban, Á.L. (2025). Live fast, die young, leave a good-looking offspring: longevity and reproduction tradeoffs in rural Spain (1536-1965). Evolution & Human Behavior, 46(2), 106673.

HBES Executive Council Statement in Defense of Science

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HBES stands in defense of the values of scientific inquiry and in opposition to censorship and the defunding of peer-reviewed research projects in the US whose grants were awarded by agencies and processes that have been subject to congressional oversight for decades. We support worldwide rallies in March 2025 calling for continued public funding of scientific research.

The Human Behavior and Evolution Society (HBES) is an international, nonprofit, non-partisan organization that welcomes scholars with diverse backgrounds, viewpoints, and areas of expertise who investigate the evolutionary bases of psychology and behavior. Our mission is to foster and support evolutionary research and researchers around the world, and we stand in support of efforts in all countries to protect scientific research and researchers from interference and threats to scientific freedom. Importantly, research projects on many phenomena that are key to the study of human evolution, such as sex and gender differences and cultural diversity, face the possibility of systematic defunding in the US, directly impacting the work of HBES members.

Historically, the United States Government has allocated funds to government agencies such as the National Science Foundation (NSF), National Institutes of Health (NIH), U.S. Agency for International Development (USAID), and many others, to finance research that has been a foundation for scientific knowledge and the health and prosperity of U.S. citizens. Hindering the disbursement of approved funds and disrupting the operations of these agencies jeopardizes the U.S. scientific enterprise, and the health and well-being of U.S. citizens.

HBES calls for an immediate return to the bipartisan overview of, and investment in, scientific research that has long characterized US policy.

Signed,

The HBES Executive Council

A large extended family

The relatives of gay individuals do not have more offspring. The old evolutionary enigma unresolved?

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– by Jakub Fořt & Jan Havlíček

Why do some individuals direct their sexual behavior exclusively toward individuals of the same sex? This is often viewed as an “evolutionary conundrum” or “evolutionary enigma” because same-sex attraction does not lead to reproduction – and yet it persists. Robust evidence from all around the world shows that individuals attracted only to same-sex persons have less offspring than other-sex attracted individuals. In fact, in some samples their reproductive outcome is close to zero1,2. It is also known that sexual orientation is partly determined by the genetic makeup, with the contribution of heritability estimated at around 30%. Genome-wide association studies indicate that the number of genes involved is large and each has just a small effect. In short, there is no single gene responsible for sexual orientation3. The question is then: how come homosexuality-associated alleles are maintained in human population if they decrease the fitness of gay individuals?

In recent decades, scientists have proposed several evolutionary theories to explain this apparent evolutionary paradox. The sexually antagonistic genes hypothesis proposed by Camperio Ciani and colleagues4 is based on the principle of sexually antagonistic selection. In other words, they claim that the same alleles that lower fitness in one sex may confer a reproductive advantage to the other sex. Specifically, their hypothesis predicts that the same alleles that cause male homosexual orientation increase the fertility of gay men’s heterosexual female relatives. In their paper, they recorded higher fertility rates in the mothers, maternal aunts, and maternal grandparents of Italian gay men (compared to relatives of heterosexual men). These results were replicated, or partly replicated, not only by the same team but also by other researchers and in other populations including, for instance, the islanders from Samoa5.

In spite of the promising nature of these early findings, more recent studies did not find evidence in support of this hypothesis6. Further doubts emerged when meta-analyses7 revealed no higher reproductive outcomes in the mothers of gay, as opposed to heterosexual, men. A re-examination of the earlier studies had moreover shown that some of the previous findings may have been an artifact of an uncontrolled variable. In particular, they may have been due to the fact that, as compelling evidence shows, gay men have more older brothers than heterosexual men do6. This is called the fraternal birth order effect. When the early data that seemingly confirmed a higher fertility of the mothers of Samoan androphilic (male-attracted) males were reanalyzed while controlling for the fraternal birth order effect, it turned out that the mothers of androphilic males have fertility rates comparable with those of heterosexual men’s mothers8. In other words, part of the original evidence that seemed to support the sexually antagonistic genes hypothesis was just an artifact of androphilic males having more older brothers.

The principle of the sexually antagonistic genes hypothesis can also be applied to lesbian women and their male relatives. Women have been too often overlooked by research into sexual orientation, its biological underpinnings, and possible ultimate causes. As a result, the abovementioned fraternal birth order effect has been mostly investigated only in the context of the biological development of male sexual orientation. New studies that include female samples have nevertheless shown that the effect is present not only in gay men but also in lesbian women. Still, few studies so far have examined the pattern of familial fertility in lesbian women: the possible role of sexually antagonistic selection in maintaining female homosexuality is thus underexplored. In our study, we have therefore included a female sample.

In a recent issue of the Evolution of Human Behavior, we have published a study where we tested the hypothesis in a new Central European sample. In an online survey, we asked our participants how many biological offspring, siblings, maternal and paternal cousins, aunts, and uncles they have. We have collected data from 693 gay men, 265 lesbian women, 843 heterosexual men, and 331 heterosexual women. After controlling for sociodemographic variables, we have confirmed that gay men (M = 0.11) had less offspring than heterosexual men did (M = 0.72, OR = 0.17) and the same held of lesbian women (M = 0.11) compared to heterosexual women (M = 0.74, OR = 0.26). This was not surprising: it is an essential assumption of all evolutionary theories of exclusive homosexuality.

In men, we recorded a slightly (but significantly) higher fertility in gay men’s paternal grandparents (OR = 1.09) but no other significant fertility differences. Most notably, neither the mothers nor the maternal aunts of gay men had higher fertility than those of heterosexual men. In women, we found a slightly higher fertility only in lesbian women’s mothers (OR = 1.09) – nonetheless, this effect disappeared when the fraternal birth order effect (present in our sample for both men and women) was controlled for.

In line with other recent studies from WEIRD societies, our findings thus do not support the sexually antagonistic genes hypothesis. One could object that in Western societies, sexual behavior had, in the course of the 20th century, become largely decoupled from reproduction. That is why it is important to note that some results from high-fertility non-WEIRD societies likewise fail to support the sexually antagonistic genes hypothesis8.

Does it mean the hypothesis should be abandoned? Our answer is cautiously negative. In particular, we believe that focus on the reproductive outcomes of homosexuality-associated alleles in the direct kin of homosexual individuals is not the optimal way of testing the sexually antagonistic effects on the genetic level. Based on genetic data, Zietsch and colleagues9 found that alleles associated with homosexuality actually do confer reproductive advantages. Specifically, women who have alleles that predispose men to homosexual behavior tend to have more opposite-sex sexual partners and, vice versa, men who have alleles that predispose women to homosexual behavior tend to have more opposite-sex sexual partners and more children.

From an evolutionary viewpoint, exclusive homosexuality remains an unresolved question. Our results, which are based on fertility data of the direct kin of homosexual and heterosexual individuals, are not consistent with the sexually antagonistic genes hypothesis but, based on genetic data, we would like to argue that sexual antagonism plays a role in maintaining homosexuality in human population, probably jointly with some other factors that have been proposed. Perhaps exclusive homosexuality is just an extreme case of an otherwise adaptive phenomenon of partial same-sex attraction and bisexuality, that is, behaviors whose advantageousness has been demonstrated in non-human animals10.

Read the original article: Fořt J., Valentova, J., Hudáčová, K., Kunc, B., & Havlíček, J. (2025). An evolutionary perspective on homosexuality: testing the sexually antagonistic genes hypothesis through familial fertility analysis. Evolution & Human Behavior, 46(1), 106649.

A man doing a flip of a rock into the ocean

Is men’s risk proneness still subject to sexual selection in modern environments?

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– by Yohsuke Ohtsubo

Men are riskier than women, which is one of the most robust findings in psychology. When compared to women, men tend to report that they seek more risks in psychometric studies. In laboratory studies, men tend to show riskier behaviors, such as expanding a computerized balloon to the point of exploding to earn more money. Outside of the lab, men tend to drive faster (often causing more traffic accidents), engage more in extreme sports (e.g., cliff diving), and more often become involved in homicidal conflicts.

A common evolutionary explanation for this robust sex difference is sexual selection: Men’s risk proneness was selected for because it conferred upon men (but not women) fitness advantages in intrasexual competition and/or intersexual mate choice. However, sexual selection is associated with at least two “contradictory” predictions. Men in good conditions who are able to take risks (e.g., cliff diving) appeal to women by taking the risks. Thus, it is expected that risk-seeking men enjoy higher reproductive success (i.e., having more children). Alternatively, it is possible that men in poor conditions who have less to lose are more likely to take risks (e.g., crimes). In this case, risk-seeking men, due to their poor conditions, should have fewer children than men in better conditions, but they are still better off taking the risks because they would otherwise end up childless.

In our study, we were interested in whether risk-prone men tend to have more offspring than risk-averse men in modern environments (i.e., contemporary Japan and the US). However, we were aware of the possibility that we could end up with null results (i.e., no association between men’s risk proneness and reproductive success) because sexual selection operated in the past—our species’ ancestral environments. If modern environments are so much different from the ancestral environments, risk proneness may no longer confer any fitness advantage on men.

Given the possible evolutionary mismatch between the modern and ancestral environments, one may find this investigation itself a risky endeavor. However, as we acknowledged in the paper, we had preliminary evidence—our unpublished study involving middle-aged Japanese men showed that their retrospective reports of risk proneness were positively correlated with the number of children they had. That said, this investigation might have been risky because, partly due to the low reliability of recall data, the correlation was quite small (approximately 0.15).

Nevertheless, our curiosity outweighed the fear of null results. We collected data from 1,205 Japanese individuals (601 men and 604 women) aged 45 to 55 years old. Among other things, we measured their retrospective risk proneness during their 20s and 30s and the number of children they had. We assumed that the number of children reported by this age group comes very close to their lifetime reproductive success because less than 5% of fathers and only 0.19% of mothers of newborns in Japan are 45 years and older. We successfully replicated the small correlation for men. The retrospective risk proneness × number of children correlation was small but significant in men (0.123), while it was virtually zero in women. More importantly, the difference between men’s and women’s correlations (0.123 vs. 0.001) was significant (many thanks to an a priori power analysis!).

Encouraged by the Japanese results, we went ahead with another round of data collection in the US. This successfully replicated the Japanese results—men’s retrospective reports of risk proneness were significantly correlated with the number of children they had (0.143), while the same correlation for women was not significant (−0.002). And, again, the men’s correlation was significantly larger than the women’s!

In both countries, we used the domain general risk proneness score in the main analysis. Sample items included “I took risks regularly” and “I preferred to avoid risks” (reverse-coded item). However, for exploratory purposes, we also included domain-specific risk measures, which measured respondents’ willingness to take different types of risks: recreational risks (e.g., rock climbing, scuba diving), health risks (e.g., smoking), career risks (e.g., quitting a job without another to go), financial risks (e.g., gambling), safety risks (e.g., fast driving) and social risks (e.g., publicly challenging a rule or decision). Interestingly, only recreational and safety risk-taking consistently exhibited a similar pattern as the domain general risk proneness: men’s, but not women’s, recreational and safety risk scores were significantly correlated with reproductive success.

These results are generally consistent with the notion that men in good conditions take more risks, and their risk-proneness is associated with higher reproductive success. However, isn’t it possible that risk proneness is associated with higher reproductive success because only successful risk takers are involved in our data? Unsuccessful risk takers might have died in risky activities, such as rock climbing and cliff diving. The unmeasured cost of dying and the measured reproductive benefit may cancel each other out. Our unpublished follow-up data collected in Japan may have some clues on how to address this problem. It shows that men with good records in high school and college sports reported being more risk-prone during their school days than a control group of average achievers. Physically fit athletic men may engage in risky behaviors, such as extreme sports, while experiencing a lower risk of injury. Obviously, this issue needs more studies.

Despite certain limitations, the evidence from our research is useful for evolutionary psychologists to respond to a common criticism against evolutionary psychology. It is sometimes criticized as being a “just-so-story”—there is no direct evidence that this behavioral trait underwent sexual selection. That’s true. However, it is certainly more persuasive to say, “We know this is the case in modern environments, so perhaps it was like this in the past” than to say, “Perhaps it was like this in the past.” Although our research is admittedly a small first step, we think it still increased our knowledge about the role sexual selection has played (and seemingly continues to play) in men’s risk proneness.

Read the original article: Sakamoto, R., & Ohtsubo, Y. (2025). Men’s but not women’s risk proneness in early adulthood is associated with lifetime reproductive success: Evidence for sexual selection in modern environments. Evolution and Human Behavior, 46(1), 106654.

Fertility, hormonal contraceptives and competitiveness: why can’t we agree?

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– by Lindsie Arthur

Competition is woven into nearly every aspect of human life, often in ways we don’t consciously notice. Whether striving for promotions, signalling status in social groups, or competing in dating markets, competition shapes our interactions and decisions. But what fuels these competitive processes?

One possibility is that hormonal fluctuations influence competition in women, particularly hormone change associated with fertility and hormonal contraceptive use. Researchers expect hormones to influence behavior because they are chemical messengers that regulate brain activity related to emotion, motivation, and decision-making. Hormone levels also change in response to internal and external conditions so that organisms can adapt to changing environmental and physiological needs.

Some researchers argue that women may exhibit greater competition for status and mates during the fertile phase, when estradiol is elevated relative to other times in the cycle. Supporting this idea, some studies find that during the fertile phase women are more likely to degrade other women, enhance their appearance, behave dominantly in economic games, and to experience peak motivation for prestige and achievement. However, not all research supports this position, as many studies have failed to find an association between fertility and a range of competitive outcomes, including appearance enhancement, competition in economic games, or self-reported intrasexual competitiveness.

Researchers have also investigated how hormonal contraceptive use may influence competitive motivation and behavior. Hormonal contraceptives prevent pregnancy by introducing synthetic hormones into the body which disrupt ovulation and the implantation of a fertilized egg. A recent review proposed that because hormonal contraceptives disrupt hormone changes across the menstrual cycle, then behaviors associated with certain times in the cycle may also be disrupted. Support for this theory is mixed. Some studies report less competitiveness compared to naturally cycling participants overall, or when comparing competitive motivation during the fertile phase. However, other work finds no differences between hormonal contraceptive users and non-users.

Given mixed results regarding the effect of fertility and hormonal contraceptive use on competitiveness, we conducted a large longitudinal diary study with 302 women (5,600 daily observations) from 22 countries. Participants completed daily surveys for at least one full menstrual cycle. Regarding competitive motivation, daily surveys assessed achievement motivation (reflecting a drive for success and self-improvement) and a general disinterest in competition. We also examined six competitive behaviors: gossip, appearance enhancement, social comparison, taking selfies, giving advice, and negatively evaluating others. These behaviors were chosen because previous research suggests that these are common strategies used by women to compete for mates and status. To estimate fertility, we used each participant’s individual menstrual cycle data to calculate daily fertility probability estimates. In addition to looking for fertility effects, our analyses allowed us to test for different patterns of responding between naturally cycling participants and hormonal contraceptive users.

Contrary to some previous research, we found no evidence that naturally cycling women became more competitive during high-fertility days. That is, we observed no mid-cycle increase in achievement motivation or any of our self-report competitive behaviors. While it is possible that there is no true association between fertility and competitiveness, we wanted to consider reasons why we failed to replicate previous findings, as well as theorise about why replication is relatively uncommon in menstrual cycle studies. Replication challenges in psychology are often attributed to researcher degrees of freedom and methodological issues. Although we agree that these are important factors to consider, differences in sample populations, study design, and environmental factors can also significantly influence findings. While diary studies provide a detailed view of individual behavior over time, the richness of this method also introduces countless contextual variables that may affect replicability. Context dependency may therefore contribute to non-replication, where psychological effects are shaped by specific situational factors that are not currently accounted for in our analyses.

Unlike fertility effects, we did replicate existing contraceptive effects, finding that women using hormonal contraceptives reported less interest in competition compared to naturally cycling participants overall. This suggests that synthetic hormones found in hormonal contraceptives may influence psychological processes related to competitiveness, though further research is required to understand the practical implications of this result. In any case, by better understanding the psychosocial effects of hormonal contraceptives, women and others who rely on this critical medicine can be empowered to make informed decisions about what medical interventions are appropriate for them.

To conclude, we believe our study highlights the need to account for contextual factors in menstrual cycle research. Although context may not be crucial in all research areas, it plays a vital role in dynamic fields like social and evolutionary psychology, where decisions and behaviors are shaped by situational factors. More research is therefore required to reasonably understand how contextual factors and methodological choices may enhance disagreement in the literature. In other words, we know too little about the ways that the social and cultural environment influences the many ways that competitiveness has been measured in previous menstrual cycle research. We encourage future research to combine methods, such as self-reports, diary designs, behavioral observations, and hormonal sampling, to better understand these processes in a range of contexts. By continuing to refine our methods and theories, we move closer to uncovering the full story of how hormones influence our motivations, behavior, and social interactions.

Read the original article: Arthur, L. C., Bastian, B., & Blake, K. R. (2024). Hormonal contraceptive use, not menstrual cycle phase, is associated with reduced interest in competition. Evolution and Human Behavior, 45(6), 106616.

The controversial origins of war and peace

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– by Luke Glowacki

When I set out to write my recent article The Controversial Origins of War and Peace: Apes, Foragers, and Human Evolution, I hoped to provide clarity on the heated debate about the role of war and peace in human evolution. Is warfare part of our evolutionary legacy, or did it emerge from the societal shifts brought about by agriculture and permanent settlements? This isn’t just an academic puzzle—it’s a question that cuts to the core of how we understand ourselves, our past, and the future of human coexistence. My goal was to peel back the oversimplification that commonly characterizes both sides and dig into the messy, complex reality of our evolutionary journey.

The debate often falls into two camps: “deep rooters” and “shallow rooters.” Deep rooters argue that warfare has ancient roots, embedded deep in our evolutionary history, possibly stretching back to our last common ancestor with chimpanzees and bonobos. They point to the brutal, coordinated violence among chimpanzees as evidence that our lineage is steeped in conflict. Deep rooters take the fact that hunter-gatherers often have war as evidence that war would have occurred among our Pleistocene ancestors. Shallow rooters, on the other hand, see war as a recent human invention, a consequence of agriculture, sedentary life, and hierarchical societies. War emerged only recently they argue, once we stopped foraging and started living in settled agricultural communities. They highlight the peaceful, cooperative lives of bonobos and some hunter-gatherers, painting a picture of a more harmonious human past.

But here’s the rub, the debate has often overlooked that both war and intergroup cooperation are parts of our evolutionary legacy, and the evidence is not nearly as clearcut as both camps sometimes assume. Simplifying human evolution into a binary of “war” or “peace” misses the fascinating complexity of our story. Chimpanzees do engage in deadly raids but using them as a direct blueprint for early human behavior ignores millions of years of evolution that followed our split. And while bonobos are often held up as paragons of peace, their societies aren’t entirely free of aggression. While lethal raids have not been reported for bonobos, they do appear to have rates of aggression that can rival chimpanzees. And neither species may be useful for understanding the last common ancestor humans shared with other apes. Further, we’ve had seven million years of evolution since sharing a common ancestor with chimpanzees and bonobos, raising questions about how much of our behavior is really inherited from a last common ancestor.

When I turned to the archaeological and ethnographic records of hunter-gatherers, the picture is  even more complex. It’s tempting to believe that warfare only appeared with the advent of agriculture, but that’s not what the evidence shows. Small-scale intergroup violence existed long before humans settled into farming communities, yet larger-scale violence appears to emerge only more recently. Just as often though, hunter-gatherers engaged in cooperative trade and alliances. And while many of them have war, war may be intermittent and even rare. Evidence of intergroup trade spanning hundreds of kilometers extends deep into the Paleolithic.

Our ancestors weren’t locked into one mode of interaction; they were adaptable, capable of both conflict and collaboration depending on the circumstances. And like humans everywhere, they likely used conflict and cooperation to obtain their goals depending on the circumstances. Because both cooperation and conflict have strong fitness relevance, both were likely important selective features in our species’ history.

What struck me—and what I hope strikes readers—is the dual legacy of war and peace in human evolution. Both violence and cooperation have been powerful forces shaping who we are. This duality resonates with the world we live in today, where acts of war and moments of profound cooperation coexist. We’re equipped for both, and it’s our environment, culture, and social structures that shapes which path we follow.

This understanding isn’t just intellectually satisfying—it’s hopeful. If peace is as much a part of our evolutionary toolkit as war, then striving for harmonious coexistence isn’t some naive dream; it’s a fundamental part of who we are. Recognizing this can shape how we approach conflict resolution and peacebuilding today. It reminds us that while conflict may be a part of our past, it is not an inevitable feature of our future.

Read the original article: Glowacki, L. (2024) The controversial Origins of war and peace: apes, foragers, and human evolution. Evolution & Human Behavior, 45(6), 106618.

Announcing HBES on Bluesky – @HumBehEvoSoc.bsky.social

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As a celebration of Darwin’s birthday, HBES is proud to announce our presence on a new social media platform – HBES now has an account on Bluesky! We will start posting news and updates under our Bluesky account name (@HumBehEvoSoc.bsky.social). This is the same as our Twitter (X) handle (@HumBehEvoSoc), except formatted for Bluesky. The account is new, so it’s still a little empty, but this will change as a we post more and more news and updates.

Bluesky is quickly becoming one of the major social media platforms. As many of you know, many people find that Twitter (X) has become increasingly unpleasant to use in recent years. This is not just political: many people have commented that it’s a less user-friendly experience than it used to be, with more ads, more spam, more bots, worse algorithms, irrelevant feeds, unwanted interjections from its owner, among other complaints. As the HBES Communications Officer, this declining experience has made me yearn for an alternative to Twitter for years. After all, HBES should be about the science, and few of us want to wade through reams of irrelevant stuff to get our information. But which platform? Bluesky seems to be coming out on top, and many HBES members have already switched. As such, I’m very happy to create the new HBES Bluesky account and start posting there.

Due to this experience, I will gradually search less and less for HBES-relevant information on Twitter and do so more on HBES Bluesky. For now, I will post all important HBES announcements on both platforms and on our Facebook account. However, I will check the Twitter feed less often because it is increasingly filled with irrelevant junk despite my efforts – I can only sort through so many ads and Twitter-sponsored intrusions to see what to re-post. Please remember that the HBES Communications Officer is a volunteer position! I will monitor engagement on both platforms, but if the worsening experience with Twitter (X) continues, then I will use it less and less unless the “engagement to irrelevance ratio” remains high. Ultimately, it comes down to where we get enough engagement to make it worth the hassle. The next HBES Communications Officer – whoever that is – will determine their own social media policy. Thank you for your understanding.

Looking forward to interacting with everyone on Bluesky!

Sincerely,

Pat Barclay (HBES Communications Officer)