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)