– by Angarika Deb & Christophe Heintz (photo credit: ChatGPT)

Just yesterday, my partner and I got into an argument regarding who has been vacuuming the last couple of times, around the house. And now that we expecting some guests, who should be the one to do it? Household arguments about undone tasks (or having done more than one’s fair share!) are a staple part of life. The actual arguments might look different from one couple to the next: some might vigorously argue aloud, providing continuous information to one another about how they are making up their minds and why; others might silently and implicitly negotiate, leaving some chores clearly unfinished to prod the other partner into picking up their slack. But underneath these, there’s a shared commonality: they’re all bargaining problems. eir slack. But underneath these, there’s a shared commonality: they’re all bargaining problems.

We can fit household bargaining onto a Nash bargaining model: imagine two partners dividing a shared pool of resources that is valuable to both, but limited in amount like leisure time. Partners can demand to split the total leisure time 50/50, or even ⅔ and ⅓ each. Both these splits seem compatible, and the house can still keep running. But if both ask for ⅔ of the total available leisure time, something in the house remains undone. The interesting question is, who gets to ask for a greater share? We suggested that each partner makes these decisions based on the fallback options they have in case the demands they make are incompatible with their partner’s, and risks ending the relationship.

This bargaining process with fallback options helps explain widespread and stable gender inequalities: women systematically have worse fallback options than their male counterparts. However, what constitutes these fallback options vary from one society to the next. Here, we studied households from two hunter-gatherer communities, the Mbendjele BaYaka based in Congo and the Agta from the Philippines, who have remarkable equality between men and women, socially, politically and domestically. We published our findings in Evolution and Human Behavior.

The lifestyles of these two groups – and other immediate-return hunter-gatherers like them – are quite distinct from our industrialised economies and settled way of life: they operate in environments with lower levels of food security; lead a nomadic existence; have very few material possessions or single-ownership of goods; and all individuals – including children – enjoy considerable autonomy over their movement and lives. Their fallback options are based mainly on their social capital—i.e., how many friends and helpers they have—since they own few material possessions.  If their marriage were to dissolve or if they were to get into a serious fight with their spouse, they could then rely upon these friends. The bargaining model we outline above, predicted that, the better one’s fallback options the greater their bargaining power and share of leisure time, within the household. To find out if that was indeed the case in these hunter-gatherers, we calculated each person’s daily average leisure hours: we observed everyone from 6 AM to 6 PM, noting down what they were doing every hour, for weeks. We also observed them giving gifts to each other, and studying how they share food in their daily lives, to establish each person’s social capital.

Our results were surprising, and contrary to what we initially predicted: individuals with higher social capital did not enjoy a higher proportion of leisure time than their spouse. Why would this be? From our ethnographic knowledge, we speculated that this was likely due to the assertive egalitarianism present in these two hunter-gatherer groups: when groups have strong norms enforcing equality between individuals, these norms can end up shaping the fallback options available to individuals and thus, modulate any individual-level power dynamics that can arise. No single individual, socially wealthy as they may be, will have the opportunity to dominate their partner into doing more work in the house, and picking up their slack.

In line with this, we found that across households, despite differences in social capital, both partners had equal amounts of leisure time, both for the BaYaka and the Agta. This was remarkable, given what we know of industrialised societies, where women usually shoulder a higher burden of household tasks. Decades of studying households across most countries, has revealed that women are putting in a substantially higher amount of time in household chores, than the men – usually 70%, but sometimes as much as 90% – even when they are employed in full-time or part-time jobs.

If you are a feminist, like the authors of this blog, it is encouraging to find that there are human societies where, not only do men and women have equal political status, but also operate on egalitarian terms in the household. Our current work provides evidence that differences in one’s social capital need not convert into individual-level power differences; and suggests the potentially important role of social norms in shaping household behaviour. Our future work will test this more directly. The gender equality thus documented, is a promising note to suggest that we in our industrialised societies can have the same, if the right kind of group-level practices and norms – such as bilocal residence after marriage, equal political voice for men and women, involvement in subsistence activities, etc – are adopted.

Read the original article here: Deb, A., Saunders, D., Major-Smith, D., Dyble, M., Page, A.E., Salali, G.D., Migliano, A.B., Heintz, C., & Chaudhary, N. (2024). Bargaining between the sexes: outside options and leisure time in hunter-gatherer households. Evolution & Human Behavior, 45(4), 106589.

– by Macken Murphy

Socially monogamous birds, like humans, often have “affairs.” In some species, these liaisons seem to serve a dual mating strategy. The females prioritize good looks in extra-pair mates (e.g., more complex ornamentation) and good parenting in primary partners (extra-pair males generally don’t invest in young). Good looks in males are thought to signal genetic benefits, providing the females with more robust—or, at least, more attractive—offspring. And so, many ornithologists take aesthetic differences between extra-pair and primary mates in these species as evidence that the females use extra-pair mating to make a combo deal: good investment at home and “good genes” from outside.

Evolutionary psychologists would have told a similar story about humans two decades ago, just with different evidence. It was popular to argue that women’s infidelity evolved due to the competitive advantage female ancestors gained from conceiving with more attractive affair partners and then raising their affair partner’s child with their more invested primary partners. A flashy series of studies from the previous several years had suggested that women’s behavior and preferences changed around ovulation, prioritizing cues to “good genes” when conception was likely. While the reasoning was less direct than that found in avian species, the initial ratification of this clever prediction lent credence to the underlying dual-mating hypothesis. If women prize different traits at ovulation compared to the rest of the month, perhaps they prize different partners as well, and recruit them towards different ends.

However, the golden age of ovulatory shifts was brief. Failed replications, skepticism about methods, and insinuations of p-hacking cast doubt on the original experiments. Then, newer, more rigorous research often found null results. This anti-climax prompted some scholars to wonder: What if the problem lies deeper, with dual-mating itself?

In 2017, David Buss and his colleagues proposed an alternative primary explanation for women’s infidelity: mate-switching. Drawing on evidence that women who cheat are more likely to be in love with their affair partners and less likely to be satisfied in their relationships than men who cheat, they argued that infidelity primarily helps women assay and seduce replacements. To paraphrase Buss, you wouldn’t quit your job before finding a new one, so why would you dump your mate before getting a better one?

(This hypothesis also has precedent in birds: One small study of cockatiels found that female extra-pair copulation lead to mate-switching in all cases, and these re-pairings were “trade-ups” in that they had higher expected reproductive success.)

However, though it was a persuasive article, its key evidence is open to multiple interpretations, and much of the empirical support for mate-switching is based on data from women who hadn’t had affairs. And, really, it’s perfectly coherent for women’s infidelity to serve a dual-mating strategy without shifting strategies around ovulation. Humans exhibit notable sexual stability across the cycle, so the ovulatory shift sub-hypothesis may not have been the best test of human strategic dualism to begin with.

A more straightforward test of dual-mating than looking for periovulatory changes would be to follow the avian research and test if affair partners are more handsome than primary partners. This pattern would suggest dual-mating since better-looking men are generally accepted to offer more genetic benefits, even if the only benefit they offer is better-looking offspring. On the other side, a similar empirical test of the mate-switching hypothesis—or, at least, its trading-up utility—would be to check whether women prefer their affair partners to their primary partners overall.

Finally, one way to test these hypotheses against each other would be to check if women see their primary partners or affair partners as better dad material. Parental attractiveness ratings are a clean way to pit these hypotheses against each other, as they clearly have dueling predictions. Dual-mating argues the primary partner is the intended father figure for offspring, and mate-switching argues it’s the affair partner. So, if women’s affairs mainly serve a dual-mating strategy, the primary partner should be the better dad. If women’s affairs primarily aid a switch to a new mate, the affair partner should be viewed as the better dad.

Given the utility and similarity of these tests, we decided to conduct them as a package. I, along with primatologist Dr. Caroline Phillips and psychologist Dr. Khandis Blake, recruited a multinational sample of 254 people who had affairs and—in a pre-registered study with open data and materials—had them rate their affair partner and their primary partner in terms of their mate value, their parental attractiveness, and their physical attractiveness.

If mate-switching drove most women’s infidelity, affair partners should have been rated as higher in mate value—but they were not. Women rated their primary partners and affair partners almost exactly equal in overall desirability. Instead, our human results followed the exact structure one would expect from strategic dualists: Affair partners were more physically attractive than primary partners and primary partners were more parentally attractive than affair partners. Our result was the best-case scenario for dual-mating and provides evidence that psychological adaptations to acquiring genetic benefits underlie women’s infidelity.

Figure caption: Interaction plot comparing men’s and women’s ratings of their primary partners and their affair partners, in terms of physical attractiveness and parental attractiveness.

However, dual-mating cannot explain all infidelity. Women in our study reported utilizing extra-pair mating as a means to various ends, including, sometimes, switching mates. Further, the primary motivation for infidelity likely varies based on ecological factors (e.g., women may primarily use infidelity to obtain additional resources in resource-scarce environments). We believe the prevalence of dual-mating in our ancestry likely varied predictably with ecology, and so our results should not be extrapolated across all contexts.

Lastly, it’s conspicuous that men followed the same pattern, cheating up in terms of looks and down in terms of parenting. This gender similarity was a bit surprising to us at first. However, since men’s affairs are broadly accepted to have evolved through producing more offspring, men who cheat, too, are “just conceiving” with affair partners and co-parenting with primary partners. Therefore, it might make sense that they prioritize the conceptive benefits signaled by good looks (e.g., fertility) in affair partners and motherly qualities in primary partners.

While we look forward to further tests of dual-mating’s relevance to human affairs, for now, our results suggest that women—and, surprisingly, men—follow a pattern common in birds: better parenting at home and better looks on the side.

Read the original article here: Murphy, M., Phillips, C.A., Blake, K.R. (2024). Why women cheat: testing evolutionary hypotheses for female infidelity in a multinational sample. Evolution & Human Behavior, 45(5), 106595.

– by Brendan Zietsch

A key question for evolutionary psychologists is: what selection pressures have shaped human traits and how do they vary and covary across individuals? Recent genomics studies have revealed a wealth of evidence that sheds light on these questions. In my paper, “Genomic findings and their implications for the evolutionary social sciences”, I aimed to bring together these findings while explaining the conceptual and technical background that is often assumed knowledge for reading the primary reports.  I also outlined what I see as the implications of these findings for psychological life history theory and for our interpretation of individual differences more generally.

The key question that genomic studies can answer is, which form of selection has shaped genetic variation in human traits: negative selection or balancing selection?  Negative (or purifying) selection removes harmful variants and depletes genetic variation. Genetic variation is maintained due to a balance between this depletion against the constant influx of new genetic variation from mutations. Balancing selection, on the other hand, refers to forms of selection that actively maintain genetic variation. It can occur when the relationship between trait value and individual fitness varies over time or place (fluctuating selection) or sex (sexually antagonistic selection), or when it depends on the rarity of the trait in the population (negative frequency-dependent selection), or when an allele’s effect on fitness depends on the other allele at the same locus (heterozygote advantage). So, the question: is the genetic variation in traits today shaped by a history of negative selection or balancing selection?

Genomics studies can be evolutionarily informative because they reveal the genetic architecture of human traits. Roughly, genetic architecture refers to the character of the genetic variation that underlies trait variation, especially the number of genetic variants that contribute to heritable variation and the how the frequencies of those variants relate to their effect sizes. Negative selection and balancing selection produce different genetic architectures (see below),. Therefore, from the genetic architecture of traits, we can make inferences about which form of selection has shaped each trait.

Certain features of genetic architecture are consistent across many traits that have been subject to genomic analysis, including traits that are of interest to evolutionary psychologists: life history traits such as age at puberty; morphological traits like waist-to-hip ratio, BMI, and height; cognitive-based traits like educational attainment; personality traits like neuroticism; and mental disorders like schizophrenia.

One common feature is that the heritability of such traits is spread evenly across thousands of genetic variants. Under no selection, or under balancing selection, we would expect that many variants might influence a trait but that a small number of these account for most of the trait variance. That’s because we know that traits are influenced by rare variants with large effect sizes, and there is no reason, other than negative selection, for this not to be true of common variants as well. As a mathematical necessity, in that case, a relatively small number of common variants with large effects would account for most of the trait variance. Instead, we see that any one variant only accounts for a tiny percentage of trait heritability, which is exactly what we would expect under negative selection.

Another feature shared among traits is that both common and rare variants contribute substantially to trait heritability. Several lines of evidence suggest that rare variants contribute disproportionately to trait variance, relative to what is expected under neutral selection or balancing selection, where virtually all the variation is expected to be accounted for by common variants. This pattern is expected under negative selection, because selection is less effective against rare deleterious alleles than common deleterious alleles. Modelling shows that balancing selection can only maintain variation at intermediate frequencies.

A third feature of genetic architecture observed across traits is that variants’ effect sizes are negatively associated with their minor allele frequency. Rarer variants tend to have larger effects than common variants, which invariably have tiny effects. The only known explanation for this pattern is that selection against harmful variants (i.e. negative selection) eliminates any common variants with large (or even moderate sized) effects, whereas rarer variants, being less visible to selection, are able to remain at low frequency in the population even with larger effects.

A fourth feature is that younger alleles (i.e. arose by mutation more recently) explain more heritability per-locus. This is expected under negative selection: deleterious alleles that have not been around as long have had less time to be eliminated by natural selection. Under balancing selection we would expect the opposite, since it would maintain variation at loci that affect the trait under selection for longer than under evolutionary drift.

These observations constitute pervasive evidence that the genetic variation in complex traits has been shaped by negative selection, and provide no evidence that it has been shaped by balancing selection. This conclusion is backed by formal tests for negative and balancing selection. These tests aggregate evidence across significantly trait-associated variants identified in genomewide association studies. They reveal that traits of interest to evolutionary psychologists show significant evidence of having been shaped by negative selection and significant evidence in the opposite direction of the criteria regarding balancing selection.

Overall, these findings mean we should not reach for balancing selection as an explanation of individual differences, as has been very common in the evolutionary social sciences. Balancing selection has been argued to have maintained a plethora of individual differences including promiscuous and monogamous individuals, cheaters and cooperators, progressives and conservatives, risk takers and hesitators, long-term planners and short-term opportunists, and aggressive hawks and peaceful doves. Indeed, various authors have argued that variation in personality traits in general is maintained by balancing selection. Genomics findings suggest that such explanations are highly unlikely.

The findings also have implications for psychological life history theory, insofar as proponents have argued that genetic covariation among traits is aligned along a fast-slow life history dimension due to balancing selection. If balancing selection has not shaped genetic (co)variation in traits, as the evidence suggests, then this claim does not get off the ground. In the paper I also discuss implications for dimensional theories of personality variation. In short, I argue that if personality variation is the result of a mess of countless genetic variants across the whole genome, many of which are rare in the population or even private to the individual, variation in personality probably does not have a simple dimensional structure (e.g. the Big Five). Rather individuals probably vary in every way possible. The Big Five may just reflect the dimensions of variation that matter to perceivers. ] We are most interested in a relatively narrow segment of all the ways people vary – we have words for (and make personality questionnaire items primarily about) the Big Five personality factors because these are relevant to our social and self-perceptions. But we don’t have words relating to blink rate, for example, even though it’s a behaviour that is socially visible (though usually unnoticed) and varies widely between individuals. The same would apply to countless other ways individuals vary that are not socially relevant or important. So all these other ways individuals vary do not make it into our personality models.

In all, the wealth of recent genomic findings gives strong insights into the history of selection on the traits we are interested in as evolutionary psychologists, as well as pointing to surprising new ways of interpreting individual differences.

Read the original article here: Zietsch, B.P. (2024). Genomic findings and their implications for the evolutionary social sciences. Evolution & Human Behavior, 45(4), 106596.

– By Laure Spake & John Shaver

In post-industrialized settings, religious parents tend to have more children than non-religious parents. But in these same contexts, having large families is costly – the more children a person has, the fewer resources there are for each child. How are religious families able to support their large families, despite the potential costs to their children?

One possible explanation is that religious families get more support from their networks, which then allows them to convert this support into resources for their children. Considering that partaking in religion is associated with increased cooperation between individuals, we have proposed that this cooperation extends to childcare, and this enables religious individuals to successfully support larger families.

To disentangle how religion might be associated with support to mothers, we started by exploring these associations in post-industrialized countries using analyses of existing datasets from New Zealand and England. In both settings, religiosity was clearly associated with fertility: religious individuals had more children. In New Zealand, religious affiliation and frequency of attendance at a house of worship predicted that a childless individual had provided childcare help to others. In England, a mother’s frequency of attendance at church was associated with receiving help from her social network and co-religionists. However, because these datasets were designed to study other research questions, the measurement of support to mothers was quite coarse. We therefore designed a new data collection to collect higher quality information on how others provide support to mothers.

We disseminated an online survey to women in the US and the UK who had at least one child under of 5 years of age in 2020. The survey was better able to address our research questions by asking about religious affiliation and behavior, social relationships, all children born to the woman. Most importantly, we obtained data on what we call the woman’s help network: a list of all of the people whom the woman said helped her take care of her youngest child or helped her with a set of household tasks. A total of 1528 women successfully completed the survey: 919 of them lived in the UK, and 609 of them resided in the US.

Overall, 44% of UK mothers reported a religious affiliation, compared to 65% of US mothers. Amongst the mothers declaring a religious affiliation, US mothers generally had higher religiosity scores – this is in agreement with a broad literature which shows that the US has unusually high levels of religiosity for a high income country.

We explored how mothers’ religiosity related to: 1) how close they lived to and how often they contacted family members; 2) their fertility, adjusted for age at the time of the survey; and 3) the childcare and household help they received from family members. For this analysis, we focused on help received from family members because existing evidence overwhelmingly shows that family members provide the most help to mothers and exert the most influence on childbearing decisions.

We found that women with higher religiosity scores tended to have more geographically dispersed kin networks, but that despite this, they maintained similar levels of contact with them as women who were less religious (virtually or in person). It seems like religion helps to support familial cooperation, even over great distances. Religiosity was also clearly associated with the number of children a woman had, which was in agreement with our previous findings in the UK, New Zealand, and the findings of others.

There was no association between maternal religiosity and the number of people who provided her help with either childcare or household tasks. However, there were some associations between religiosity and the amount of help a mother received. In both the US and the UK, religiosity was clearly associated with receiving more help with household tasks.

In both countries, religiosity was clearly associated with receiving more help from non-partner kin. Most commonly, these helpers were either the mother’s mother or her partner’s mother. However, the third most commonly named non-partner helper was a mother’s older female child. When this helper was named to the help network, they provided the highest average amount of help with both childcare and household tasks.

The mother’s partner’s behaviour, however, was not uniformly affected by maternal religiosity. In the US, the partners of more religious mothers tended to provide less help, while in the UK mother’s religiosity was associated with receiving more help from partners, at least with household tasks. This was a bit surprising, as sociological research on religious fathers suggests that they tend to be more involved with their children than non-religious fathers.

The finding that family members increase their cooperative contributions to a household in response to maternal religiosity is a new and interesting one. Indeed, one explanation for why religion evolved and persists is that it can help to connect people. In post-industrialized settings, researchers have typically expected religion to connect unrelated individuals, compensating for the loss of extensive family networks that would have been available in non-urbanized, non-industrialized settings. However, our research has shown that religiosity does increase the amount of support that a mother receives from her family, though not necessarily from her partner. We have also shown that religiosity promotes cooperation differently across the US and the UK, which are fairly culturally similar contexts.

Since we conducted this study, we have collected more data on these themes across five different cultural settings in Bangladesh, The Gambia, India, Malawi, and the USA. We are busy analyzing these data to see if the relationships we found in this study persist in non-Western contexts, or whether they are influenced by market integration and/or religious tradition.

Read the original article: Spake, L., Schaffnit, S.B., Page, A.E., Hassan, A., Lynch, R., Watts, J., Sosis, R., Sear, R., Shenk, M.K., & Shaver, J.H. (2024). Religious women receive more allomaternal support from non-partner kin in two low-fertility countries. Evolution & Human Behavior, 45(3), 268-280.

– by Mia Karabegovic & Hugo Mercier

“Free” information is so ubiquitous that we rarely question the motivations for producing and sharing it in the first place. We can hop on Wikipedia to learn about any topic, however niche, check diners’ experiences on Yelp to figure out where to eat, or consult book reviews on GoodReads to pick up the best new thriller. We receive advice from friends, colleagues, acquaintances, about anything from a prospective purchase to tips and tricks which make our jobs easier. Most of this wealth of information at our disposal is contributed by others who ostensibly gain nothing for providing it.

But all of these people – from friends providing advice to Yelp reviewers – pay some cost, at least in the time it takes for them to provide us the information (how often do you leave informative reviews of the places you’ve eaten at?). Why are we so willing to pay these costs?  Our hypothesis is there is something in it for them, and that something is gratitude.

Research on gratitude has shown that being on the receiving end of prosocial behavior elicits feelings of gratitude, which reflect on the perceived cooperative partner value of the one extending the favor, and lead the receiver to be more prosocial in turn – especially when it comes to reciprocating the favor. We used a model proposed by McCullough and colleagues to check whether gratitude for information behaves as gratitude for other prosocial behavior: that it increases with the amount of benefits the receiver gets from the advice, with the cost of the sender for providing it, with the intention to transmit the information to the particular receiver, and with whether it is provided gratuitously (i.e. without expecting immediate rewards).

Our first study presented online participants with different scenarios, such as imagining they needed advice about taxes or help in finding a rare postage stamp. We asked them to choose which advisor they would feel more grateful to out of pairs in which one either provided more benefits, paid a higher cost, intentionally provided the information, or did not ask for immediate reciprocation – and found that gratitude followed the theoretically predicted pattern.

When there was no difference between informants on the four dimensions, participants showed more gratitude to those who provided the information first. This “redundancy effect” might help to explain why journalists often scramble to break a story first and get the “scoop”: apart from selling more papers or getting more clicks on the news in question, they might also gain more future favor via gratitude. Competing to be “first” could play a role in disseminating unchecked or incomplete information, especially in competitive situations like the news sector or scientific publishing.

While our first study showed that the model of gratitude is generally applicable to the case of information, there are certain features which set information apart from the kind of prosocial behaviors that have been shown to elicit gratitude.

One peculiarity of information is that it can generally be shared with either a few or many people at once without losing its value to the recipients. For example, knowing about epistemic gratitude will likely benefit you equally whether two or two hundred people read this blog post. However, gratitude could still differ depending on the size of the audience because of perceived intentionality, which is what we found in a study pitting information shared with an individual versus a group: our participants reported feeling grateful towards those who provided the information to them in private more often than those who shared it with a larger group. This kind of strategy is sometimes employed in personalized marketing, which might incite more gratitude than usual ads with broader targets, because people might feel more grateful for messages that appear as if they have been sent to them specifically.

Secondly, unlike a gifted apple that can be eaten only once, knowledge can be reused and passed on to others. Do we favor the providers of such “gifts” over those who provide the same benefits, but only to us? In a follow-up experiment, we showed that people tend to be more grateful to those whose advice they can further employ to boost their own reputation by spreading, as opposed to advice which only carries benefits to themselves. Combined with the result of the last experiment, this might explain why rumors are often propagated from one person or very small group to the next, as it would maximize the gratitude of those whom the rumor is shared with.

Some other, broader implications of our results deserve a mention. One of the most salient benefits of information is to change our minds. We should thus feel more grateful for (accurate) information that challenges our views. But this might not be true for all kinds of beliefs, however. For political opinions, where accuracy plays a smaller role, people might feel more grateful to those who provide them with information that can justify their views, rather than challenge them.

People who share information do not always seek gratitude – sometimes, they want to appear competent. There are interesting tradeoffs in the way we might present the costs of information acquisition in this regard. Our study shows that, to maximize gratitude, we should stress the cost (or effort), while the same might not be true for competence. Taken to the extreme, this might help explain why some religious figures are presented as having paid a very high cost to deliver their message (Jesus Christ and the gospel being the paradigmatic example), while by contrast scientists might be more keen on describing their discoveries as flashes of insight. A possible strategy to maximize both gratitude and competence (for which Jesus Christ is, again, a good example) could be to emphasize the costs of transmission: in this case, we can appear competent (we easily thought of something clever) and simultaneously show we care about their audience (we incurred a substantial cost to relay the information). Competence should also not be affected by the size of the audience in the same way gratitude is – as a result, people who address broader audiences might be more tempted to try to appear competent (which can be done at scale), than to elicit gratitude in their audience.

We hope that this study will stimulate more research into what motivates people to provide information ‘for free,’ and how that shapes our informational environments.

Read the original article: Karabegovic, M., Wang, L., Boyer, P., & Mercier, H. (2024). Epistemic gratitude and the provision of information. Evolution & Human Behavior, 45(3), 252-260.