Abstract core knowledge may shape the basins of cultural attraction: romantic kissing as a case study

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– by Hossein Samani & Ashley J. Thomas

It may be easy for many readers to assume that romantic kissing (lip-to-lip contact between romantic partners) is universal. While romantic kissing is common around the world, it is far from universal. On the other hand, rather than spreading from a single point of origin, ethnographic and historical evidence suggests that kissing has been ‘invented’ independently at different times and places. This combination—widespread but not universal, and seemingly reinvented multiple times—is what we set out to explain. Our central claim is that romantic kissing is best understood as a cultural invention that reappears because it aligns with how humans intuitively understand intimacy. We argue that similarities in how everyone understands intimacy create a sort of gravitational pull, or “basin of attraction,” that makes some ways of signaling intimacy (like kissing) much more intuitive, and therefore more likely, than others.

To make sense of a cultural practice like romantic kissing, we broke down the question into two parts: (1) What problem does this practice solve? (2) Why does it take this particular form? Consider writing. Writing systems around the world differ in countless ways, but they all solve similar problems: they let us record and transmit information across time and space. The features of writing systems are shaped by the limits of our minds and bodies. Though diverse, writing systems around the world tend to be composed of relatively simple, easy-to-distinguish shapes that are easy to produce. We can ask the same questions about kissing. What is it for? And why lip-to-lip contact rather than the endless other actions that could signal romance? Our answer is that romantic kissing is particularly intuitive to all humans as a way to communicate and intensify intimate bonds, and it serves to convey a special kind of intimate bond, i.e., romantic love, where and when romantic love is important.

We start with the idea of “cultural attractors.” This idea, from theories of cultural evolution, argues that cultural evolution is affected by the structure of the human mind. Practices that resonate with existing biases, cognitive tools, or innate representations are easier to invent, imitate, and remember. While this process does not determine practices, over time, cultures tend to converge on practices that are easier for people to reproduce and understand. Crucially, this process doesn’t determine exact behaviors. It doesn’t include the specific practice of kissing. Instead, it shapes the landscape so that some practices that are more intuitive to understand and easy or more pleasurable to reproduce create ‘gravitational pulls’. Romantic kissing, we argue, is one such practice.

To understand why, we incorporate the idea of ‘core knowledge’ which explains how humans—starting in infancy—make sense of the world, including social relationships. Over the past several decades, research on infants has challenged the idea that babies begin life with only sensory impressions and gradually build toward abstract concepts. Instead, there is now strong evidence that infants come equipped with what’s called core knowledge: domain-specific systems that represent abstract aspects of the world—objects, agents, numbers, space. It enables infants to make sense of situations or entities the first time they encounter them and to learn quickly about their specific environment. Recent evidence suggests these ideas can be extended to social relationships. Importantly, even infants seem to distinguish between friendly and socially intimate relationships. Specifically, they link physical closeness, such as using the same spoon, to emotional closeness, such as comforting someone. In these studies, infants predicted that a woman who shared a single orange slice with a puppet by eating some herself, putting it in the puppet’s mouth, and then back in her mouth, would be the one to comfort the puppet when it was sad. We argue that because very young infants recognize this, it is likely universal, and this is why romantic kissing is intuitive.

Next, we asked about the purpose of romantic kissing. Romantic love itself seems to appear in many cultures, but its importance and distinctiveness vary. In some places and historical periods, romantic love is central to marriage, identity, and life decisions. In others, it’s less salient than, say, loyalty to kin or community, or it’s not strongly separated from other forms of affection. In societies where romantic love is not as sharply defined or highly valued, the specific need that kissing solves is less present. Other practices—such as different forms of affection, ritual, or sex—may carry the load instead. Conversely, as societies change—economically, demographically, ideologically—and romantic love becomes more salient, we would expect to see increases in both how much people talk about romantic love and how central romantic kissing becomes as a symbol of it. Our proposal is that romantic kissing becomes common and symbolically powerful in cultures where romantic love is treated as a distinct and important relationship category and where people need a clear, intuitive way to express and signal that specific bond.

Given that humans share intuitions about intimacy, lip-to-lip kissing—with very close bodily contact, synchrony, and the exchange of saliva—fits those needs extremely well. We argue that people could easily learn that kissing is a way to communicate that “this relationship is special.” Kissing itself also likely triggers intense feelings of being moved or deeply connected, which further reinforces its role. In that sense, romantic kissing functions as a cultural technology: it leverages universal intuitions to address a local social problem—how to signal, deepen, and display romantic love.

It is also possible that romantic kissing is costly. Kissing is not just intimate; it’s risky. It involves the face and mouth, carrying a risk of injury (from biting) and pathogen transmission through saliva sharing. From a signaling perspective, that’s exactly what you might want in a commitment device: something you wouldn’t do with just anyone. From an evolutionary perspective, romantic love can be a high-stakes relationship because it involves commitment to a single person. Costly signals may help distinguish genuine commitment from casual interaction.

In the end, we don’t think romantic kissing is “in our genes” in any simple sense. Nor do we think it’s a mere historical accident. Instead, we see it as a recurring cultural solution built on universal knowledge of what it means to be close to someone. By studying infants, cross-cultural data, and historical change together, we can begin to see why certain practices—like kissing—are common but not universal.

Hossein Samani and Ashley J. Thomas. 2026. Abstract core knowledge may shape the basins of cultural attraction: romantic kissing as a case study. Evolution and Human Behavior 47 (2): 106810.

Do the damaging effects of the Epstein Barr virus, the cause of the Kissing Disease, weaken the idea that kissing is an adaptation favored by sexual selection?

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– by Paul Ewald

In modern societies kissing is a widespread and important aspect of social interactions. It appears to be a sexually selected adaptation that facilitates mate evaluation, pair bonding, and/or sexual activity. This evolutionary explanation is paradoxical because kissing seems unnecessary for mate selection and is associated with transmission of diseases, which might outweigh the evolutionary advantages of kissing. The most widely recognized of these diseases is infectious mononucleosis, commonly known as the Kissing Disease. Although people with mononucleosis experience fever, body aches, and extreme fatigue, the illness itself is almost never life-threatening. People generally recover within a few weeks without any specific treatment. If that were the whole story, suffering infectious mononucleosis as a consequence of kissing would not be compelling evidence against the hypothesis that kissing is a sexually selected adaptation. However, the mounting knowledge about EBV-associated diseases indicates that kissing has more serious consequences.

The Epstein Barr virus was first accepted as the primary cause of infectious mononucleosis in 1968. About a decade later its major route of transmission in affluent populations was generally recognized to be salivary exchange during intimate kissing. Over the past half-century, EBV has been associated with severe diseases such as cancers, multiple sclerosis and systemic lupus erythematosus. Together with infectious mononucleosis, these diseases would seem to generate fitness costs that could offset or even completely negate the mating advantages of kissing, and make an adaptive explanation of kissing questionable.

If these diseases were not present during the evolutionary history of humans, this concern would disappear.

Comparisons across populations help clarify whether intimate kissing has been associated with these diseases during our evolutionary history and therefore with a high fitness cost. Specifically, if these diseases tend to occur only in modern populations with particular attributes, then their presence would not weaken the hypothesis that kissing evolved as a sexually selected adaptation.

The existing evidence indicates that infectious mononucleosis occurs primarily in modern affluent populations in which environmental disinfection is emphasized. In areas with poor hygiene, EBV infections tends to occur prior to adolescence and rarely causes infectious mononucleosis. When infectious mononucleosis does occur, it tends to be more mild than when it occurs in affluent populations. The general explanation for this difference is that kids tend to be exposed to low doses of EBV in areas with poor hygiene and therefore develop some immunity that protects them from the high doses that are associated with intimate kissing. Comparative evidence therefore indicates that infectious mononucleosis has been largely a recent consequence of EBV infection resulting from hygienic activities that increase the probability of severe infections from kissing. This conclusion has ramifications for understanding the fitness costs of kissing that arise from the more serious EBV-associated diseases.

EBV has been strongly associated with breast cancer. Comparisons of EBV positivity in breast cancers relative to control breast tissue together with effects of EBV on cellular proliferation indicate that EBV plays a causal role in about 20% of breast cancer. EBV is also now generally accepted as a cause of multiple sclerosis. The associations of EBV with breast cancer and multiple sclerosis are statistically significant only among patients who had experienced infectious mononucleosis. Because mononucleosis is a modern disease associated with delayed EBV infection, the associations of these diseases with mononucleosis indicates that they too are modern diseases associated with delayed EBV infection.

EBV is now a generally accepted cause of about half of Hodgkin’s lymphoma. Breast cancer is associated with young adult Hodgkin’s lymphoma, which tends to occur within two to three years after infectious mononucleosis in EBV positive tumors. EBV-negative tumors show no association with time since infectious mononucleosis. Like breast cancer, Hodgkin’s lymphoma therefore is associated with the more severe EBV infections in modern affluent populations.

EBV is now generally accepted as a cause of systemic lupus erythematosus. Although studies have not assessed whether lupus is restricted to individuals who have had infectious mononucleosis, lupus, like multiple sclerosis, tends to occur in affluent modern populations. The main genetic risk factor for EBV-associated multiples sclerosis is also a risk factor for lupus. It codes for a protein that EBV uses as a receptor to enter cells and became prevalent about 5000 years ago. This information suggests that lupus, like multiple sclerosis, would not have been a factor disfavoring kissing during the broader span of human evolution.

These findings indicate that the relationship between kissing transmitted EBV and these severe diseases is relatively recent, and that these diseases therefore did not generate fitness costs that would have selected against intimate kissing during the evolutionary history of Homo sapiens. The flip side of this conclusion is that intimate kissing is now more unsafe than it used to be during our evolutionary history, and humans have not evolved to be sufficiently wary of the present-day dangers of intimate kissing.

Paul W. Ewald. 2026. Epstein Barr virus, infectious mononucleosis and associated diseases as contributors to the costs of intimate kissing. Evolution and Human Behavior 47 (2): 106817.

When animals “kiss”: what nose-to-nose contact can tell us about behaviour and evolution

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– by Sophie Lund Rasmussen

Kissing is often considered one of the most human of behaviours.

It feels intimate. Emotional. Social. And perhaps even a little mysterious. Why do we do it?

Interestingly, lip-to-lip kissing is actually very rare in the animal kingdom—only documented in humans, chimpanzees, and bonobos. But if we take a step back and look more broadly, many other mammals engage in a different kind of close-contact behaviour: nose-to-nose contact.

At first glance, it looks disarmingly similar. Two animals gently touching faces. It’s easy—perhaps inevitable—to interpret it as affection.

But as is often the case in behavioural biology, what we think we are seeing and what is actually happening may be very different.

A behaviour hiding in plain sight

Nose-to-nose contact is surprisingly widespread across mammals—from solitary hedgehogs to highly social bats, pigs, and even eusocial naked mole-rats. Yet, despite being so visually obvious, it has received remarkably little scientific attention.

That gap was the starting point for this study.

By taking a comparative approach—bringing together published observations and field insights across mammal species with very different social systems—I wanted to ask a simple question:

Does nose-to-nose contact serve the same suggested purposes across species—or is it something entirely different depending on context and levels of social organisation?

Information exchange

One of the clearest patterns that emerges is that the same physical behaviour can have very different functions depending on the species.

At its core, nose-to-nose contact is about information exchange.

Mammals rely heavily on chemical cues. Through close contact, individuals can assess each other’s identity, health, reproductive status, and even social rank.

But what animals do with that information varies dramatically.

The awkward encounter of hedgehogs

Take the European hedgehog.

If you imagine two hedgehogs touching noses, it is tempting to interpret it as something friendly—even affectionate. But the reality appears far less romantic.

Hedgehogs are solitary animals. They generally avoid one another and only interact under specific circumstances, such as mating or when feeding at feeding stations with cat food provided in residential gardens.

When nose-to-nose contact does occur, it seems almost accidental—arising during mutual sniffing rather than as an intentional social gesture.

And their reaction is striking.

After touching noses, hedgehogs often recoil, freeze, and appear momentarily “switched off,” as if overwhelmed by the sensory input. They may remain in this state for up to a minute, seemingly unaware of their surroundings.

From an evolutionary perspective, this is puzzling.

Why engage in a behaviour that temporarily leaves you vulnerable to predators?

One possibility is that the information gained is so valuable that it outweighs the risk. The contact may provide a rapid and concentrated burst of chemical signals—allowing individuals to quickly assess whether another hedgehog is a threat, a potential mate, or something to avoid.

Not exactly a romantic moment.

From information to social bonding

In more social species, however, nose-to-nose contact takes on a very different role.

In bats, for example, similar behaviours appear to function as greeting rituals and help reinforce social bonds when individuals reunite.

In pigs, nose-to-nose contact is part of a broader repertoire of “social nosing” behaviours. These interactions are associated with reduced aggression and even improved growth rates in piglets—suggesting a link between tactile social contact and physiological benefits.

Here, the behaviour is not just about gathering information—it is also about maintaining relationships.

In other words, something that may begin as a sensory mechanism could, over evolutionary time, become a social tool.

When contact becomes control

The story becomes even more intriguing in eusocial species like the naked mole-rat.

In these animals, nose-to-nose contact is part of a behaviour known as “shoving,” where dominant individuals—particularly the breeding female—physically push subordinates.

This behaviour plays a role in maintaining the colony’s social structure, including suppressing reproduction in non-breeding individuals.

What looks like a simple face-to-face interaction is, in this case, tied to power, hierarchy, and reproductive control.

It’s a striking example of how a behaviour can be repurposed—what evolutionary biologists call exaptation—from one function (information exchange) to something entirely different (dominance enforcement).

A continuum of meaning

Taken together, these examples suggest that nose-to-nose contact exists along a continuum depending on the level of social organisation:

  • In solitary species: primarily incidental and sensory
  • In social species: affiliative and communicative
  • In eusocial species: regulatory and hierarchical

The behaviour itself remains broadly similar. But its meaning—and its evolutionary function—appears to shift depending on the social and ecological context.

Why this matters

At first glance, studying nose-to-nose contact might seem like a niche curiosity.

But it highlights something much broader.

Behaviour is not fixed. It is flexible, context-dependent, and shaped by both evolutionary history and current social environments.

The same physical action can mean entirely different things depending on who is doing it—and why.

Sophie Lund Rasmussen 2026. Exploring nose-to-nose contact in mammals. Evolution and Human Behavior, Volume 47, Issue 1, 2026, 106809.

What the replication of experiments does and doesn’t achieve

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– by Stuart West & Max Burton-Chellew

Replication or repeating of experiments is a key part of the scientific methodology. It increases your trust in that result. It shows that the result was not just due to some unnoticed bias or a particular setup. This helps to filter out false positives and expose questionable research practices. That is great.

Suppose, however, that you had developed a hypothesis for a result. In that case, repeating the same experiment might not increase your confidence in that hypothesis. If your aim is hypothesis testing then it can be more useful to alter the experiment, so that you can test between competing hypotheses or make a strong attempt to falsify a hypothesis.

Our paper was about what happens if these two points get confused. This could happen anywhere, but we illustrate the potential problem with the literature on public goods games.

The public goods game is an economic experiment designed to investigate the tension between an individual’s selfish interest and the interest of the group. Individuals can contribute money to a group fund. The group fund is multiplied by a factor and then divided between players. Crucially, the amount it is multiplied means that everyone gets back less from their contribution than they initially contributed. Consequently, while everyone would do best if everyone contributed maximally, individuals maximise their personal income by not contributing.

But when people play the public goods game, they do contribute. On average, individuals contribute a significant amount. If they play the game multiple times they contribute less, but they keep contributing. This is an incredibly strong result – the experiment has been repeated >100 times. The common conclusion is that because individuals contribute >0%, this shows that humans have ‘prosocial preferences’ which make them cooperate even when it is not in their best interest.

Can you see a potential problem with this conclusion? Individuals would only appear to behave as if they are trying to maximise their personal gain by contributing nothing (0%). Any other amount (>0%) can be argued to reflect pro-social preferences. And it is possible to come up with a huge number of alternative hypotheses for why individuals might contribute >0% (see figure above). ‘Pro-social’ preferences is a hypothesis not an unavoidable conclusion. Alternative hypotheses include, but are not limited to:

  • Individuals are prone to playing as if it was the real world, where there would be repeated interactions, and hence the opportunity for reciprocal helping.
  • Individuals might consider extreme strategies like 0 or 100% as risky.
  • Individuals might start a bit confused and want to use the game to learn how to best maximise their gain.
  • Individuals might want to explore the options available.

So, while repeating the basic public goods game experiment was great for proving the robustness of the initial result, it doesn’t help distinguish between competing hypotheses. To do that, you need to adjust your experiment. You could play the game in a way that removes the potential concern for others. For example, by making individuals play with computers or by not letting them know about the game they are playing (contributions are to a ‘black box’). Or you could change the game so that the best selfish option is to contribute >0%. Or could analyse individual behaviour and see what makes them alter their contribition. And so on.

Another way of thinking about this issue is that we need to get to the right answer by as rigorous a scientific method as possible. This can involve actively developing different controls and null hypotheses. We need to work hard to try to falsify hypotheses and test competing hypotheses.

That is our main point, and you can stop here. If you want to know the gory details from public goods games, how different controls can be developed, and how to test competing hypotheses then go and read our paper. You might also be wondering what would happen if this approach overturned the accepted conclusions. No spoilers, but there is an obvious comparison to a folktale by Hans Christian Anderson: The Emperor’s New Clothes.

Stuart A. West & Maxwell N. Burton-Chellew. (2026). Replication of experiments and the canonisation of incorrect conclusions. Evolution & Human Behavior, 46, 106749.

HBES 2026: Registration is now open (and on-campus accommodation requests are live)

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Dear HBES colleagues,

Below find important information for the HBES conference:

Registration: The registration for the 2026 meeting in Rabat, Morocco, is now open! Please visit the website for information regarding registration, or directly access the registration form. The early bird period will conclude on the 29th of March 2026. Online attendance is possible. If you wish to attend but not present at the CES conference, you can do so at a fixed discounted fee (use the HBES registration form to do so). You can also buy up to 2 tickets for the conference gala dinner, which will take place at the beautiful Rabat Story Hotel – Le Carousel (shuttle from campus included) on the evening of May 16th. Also available are two excursions to Roman ruins–the Chellah in Rabat and the Volubilis ruins closer to Meknès (both UNESCO World Heritage Sites)–planned for May 14th, to which you can also register (transportation and guide included).

Accommodation: The conference is hosted on the Rabat (Salé) campus of the UM6P. The campus offers a range of accommodation options, including the campus hotel and student residences. Prices vary based on the chosen option. Student residences are highly recommended for student participants as they are very cheap. Both are within walking distance of the event venue (approximately 3 minutes). Please note that children, while admitted on campus, are not allowed to reside in the on-campus accommodations. You can book your on-campus accommodation with this form (subject to availability).

Off-campus accommodations are available (see the website for suggestions). A regular shuttle service will pick participants near the Onomo Hotel downtown Rabat (in the Hassan neighbourhood: location here). The stop is also close to a tramline linking with hotels located in Agdal (such as the Marriott or First Suites hotel; about 10 minutes). An additional shuttle service may be provided based on the preferred location of the participants. Please note that the shuttle service is located about 20-30-minute walk uphill if you choose to reside in the Old Medina.

We are also issuing an additional call for Special Activities (e.g., workshops, training sessions, roundtables, networking events, interest-group meetups, and other community-building programming). If you would like to propose a Special Activity, please review the guidelines below and submit a proposal by email (to: HBES2026@um6p.ma). Submissions are due March 15th.

Childcare services: A free childcare center will be organised for the conference, for children from 11 months to 6 years of age. To book a space contact the organizers directly at HBES2026@um6p.ma by March 16th 2026. A discreet changing and breastfeeding space will also be provided (no booking required).

Call for activities proposals: You are invited to submit proposals for two activities to be held during the conference. All activity proposals and further questions should be sent to HBES2026@um6p.ma More details on Special Activities (including formats, expectations, and submission instructions) are included below and available at this link.

Warmly,

Zach Garfield, Ed Seabright, Sarah Alami, Mathieu Charbonneau, HBES 2026 Organizing Committee
Conference website

HBES2026 – Call for activity proposals

You are invited to submit proposals for two activities to be held during the upcoming HBES 2026 conference:

1.⁠ ⁠Lunchtime Panel Discussion

We invite proposals for a panel discussion (about 90 minutes) to be held during lunch, tentatively on the final day of the conference (May 16th). This panel should address an important topic for the discipline that is not strictly scientific but highly relevant, such as issues of representation and inclusion, ethics or equity in the field, history of the field, and broader issues shaping disciplinary culture. Proposals will be reviewed, and only two panel submissions will be selected for inclusion in the program.

Deadline: March 15th, 2026

2.⁠ ⁠Lunch Across Societies – Joint with CES

Held on the joint/overlapping day with CES (May 13th), Lunch Across Societiesoffers an informal space for small-group scholarly exchange aimed at fostering cross-societal dialogue and intellectual exchange in an informal setting. We welcome proposals for brown-bag–style interactive sessions (about 90 minutes), including methods clinics, special-interest or thematic discussions, discussions aimed at supporting junior researchers in their networking and professional development, or any other community-building discussions. Proposals will be reviewed, and a limited number of activities will be selected.

Deadline: March 15th, 2026

Submission Guidelines for 1 and 2 

Please include a short description (max 250 words) of your proposed activity or panel, outlining the following:

  • ⁠Format and topic
  • ⁠Intended audience and relevance
  • ⁠Any logistical needs

All activity proposals, video submissions, and further questions should be sent to HBES2026@um6p.ma. Please include your name and HBES2026 – Call for activities subject line.

Whom Should We Help? People Track Shared Fate to Solve Cooperation Dilemmas

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– by Diego Guevara Beltran

English

Imagine you live in a small community where food is often scarce; storms, droughts, pests, and fires can destroy crops, disease and injuries can keep you from working, and help is often the difference between getting by and going hungry. Every day, you face decisions about cooperation: Whom should I help? How much should I give? And who will help me when I need it? These are not abstract moral questions. They are practical problems that humans have faced throughout evolutionary history. And, they raise a fundamental question about evolution and human behavior: how do people decide whom to help when cooperation is costly and resources are limited? Two long-standing answers are kinship and reciprocity. But these explanations leave some important things unresolved. People often cooperate with non-kin, even when reciprocity is uncertain. Not only that, but in natural fertility populations, people often have more available kin (e.g., cousins, siblings, in-laws) than they could realistically afford to cooperate with. So, what psychological mechanisms allow people to flexibly navigate these partner-choice dilemmas? Drawing from biological markets and fitness interdependence theory, my colleagues and I argue that a key part of the answer lies in how people perceive shared fate.

Shared fate is the extent to which people believe that another person’s outcomes—good or bad—will affect their own. If your sibling falls ill, that may directly affect your workload, your food security, your coalitional strength, or your childcare needs. If your fishing partner succeeds, you may eat better too. In contrast, if an acquaintance struggles, their misfortune may have little consequence for you. From an evolutionary perspective, shared fate is a psychological estimate of fitness interdependence—the degree to which two individuals’ survival and reproduction are yoked. The central idea is this: people should be more willing to help those with whom they share positive fitness stake. But where do these perceptions come from, and what cues do people use to infer shared fate?

To address this question, I conducted fieldwork among the Mayangna, an indigenous small-scale society in northern Nicaragua. The Mayangna rely primarily on horticulture, subsidized by fishing, domestic animals, limited hunting, and sparse or seasonal wage labor. For the Mayangna, as is the case for many subsistence societies, recurrent risks such as food scarcity, illness, and disasters are part of everyday life. Absent wage-labor security or institutional support, cooperation is often the best available insurance against life’s many challenges. We interviewed 146 adults and asked them about their relationships with three types of people: an acquaintance, a cousin, and a sibling. For each relationship, we measured: (1) Ten sources of interdependence, such as relatedness, childhood co-residence, eating together, farming, hunting or fishing, labor sharing (i.e., household construction), shared religion, and experiencing disasters (i.e., floods/hurricanes) together; (2) Perceived shared fate, measured with items like “What is good for [target] is good for me” and “When [target] succeeds, I feel good”; and (3) Cooperation across seven fitness-relevant domains, including giving meat or fish, sharing crops, cooking or preparing meals, helping children, helping with household construction tasks, assisting after disasters, and helping someone harmed by outsiders.

We then tried to address a straightforward question: which sources of interdependence shape perceptions of shared fate, and do those perceptions guide cooperation? As you might suspect, not all interdependence cues are made equal. Most sources of interdependence were correlated with higher shared fate between participants. But, when we examined which cues were most diagnostic across relationships within participants, three stood out: relatedness, commensality (regularly eating together), and shared subsistence activities (i.e., planting/harvesting, and hunting or fishing). These are not arbitrary cues. They are attributes or activities that directly tie people’s outcomes together. Kinship links long-term fitness interests; while eating, farming, and foraging together not only allow people to pool risks, but such activities may also communicate to partners that we value their welfare above others. Importantly, other plausible cues—such as childhood co-residence or sharing the same religion—were not associated with shared fate once the core sources were accounted for. This suggests that shared fate is not simply about tracking social closeness or group affiliation per se. Shared fate is about tracking fitness-relevant cues of interdependence.

Perceived shared fate, in turn, was strongly associated with cooperation. People were more likely to help those with whom they perceived higher shared fate across every domain we measured—from sharing food to helping after floods. What’s more, shared fate statistically mediated the association between relatedness and helping. This suggests that shared fate is a key psychological mechanism by which kinship structures much of the cooperation observed within Mayangna communities—and perhaps across other human societies too. Finally, we wanted to test whether shared fate was associated with actual costly behavior, not just self-reports. We ran a follow-up study with a subset of participants, and we gave them a real choice: keep money for themselves or use it to buy rice for a specific partner. Shared fate was strongly associated with choosing to give up money to help someone else.

Although one interpretation of this work is that our findings simply reaffirm the importance of kinship for cooperation, we argue that such conclusion would be too narrow. The broader implication is that humans appear to possess a flexible psychological system for estimating cues of fitness interdependence, one that integrates multiple cues according to ecologically relevant affordances. In societies where cooperation is largely structured by kinship, as is the case for the Mayangna, shared fate should track kinship closely. In societies where fitness depends on cooperation with non-kin—such as friends, neighbors, or labor/exchange partners—shared fate should track such corresponding cues, including a history of shared labor, risk pooling, or even cooperation itself.

In sum, perceived shared fate may offer a simple solution to partner choice dilemmas: help those whose welfare is linked to yours. Following this heuristic allows individuals to mitigate opportunity costs, protect themselves from the costs of defection, and reap the rewards of cooperation. Understanding how people come to perceive shared fate with others could help us understand not only cooperation in small-scale societies, but also friendship, social support, and collective action in response to larger scale disasters. When people feel that their futures are intertwined, cooperation follows. When that sense of shared fate erodes, so too does willingness to help. Understanding shared fate could be as relevant for understanding how people in modern nations deal with the shadow of disasters (e.g., climate change, intergroup conflict) as it is for small-scale societies past and present.

Spanish

¿A quién deberíamos ayudar? Las personas monitorean el destino compartido para resolver dilemas de cooperación

Imagina que vives en una comunidad donde los alimentos suelen ser escasos; tormentas, sequías, plagas e incendios pueden destruir las cosechas; las enfermedades y las lesiones pueden impedir que trabajes; y la ayuda suele marcar la diferencia entre salir adelante o pasar hambre. Todos los días enfrentas decisiones sobre la cooperación: ¿a quién debería ayudar? ¿Cuánto debería dar? ¿Y quién me ayudará cuando yo lo necesite? Estas no son preguntas morales abstractas. Son problemas prácticos que los seres humanos han enfrentado a lo largo de la historia evolutiva. Y plantean una pregunta fundamental sobre la evolución y el comportamiento humano: ¿cómo deciden las personas a quién ayudar cuando cooperar es costoso y los recursos son limitados? Dos respuestas clásicas son el parentesco y la reciprocidad. Pero estas explicaciones dejan asuntos importantes sin resolver. Las personas a menudo cooperan con personas nada aparentadas, incluso cuando la reciprocidad es incierta. Además, en poblaciones de fertilidad natural, las personas suelen tener más parientes disponibles (por ejemplo, primos, hermanos, cuñados) de los que podrían permitirse ayudar de manera practica o realista. Entonces, ¿qué mecanismos psicológicos permiten a las personas navegar de forma flexible estos dilemas sociales? A partir de los mercados biológicos y de la teoría de la interdependencia de aptitud (fitness), mis colegas y yo sostenemos que una parte clave de la respuesta reside en cómo las personas perciben el destino compartido.

El destino compartido es el grado en que las personas creen que los resultados de otra persona —buenos o malos— afectarán los propios. Si tu hermano se enferma, eso puede afectar directamente tu carga de trabajo, tu seguridad alimentaria, tu fortaleza de grupo o tus necesidades de cuidado infantil. Si tu compañero de pesca tiene éxito, tú también podrías comer mejor. En contraste, si un conocido pasa por dificultades, su infortunio puede tener pocas consecuencias para ti. Desde una perspectiva evolutiva, el destino compartido es una estimación psicológica de la interdependencia de aptitud: el grado en que la supervivencia y la reproducción de dos individuos están ligadas. La idea central es simple: las personas deberían estar más dispuestas a ayudar a quienes comparten con ellas una apuesta positiva de aptitud. Pero ¿de dónde provienen estas percepciones y qué señales utilizan las personas para inferir el destino compartido?

Para abordar esta pregunta, realicé trabajo de campo entre los Mayangna, una sociedad indígena de pequeña escala en el norte de Nicaragua. Los Mayangna dependen principalmente de la horticultura, complementada por la pesca, animales domésticos, caza limitada y trabajos asalariados escasos o estacionales. Para los Mayangna, como para muchas sociedades de subsistencia, riesgos recurrentes como la escasez de alimentos, la enfermedad y los desastres forman parte de la vida cotidiana. En ausencia de seguridad laboral asalariada o de apoyo institucional, la cooperación suele ser la mejor póliza de seguro disponible frente a los múltiples desafíos de la vida. Entrevistamos a 146 adultos y les preguntamos sobre sus relaciones con tres tipos de personas: un conocido, un primo y un hermano. Para cada relación medimos: (1) diez fuentes de interdependencia, como el parentesco, la co-residencia durante la infancia, comer juntos, cultivar, cazar o pescar, compartir trabajo (es decir, construcción del hogar), religión compartida y experimentar desastres juntos (es decir, inundaciones y huracanes); (2) el destino compartido percibido, medido con ítems como “Lo que es bueno para [esta persona] es bueno para mí” y “Cuando [esta persona] tiene éxito, me siento bien”; y (3) la cooperación en siete dominios relevantes para la aptitud, incluyendo dar carne o pescado, compartir cosechas, cocinar o preparar alimentos, ayudar con el cuidado de los niños, colaborar en tareas de construcción del hogar, asistir después de desastres y ayudar a alguien perjudicado por personas externas.

Luego abordamos una pregunta directa: ¿qué fuentes de interdependencia de aptitud moldean las percepciones de destino compartido, y esas percepciones guían la cooperación? Como cabría esperar, no todas las señales de interdependencia son iguales. La mayoría de las fuentes de interdependencia se correlacionaron con un mayor destino compartido entre participantes (es decir, aquellos que mencionaron altas fuentes de interdependencia en general mencionaron percepciones de destino compartido más alto). Sin embargo, al examinar cuáles señales eran más diagnósticas a través de las relaciones dentro de cada participante, destacaron tres: el parentesco, la comensalía (comer juntos de manera regular) y las actividades de subsistencia compartidas (es decir, sembrar y cosechar, y cazar o pescar). Estas no son señales arbitrarias. Son atributos o actividades que vinculan directamente los resultados de las personas. El parentesco conecta intereses de aptitud a largo plazo; mientras que comer, cultivar y recolectar juntos no solo permite compartir riesgos, sino que también puede comunicar a nuestros socios que valoramos su bienestar por encima del de otros. De manera importante, otras señales plausibles —como la co-residencia en la infancia o compartir la misma religión— no se asociaron con el destino compartido una vez que se tuvieron en cuenta las fuentes centrales. Esto sugiere que el destino compartido no se trata simplemente de monitorear la cercanía social o la afiliación grupal per se. Se trata de monitorear señales de interdependencia relevantes para la aptitud.

El destino compartido percibido, a su vez, se asoció fuertemente con la cooperación. Las personas fueron más propensas a ayudar a quienes percibían con mayor destino compartido en todos los dominios que medimos, desde compartir alimentos hasta ayudar después de inundaciones. Además, el destino compartido medió estadísticamente la asociación entre parentesco y la ayuda. Esto sugiere que el destino compartido es un mecanismo psicológico clave mediante el cual el parentesco estructura gran parte de la cooperación observada en las comunidades Mayangna —y quizás también en otras sociedades humanas. Finalmente, quisimos probar si el destino compartido se asociaba con conductas costosas reales, no solo con autoinformes. Realizamos un estudio de seguimiento con un subconjunto de participantes y les dimos una elección real: quedarse con el dinero o usarlo para comprar arroz para una persona en específico. El destino compartido se asoció fuertemente con la decisión de renunciar al dinero para ayudar a la otra persona.

Aunque una interpretación de este trabajo es que nuestros hallazgos simplemente reafirman la importancia del parentesco para la cooperación, sostenemos que esa conclusión sería demasiado estrecha. La implicación más amplia es que los humanos parecen poseer un sistema psicológico flexible para estimar señales de interdependencia de aptitud, que integra múltiples señales de acuerdo con las oportunidades ecológicamente relevantes. En sociedades donde la cooperación está ampliamente estructurada por el parentesco, como ocurre entre los Mayangna, el destino compartido debería seguir de cerca al parentesco. En sociedades donde la aptitud depende de la cooperación con personas poco o nada aparentadas —como amigos, vecinos, o socios de trabajo o de intercambio— el destino compartido debería monitorear señales correspondientes, incluyendo una historia de trabajo compartido, la puesta en común de riesgos o incluso la cooperación misma.

En suma, el destino compartido percibido puede ofrecer una solución simple a los dilemas de cooperación: ayudar a quienes tienen su bienestar ligado al propio. Seguir esta heurística permite a las personas mitigar costos de oportunidad, protegerse de los costos de la deserción y obtener los beneficios de la cooperación. Comprender cómo las personas llegan a percibir el destino compartido con otros puede ayudarnos a entender no solo la cooperación en sociedades de pequeña escala, sino también la amistad, el apoyo social y la acción colectiva frente a desastres de mayor escala. Cuando las personas sienten que sus futuros están entrelazados, la cooperación sigue. Cuando ese sentido de destino compartido se erosiona, también lo hace la disposición a ayudar. Comprender el destino compartido puede ser tan relevante para entender cómo las personas en sociedades modernas enfrentan la sombra de los desastres (por ejemplo, el cambio climático o los conflictos intergrupales) como para las sociedades de pequeña escala del pasado y del presente.

Link to article

Diego Guevara Beltran, Jessica D. Ayers, Lee Cronk, Daniel P. Balliet, Jeremy Koster, & Athena Aktipis. (2026). Sources of fitness interdependence associated with shared fate and cooperation in a small-scale horticultural society. Evolution & Human Behavior, 47, 106802.

Decreased Sexual Motivation during the Human Implantation Window

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– by James R. Roney

A lot of research has investigated shifts in women’s psychology and behavior across different phases of the menstrual cycle. One pattern that has emerged in multiple studies is an increase in sexual desire during the “fertile window,” which is defined as the cycle days when unprotected intercourse can lead to conception. In humans, the fertile window extends from about 5 days before ovulation through the day of ovulation itself, with the ovulation day usually falling near the mid-point of a typical menstrual cycle.

Why should desire go up during the fertile window? Motivational priorities theory argues that hormonal signals tend to increase sexual motivation when the net fitness benefits of sex were highest during human evolution. Fitness benefits were outcomes associated with increased gene replication, whereas fitness costs were outcomes associated with decreased gene replication; net benefits were benefits minus costs. Sex would have had fitness costs for our female ancestors: it could have caused injury, infection, social disapproval from competitors, or just distraction from other behaviors. Conception would have been a fitness benefit from sex: indeed, gene replication is not possible without it. Because conception was possible only during the fertile window, however, there was an increase in the benefit side of the fitness equation during the fertile window that may have acted as a selection pressure to increase sexual motivation at that time.

This simple cost-benefit analysis may explain why most of the evolutionary literature on menstrual cycle shifts—including my own research—has focused primarily on the fertile window. But what if certain fitness costs of sex were not constant across the cycle, as perhaps generally assumed, but instead increased during specific time windows? My co-authors and I explored that possibility, reasoning that increased infection risk during the “implantation window” may have selected for reduced sexual motivation at that time.

The implantation window refers to those days when the endometrium (the uterine lining) is receptive to attachment by an embryo. (The image at the top of this post illustrates the attachment process.) Part of being receptive entails a reduction of immune responses within the uterus to prevent the immune system from attacking an embryo if conception has occurred. However, that same immunosuppression may make women more vulnerable to sexually transmitted infections (STIs) at this time. As reviewed in our article, progesterone appears to be a key signal that helps to cause such immunosuppression, and both experiments in nonhuman species and correlational research in humans supports increased susceptibility to some types of STIs when progesterone is elevated. That increased infection risk should have increased the cost side of the benefits minus costs fitness equation during the implantation window, thus acting as a selection pressure to reduce sexual motivation at that time.

Based on the above reasoning, we tested whether measures of women’s sexual motivation were lower during the estimated implantation window. The implantation window is estimated to run from about 5 to 9 days after ovulation in humans, during an infertile region of the cycle called the luteal phase. We used data from three daily diary studies that had previously been conducted in my lab. In each study, women responded to daily online surveys across full menstrual cycles. The surveys included measures of subjective sexual desire and reports of whether women masturbated, which were chosen as our primary measures of sexual motivation. Hormonal estimates of the day of ovulation were also available in each study; by counting forward from the day of ovulation, we could estimate the implantation window days within each cycle.

Across the three studies, there were 2576 survey responses across 102 ovulatory menstrual cycles sampled from 83 women. Our data analyses used statistical techniques to estimate, within the same women, whether responses differed on implantation window days relative to other cycle regions. (Imagine, for instance, for one woman, subtracting her average desire during the implantation window from her desire on all other days, and then averaging that difference across all women; our multi-level regression models made conceptually analogous comparisons.)

Our results provided evidence for lower self-reported sexual desire during the implantation window relative to the remaining days of the cycle. This was true in each of the three studies and also in the combined sample. Importantly, desire in the combined sample was lower in the implantation window relative to cycle regions that excluded days within the fertile window. That result demonstrates that lower desire during the implantation window was not simply an artifact of comparisons with elevated desire during the fertile window. Results for our behavioral measure of sexual motivation were slightly less clear, perhaps because about 40% of our sample reported no incidents of masturbation. Nonetheless, in the combined sample, there were one-third lower odds of masturbation during the implantation window relative to all other cycle days.

These findings add a new wrinkle to research on cycle phase shifts in women’s sexuality. Hormonal mechanisms may generate changes in sexual motivation within two conceptually significant windows of time: increased desire within the fertile window when conception is possible, and decreased desire during the implantation window when infection risk is elevated. There may be alternative explanations for why desire is reduced during the implantation window—some of which we considered in our article—and we hope these findings encourage further research into whether there are adaptations that modulate sexual motivation based on implantation window timing.

Roney, J. R., Simmons, Z. L., Mei, M., Grillot, R. L., & Emery Thompson, M. (2025). Decreased sexual motivation during the human implantation window. Evolution and Human Behavior, 46, 106761.

Agent-based simulations of the evolution of psychopathy

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– by Dražen Domijan

Agent-based simulations offer valuable insights into processes that are difficult or impossible to observe directly. They enable systematic variation of relevant factors and examination of their influence on the behavior of simulated agents. For example, one can examine and compare how the same trait evolves under favorable conditions in an environment with abundant resources, or in a harsh environment where resources to sustain life are scarce. In this work, we were inspired by a recently developed agent-based model by Dr. Martina Testori and her colleagues. We are grateful to Dr. Testori for openly sharing their simulation code, which increases the transparency and reproducibility of computational studies and also significantly reduces the time required to develop our extended model.

Testori et al. adopted a public goods game in which generous and selfish agents interact to produce resources that are shared equally among all members of the society. Agents then reproduce and are replaced by their offspring in the next generation. The number of offspring each parent produces is proportional to their fitness. If an agent’s fitness is below the survival threshold, the agent dies without leaving offspring. Reproduction is governed by a probabilistic mechanism that incorporates genetic propensity for selfishness and behavioral contribution to the development of selfish and generous personality traits. This mechanism enables offspring to inherit their parents’ phenotype or to switch to the opposite phenotype with a certain probability of mutation.

To simulate psychopathic traits in more detail, Testori et al. introduced two additional behavioral components of the psychopathic personality, beyond mere selfishness. Firstly, psychopathic individuals tend to engage in risky behaviors that may lead to their premature deaths and removal from society. In the model, this is represented by a parameter called the mortality rate, meaning that a fraction of psychopaths were eliminated from the game before they had the opportunity to reproduce. Secondly, psychopathic individuals tend to engage in destructive acts that harm public goods, thereby imposing a community cost on available resources. The mortality rate and community cost are free parameters that are allowed to vary independently, enabling their effects on the evolution of psychopathy to be assessed in simulations.

When we began work on this project, our attention was immediately drawn to the finding that, in most reported simulations in the Testori et al. study, selfish agents dominated society. In some combinations of mortality rate and community cost, they constitute more than 80% of the population. However, this is not a plausible evolutionary outcome, as a recent meta-analysis suggests that the frequency of psychopaths in the human population is quite low. Estimates range from 1.2% to 4.5%, depending on the instrument used to assess psychopathy. Some authors even suggest a more conservative estimate of around 1% in the general population. This raises an important question: what societal forces keep the fraction of psychopathic individuals at such low levels?

Inspired by the work of Hiroaki Chiba-Okabe and Joshua Plotkin, we hypothesized that institutions could serve as a stabilizing factor that alleviate the adverse consequences of the actions of selfish agents. Here, institution should be understood in the broadest sense, involving any set of rules that regulate interactions among members of a society. In our model, the role of the institution is to act as an external agent that partially reallocates resources from psychopathic agents to cooperators, thus helping to reduce the resource inequality generated in the public goods game. In this way, the institution increases the fitness of generous agents who are disadvantaged in exchanges with selfish agents. We devised several scenarios in which the institution imposes punishment on selfish agents, or combines punishment of selfish agents with rewards for generous agents. We also considered the effect of conditional cooperation, that is, a scenario in which cooperators adjust their contributions to the public goods game by taking into account information about the level of cooperation in the previous generation.

Our simulations showed that both punishing selfish agents and rewarding generous agents were necessary to reduce the proportion of selfish agents in society to levels closer  to those observed empirically. Furthermore, these measures must be accompanied by a high mortality rate among selfish agents. However, although we approached our target, we were unable to achieve exactly 1% selfish agents in the final population. Our results suggest that selfish agents are highly resistant and manage to survive institutional interventions. This supports the conclusion that psychopathy may have adaptive value because it strongly contributes to fitness. This is consistent with several recent studies showing that certain features of psychopathy, especially manipulation and emotional coldness, are positively associated with fertility as a crucial component of evolutionary fitness. Interestingly, across simulations we also observed that population size increases as institutional interventions become stronger. Thus, institutional redistribution of resources contributes to population growth.

To keep our model as simple as possible, we made several simplifications and left many topics unaddressed. For example, we treated personality traits as if they were consistent with genetic makeup. However, it is possible that psychopaths alter their behavior in situations where psychopathic behavior is punished or prosocial behavior is rewarded. We modeled psychopathy as a single phenotype, although it is better described as a behavioral syndrome that can be subdivided into several distinct phenotypes. We also kept the environment constant across generations, although it may fluctuate over time. We hope our work will inspire further computational explorations of the societal and ecological conditions that shape the evolution of psychopathy.

Dražen Domijan & Janko Međedović. (2026). Evolution of psychopathy in the public goods game with institutional redistribution of resources. Evolution & Human Behavior, 47, 106771.

Engineering Fear: Protagonist Vulnerability and the Evolutionary Design of Horror

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– by Edgar Dubourg and Coltan Scrivner

What is it about some stories and situations that make them more effective at evoking fear? One way to answer this is to reverse engineer the emotion of fear.

Our ancestors would have come across a number of hostile people and animals throughout their lives. Quickly computing the odds of survival in the event of a conflict would have been an extremely valuable cognitive ability. The score for each side’s ability to win an all out fight has been conceptualized as formidability and has been successfully applied to understand emotions like anger. We argue that formidability may also help us understand the emotion of fear. Perceiving an adversary whose formidability far outmatches your own should trigger a computational appraisal of poor survival odds, perhaps felt as vulnerability. Fear should be one output of that appraisal, helping to coordinate defensive attention, arousal, and avoidance. Given this, stories that present a formidability asymmetry between the hero and villain such that the villain is powerful and the hero is weaker and more vulnerable should be better at evoking fear.

Horror fiction is the genre most closely associated with the emotion of fear — so much so, that the ability and intent to scare the audience is often found in definitions of the genre. If our hypothesis about formidability asymmetry and fear is correct, then the basic format of a horror story should be a powerful antagonist facing a vulnerable protagonist.

According to this hypothesis, it’s not only a weak protagonist or only a powerful antagonist that makes a story horrifying. Rather, it is the overall vulnerability of the protagonist. In Alien, (1979), a nearly perfect extraterrestrial predator stalking a commercial spaceship crew produces sustained fear because the crew is unprepared, poorly armed, and trapped, with limited information and no reliable escape. Predator (1987) presents a similarly powerful and predatory antagonist stalking a group of people. However, this group of people are an elite paramilitary team led by a character who is portrayed by former Mr. Universe, Arnold Schwarzenegger. As a result of the powerful protagonists, Predator feels much more like an action movie than a horror movie. For the same reason, superhero movies that feature powerful antagonists and loads of violence typically feel more like action rather than horror. Likewise, many films with low-formidability protagonists (e.g., Kevin from Home Alone) can end up in the comedy realm when the antagonists are also low formidability.

To test this hypothesis at scale, we relied on an automated annotation method that leverages the latent cultural knowledge of large language models. Rather than manually coding films one by one, we used a structured prompting procedure to have a language model estimate key narrative features for each movie, such as the formidability of both the protagonist and antagonist, the persistence of the threat, and the hostility of the environment.

Analyzing 691 films spanning ten broad film genres including horror, action, thriller, fantasy, and romance, we found a clear structural partition. Horror films were uniquely characterized by weak protagonists, strong antagonists, persistent threats, and hostile environments. Action, fantasy and thriller films, by contrast, often featured formidable antagonists but paired them with capable or well-equipped protagonists, resulting in much lower protagonist vulnerability. In other words, what separates horror from other genres is not the presence of danger, but the systematic imbalance between who threatens and who must endure the threat.

This raised a natural follow-up question: does protagonist vulnerability predict fear even outside the horror genre? To address this, we turned to viewer-generated data. Among films not labeled as horror, some are nonetheless widely described as frightening. Using IMDb’s keyword system, we compared non-horror movies that were tagged with fear-related terms to those that were not. The result was strikingly consistent: non-horror films associated with fear keywords showed significantly higher vulnerability scores. This suggests that the same structural ingredients that define horror also predict fear at a finer narrative level, beyond genre labels themselves.

Finally, we asked whether protagonist vulnerability tracks not just perceived fear, but also its associated physiological responses. Using data from an independent project that measured viewers’ heart rates while watching horror films, we found that horror movies with higher vulnerability scores reliably elicited stronger increases in heart rate. In short, narratives that place weak protagonists in the path of overwhelming threats engage the body’s threat-management systems more intensely.  All these tests held even when accounting for the film’s release year and remained robust across alternative ways of computing protagonist vulnerability.

Together, these findings, reported in Evolution and Human Behavior, show that protagonist vulnerability captures a core input condition of fear in movies across genre boundaries, subjective judgments, and physiological responses. More generally, this perspective helps explain why certain narrative tropes recur so reliably in horror fiction. Characters in horror films are isolated, cut off from communication, or trapped in hostile environments because isolation removes social and material resources that would otherwise reduce vulnerability. Settings such as remote cabins, deserted hotels, confined spaces, or unfamiliar worlds systematically limit escape and amplify exposure to harm. Even when protagonists are adults, horror often strips them of weapons, authority, or knowledge, placing them in situations where resistance is ineffective and outcomes feel uncontrollable. From this perspective, many classic ingredients of horror such as darkness, confinement, isolation, helplessness, are efficient ways of engineering vulnerability.

More broadly, these results shed light on how the fear system is elicited. They suggest that fear is not triggered simply by the detection of threat-related cues, but by an assessment of whether harm is plausible given the situation and capacities of the individual facing the threat. Horror fiction is particularly well suited to revealing these input conditions because it systematically exaggerates and isolates them. By holding constant the nature of the threat while varying the vulnerability of protagonists, fictional narratives create controlled contrasts that would be difficult or unethical to study in real life. More generally, cultural products that are explicitly designed to evoke specific emotions provide a powerful and efficient way to study the input conditions of evolved psychological mechanisms: when they succeed across audiences and contexts, they offer converging evidence about how those mechanisms are tuned to the structure of the world.

Edgard Dubourg & Coltan Scrivner. (2026). Vulnerability and the computational logic of fear: insights from the horror genre. Evolution & Human Behavior, 47, 106813.

Is He a Good Dad, or Does He Just Have His Mother’s Genes?

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– by Thomas Felesina

The male nipple is a classic conversation starter (at least in the evo biology faculty lounge). The nipple is a feature that is functional and essential for reproductive success in females,[1] yet non-functional in males, perhaps decorative at best. We don’t spend much time agonizing over the “evolutionary function” of the male nipple. We generally accept that it exists not because it provides a fitness advantage to men, but because men and women are built from the same genetic blueprint. Men have nipples because women need them, and decoupling the genetic architecture for chest anatomy between the sexes is evolutionarily “expensive” and largely unnecessary since it imposes few costs for men. To get rid of male nipples, a mutation would first need to occur that says something like, “grow nipples ONLY if female” or “suppress nipples ONLY if male.”

However, when we move from anatomy to psychology and behavior, we often lose sight of this logic: forgetting that the shared genome governing the male and female body also governs the male and female mind. We have 23 pairs of chromosomes. Aside from the sex chromosomes (X and Y), the other 22 pairs, the autosomes, are effectively identical between the sexes. This means that most genetic variants that influence our brains and our behavior are the same for both males and females.

In my recent paper, I show that for many complex traits, the genetic correlation between the sexes (referred to as rMF) is extremely high. The genetic correlation is extremely high because when sharing genetic architecture, you cannot easily pull one sex in an evolutionary direction without dragging the other sex along with it (referred to as a correlated response). Consequently, when selection favors a trait in females, the mean trait value may increase in males as well, even if it provides no direct benefit or is slightly costly. This is the genetic equivalent of a sidecar on a motorcycle: if the driver (the sex under strong selection) turns left, the passenger (the other sex) goes left, too.

And so, when we ignore the correlated responses to selection acting on one sex, we risk falling into a trap of proposing two separate adaptive explanations: one for men, and one for women, when a single explanation might do, or at the very least, be a significant factor in the initial evolution of a trait in one of the sexes.

For instance, evolutionary psychology emphasizes that females should be “choosy” because the biological costs of a bad mating decision are high (e.g., expensive eggs, pregnancy, lactation). Males, with lower obligatory investment, are theoretically predicted (and empirically observed among many animals) to be less choosy. Yet, human men are surprisingly discriminating compared to most male mammals. Is this because men faced their own unique selection pressures to be picky? Perhaps. But it is arguably more parsimonious to suggest that strong selection for choosiness in women, driven by prolonged child-rearing demands, shifted male psychology in the same direction via our shared genome.

Take another example: humans are outliers among mammals; our fathers are unusually involved in child-rearing. Perhaps paternal care in humans increased offspring survival or improved mating opportunities, but it would be remiss to ignore the selective pressure on human mothers to be nurturing and attentive to highly dependent, altricial infants. Natural selection would have aggressively favored genetic variants that promoted responsiveness to infant cues, patience, and bonding in mothers. Because these “parental” genes likely reside on the autosomes, they are inherited by sons just as they are by daughters, leading to the emergence of paternal care. And so, the baseline capacity for human paternal care may be, at least initially, a genetic byproduct of intense selection on maternal care. Men may be nurturing dads partly because they are the sons of nurturing mothers.

While my review of the quantitative genetic literature found that high genetic correlations (often nearing 1.0) are the norm for human behavior, there are, of course, exceptions. For instance, research on extra-pair mating (infidelity) in humans has shown a surprisingly low genetic correlation between the sexes. This suggests that for cheating behavior, men and women may indeed have evolved under distinct, sex-specific pressures, allowing their genetic architectures to decouple.

Why This Matters for the Future of the Field

Acknowledging the shared genome doesn’t mean denying sex differences. Men and women differ significantly in height, yet the genetic correlation between the sexes for height is near-perfect. As I discuss in the paper, while the set of genes influencing a trait is largely the same for both sexes, the magnitude of their effect can differ for a variety of reasons; for instance, the distinct hormonal environments of men and women can amplify or dampen the effect of genes.

With this in mind, as evolutionary social scientists, we should adopt a new default assumption: unless we have evidence to the contrary, we should assume the genetic correlation between sexes is high. To claim that a trait evolved independently in both men and women requires us to devise two separate adaptive accounts, one for each sex, while also assuming the successful evolution of complex mechanisms to decouple their genetics. In contrast, assuming a high genetic correlation requires only one adaptive explanation: selection acted strongly on one sex, and the other simply inherited the trait. The latter explanation demands fewer assumptions, aligns with the biological reality of our shared genome, and should therefore be our starting point.

By respecting the constraints of our shared DNA, we can build more rigorous, parsimonious, and biologically grounded theories of human behavior. Sometimes, a dad is a dad because he evolved to be one. But sometimes, he’s a dad because his mother was a mom.

[1] While the functional nipple is a female trait, it is essential for the reproductive success of both sexes.

Thomas Felesina. (2026). The shared genome constraint: why between-sex genetic correlation matters for evolutionary social science. Evolution & Human Behavior, 47, 106773.