– 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.

– 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.

– 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.

– 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.

– by Martina Francesconi & Elisabetta Palagi

Sex is often thought of as a purely physical activity, driven by movements, touch, and physiological arousal. Yet in humans, sexual interactions are also deeply communicative. Eye contact, facial expressions, and subtle emotional cues shape how partners experience intimacy, influencing feelings of pleasure, connection, and satisfaction. Nonverbal communication, in particular, appears to play a central role: during sex, faces and bodies often “speak” more loudly than words.

But are these communicative dimensions of sexuality uniquely human? Or do they have deeper evolutionary roots?

To explore this question, we turned to one of our closest living relatives: the bonobo (Pan paniscus). Bonobos are famous for their rich sexual repertoire. Unlike most other primates, they engage in sexual interactions across ages and sexes, and not only for reproduction. Sex in bonobos also serves important social functions, helping to reduce tension, repair relationships, and strengthen social bonds. This makes them an ideal model species for investigating the evolutionary origins of sexual communication.

In humans, facial expressions during sex are often interpreted as spontaneous manifestations of pleasure. Yet growing evidence suggests that they may also play an active communicative role, helping partners coordinate emotionally and behaviorally. Testing this idea directly in humans is extremely difficult for obvious ethical and practical reasons. In non-human primates, observing spontaneous sexual interactions with fine-grained temporal precision is simply not feasible. Bonobos offer a rare opportunity to overcome this limitation. Their sexual interactions are frequent, often face-to-face, and occur in a social context where detailed behavioral observation is possible. Importantly, bonobos regularly display a facial expression known as the silent bared-teeth display (SBT) during sex. This expression can be produced by one partner alone, or reciprocated by both partners in a rapid, automatic exchange known as Rapid Facial Mimicry (RFM), where one individual mirrors the other’s facial expression within less than a second.

This distinction allowed us to ask a key question: is it simply seeing a partner’s facial expression that matters during sex, or is it the mutual exchange, the moment when both partners’ faces fall into synchrony?

To address this question, we analyzed sexual interactions in a captive bonobo colony using high-resolution video recordings. Rather than focusing on outcomes such as mating success, we examined the moment-to-moment dynamics of sexual interactions. Specifically, we used the rate of rhythmic pelvic or genital movements as a proxy for the intensity of sexual stimulation. Faster, more frequent movements indicate higher levels of stimulation, while slower rates suggest a decline. We then examined how these movement patterns changed before, during, and after different facial expression conditions: no facial expressions, unilateral SBTs, and reciprocal facial mimicry (RFM). Crucially, our analyses focused not just on whether these behaviors co-occurred, but on timing. We asked what happens immediately when facial mimicry begins, and what happens when it ends.

Our results revealed a striking pattern. Sexual interactions involving rapid facial mimicry were characterized by the highest levels of stimulation. When both partners mirrored each other’s facial expressions, the rate of rhythmic movements reached a peak. Even more telling was what happened when this mimicry stopped. As soon as facial synchrony was disrupted, when even one partner ceased to mirror the other, the intensity of sexual stimulation sharply dropped. This decline occurred rapidly, within seconds, and persisted even when we excluded interactions interrupted by third parties or external disturbances.

By contrast, unilateral facial expressions told a different story. When only one partner displayed the silent bared-teeth expression, without being mimicked, stimulation levels did not show a consistent or robust change. In other words, expressing a facial signal was not enough. What mattered was sharing it.

These findings suggest that rapid facial mimicry does not merely reflect pleasure as a byproduct of sexual stimulation. Instead, it appears to mark moments of fine-grained socio-emotional coordination between partners.

One possible interpretation was that facial mimicry might serve as an anticipatory signal, communicating motivation to increase stimulation. However, our data did not support this idea. Stimulation did not increase after facial mimicry occurred. Rather, it was highest during mimicry and declined immediately once it ended. This pattern indicates that RFM marks the peak of sexual coordination rather than signaling a future escalation. In short, when partners are emotionally and behaviorally aligned, stimulation is highest; when facial synchrony breaks down, so does intensity. Whether these peaks in stimulation correspond to orgasm or other reward states remains an open question. Nonetheless, our findings have broader implications. Rapid facial mimicry is observed in bonobos across multiple social contexts, including social play, and is thought to reflect shared emotional states and automatic emotional resonance.

Our study suggests that such facial coordination mechanisms may also play a role in regulating sexual interactions. Given the importance of sex in bonobo social life, not only for reproduction but also for maintaining tolerance, cooperation, and social bonds, selection may have favored individuals who could finely tune their behavior to their partners’ emotional signals.

From an evolutionary perspective, this points to a deep-rooted link between nonverbal communication, emotional synchrony, and coordinated social action. The communicative role of facial expressions during human sexual interactions may therefore not be a recent cultural invention, but part of an ancient primate toolkit for social connection.

In both bonobos and humans, it seems that during intimate moments, being in sync matters as much as, if not more than, the movements themselves.

Martina Francesconi, Alice Galotti, Yannick Jadoul, Federico Giovannini, Andrea Ravignani, & Elisabetta Palagi. (2026) SEX in bonobos: The intensity of sexual stimulation sharply drops after facial mimicry. Evolution & Human Behavior, 47, 106786.