– by Ana María Fernandez, María Barbato, Michele Dufey

Jealousy is often portrayed as irrational, possessive, or simply unpleasant. Yet from an evolutionary and developmental perspective, jealousy may be better understood as a socially intelligent alarm system: an emotion that becomes activated when a valued relationship appears to be threatened by a third party. In our study, we tried to “switch on” this alarm in a controlled way, by presenting concrete decisions about how partners distribute resources between their romantic relationship and a potential rival.

This alarm system does not begin in adulthood. It can be observed early in life, when infants protest the apparent loss of a caregiver’s attention to a sibling or another person. Later, in friendships, jealousy may emerge when a newcomer threatens to replace our special place in a best friend’s affection.

Romantic jealousy follows a related logic; it is commonly understood as a response to threats of sexual or emotional infidelity. But beneath these familiar scenarios lies a broader question: what exactly is being protected? From an evolutionary perspective, romantic jealousy may function to protect exclusive access to a partner’s affection, attention, time, commitment, and investment. It is not only about sex, but about the possibility that a partner’s valued resources—emotional, social, reproductive, or material—may be redirected toward a rival.

This broader view motivated our research. For many years, studies of romantic jealousy relied heavily on hypothetical forced-choice scenarios, especially those contrasting sexual and emotional infidelity. We wanted to move beyond asking people to imagine betrayal in the abstract. Instead, we sought to create a more interactive experimental situation in which jealousy could be evoked through the allocation of resources. The economic game we used was designed to capture exactly this sense of being chosen, how much “goes to me” versus “goes to someone else” becomes a simple but powerful signal of where a partner’s attention and commitment are directed.

So, we invited romantic couples to the laboratory and used a modified economic game inspired by the dictator game. Participants were led to believe that their partners had exchanged resources with anonymous opposite-sex individuals. The game allowed us to model two different kinds of rival-related threats.

In the first condition, which we will call emotional investment, participants were informed that their partner had given most of their available resources—75 out of 100 dollars—to an opposite-sex person they had just met, rather than to the participant. This situation was designed to represent a partner actively directing resources toward a potential rival, and evoked the highest jealousy overall, and it was stronger among women than men.

In the second condition, which we will call mate poaching, participants were told that an opposite-sex person, already in a relationship, had given most of their resources to the participant’s partner, at the expense of their own partner. This was designed to represent someone else attempting to invest in, attract, or “poach” the participant’s partner, and it tended to evoke more jealousy among men than women, although the effect was weaker than expected

Importantly, we also included control conditions. These allowed us to examine whether participants were simply reacting negatively to any unequal allocation involving the opposite sex. The answer was no. Jealousy was not merely a response to unfairness, generosity, or discomfort with resource exchange. Rather, jealousy was especially heightened when the allocation implied a possible shift in relational investment: when a loved one appeared to give more to someone else than to the participant.

This is the central novelty of the study. Romantic jealousy may be triggered not only by explicit sexual or emotional betrayal, but also by subtle signs that a partner’s investment is being redirected. A simple economic decision—who receives more, who gives more, and at whose expense—can carry relational meaning. It may signal closeness, preference, availability, or threat.

Why does this matter? Because jealousy is often studied as if it belongs only to adult romantic life. Our findings suggest that jealousy may be part of a broader relational system that develops across the lifespan. From infancy to friendship to romantic love, jealousy appears when a valued bond feels vulnerable to displacement. It protects not only mating interests, but also attachment, exclusivity, affection, and the expectation that we hold a special place in another person’s mind.

By using an economic game, our study offers a new way to examine jealousy experimentally. It also helps refine the evolutionary interpretation of jealousy. Going beyond basing jealousy on sex differences or infidelity scenarios, the findings point to a more general mechanism: jealousy is activated when the exclusive investment of a significant other appears to be at risk. Jealousy-related feelings should also be placed in a constructive relational perspective. They do not reflect conflict or insecurity alone; they may also signal that closeness, attention, and commitment matter, and motivate healthy ways to protect them.

The green-eyed monster, then, may not be merely a destructive force. It can also be seen as an evolved emotional signal, alerting us when a valued bond feels less secure, less exclusive, or less reciprocally invested than we expected. In this sense, jealousy reveals something deeply human: our need not only to love, but also to feel chosen, valued, and uniquely held in the mind of another. When understood constructively, jealousy may become less a threat to relationships than an invitation to reflect on what our closest bonds require to remain secure, reciprocal, and meaningful.

Ana María Fernández, María Teresa Barbato, Michele Dufey, Belén Zavalla, María Luíza Rodrigues Sampaio de Souza. 2026. Resource allocation and romantic jealousy: An experimental test of sex differences using economic games. Evolution and Human Behavior 47(2): 106816.

– by Yago Moraes

Imagine two strangers meeting for the first time. Instead of making small talk, they either play an ancient strategy game or role-play a mild disagreement between friends. Weeks later, their impressions of one another have changed, but not in the way many theories about games would predict.

This puzzle lies at the heart of our paper, “Evolutionary and social functions of gaming”, which combines a lab experiment with computer simulations to ask: what are games for in human evolution?

Humans create games in nearly every culture and keep playing them long after childhood. This is striking because, although play is common across many mammals, gaming is different. Games have rules, goals, and winners-losers. They are also especially common among post-pubertal males and often reflect hunting, combat, and competition.

These features have inspired several explanations. Evolutionary researchers argue that games function as low-cost practice for hunting or warfare, and as arenas for status competition, helping players display skill, gain prestige, and attract mates. Social scientists, meanwhile, often emphasize a different role: games as social glue, capable of easing tension, breaking the ice, and fostering group cohesion.

My colleagues and I propose a framework that integrates these perspectives by extending Deaner and colleagues’ Allying with Coalitions Hypothesis. We suggest that games may have evolved not just as practice or entertainment, but as tools for finding valuable allies. In ancestral settings, success in activities such as big-game hunting or warfare depended on choosing reliable partners. A poor ally could be costly; a skilled and trustworthy one could be lifesaving. Games, with clear rules and measurable results, may have offered a low-risk way to judge others: Who is skilled? Who is strategic? Who cooperates well under pressure?

This idea leads to a clear prediction: if games help people judge potential allies, then playing together should speed up judgments of one another’s value and strengthen social bonds more efficiently than other activities.

To test this, we ran an experiment with same-sex pairs of strangers assigned to one of two conditions. In the gaming condition, participants played Nine Men’s Morris, a centuries-old strategy board game. In the control condition, they completed a role-play task in which two friends negotiated different preferences.

Across three sessions spanning roughly one month, participants repeatedly rated one another on two dimensions: perceived peer value (how desirable the other seemed as a potential friend or ally, including traits such as intelligence, charisma, attractiveness, and status) and relational proximity (how close and connected they felt to the other person). If gaming functions as an ally-selection mechanism, then the gaming condition should have produced faster or stronger shifts in these ratings. It did not.

Both activities changed how participants saw one another over time. Repeated interaction mattered, as expected. But gaming did not outperform the control condition. Playing a strategy game did not make strangers feel closer or judge each other more positively any faster than a structured, mildly conflictual conversation. This pattern appeared in both male and female pairs.

At first glance, this seems like a challenge to the hypothesis. But null results can still be informative. Rather than disproving the theory, the findings suggest that gaming’s social effects depend strongly on context. Not all games are equally informative. A low-stakes, one-on-one abstract board game played in a lab may be a poor model for the kinds of games that mattered in human evolution.

To explore this, we turned to simulations. In our model, virtual individuals could adopt either a gaming or non-gaming strategy. They formed alliances, completed risky cooperative tasks, survived or died based on performance, and reproduced across generations. This allowed us to test when gaming might become adaptive.

The simulations showed that gaming spread only under specific conditions. First, the initial gamer had to be above average in skill. Second, environmental risks had to remain meaningful by changing alongside population skill; otherwise, risks became too easy to overcome.

When low-skill individuals adopted gaming, the strategy usually disappeared. This is what we would expect if gaming functions as an honest signal: it benefits only those who can truly display valuable traits. The model also showed that gaming loses value when stakes are low. In safe settings where performance differences matter little, there is less reason for gaming-based assessment to evolve or persist.

This offers a plausible explanation for the experimental null results. Laboratory settings are not ancestral battlefields. No one’s survival depended on winning NMM or choosing the right lab partner.

Together, the experiment and simulations suggest a more nuanced view of gaming. Games do not reliably speed up friendship formation in low-risk, short-term interactions with strangers, at least not more than other structured social activities. But in conditions resembling ancestral life, where success in risky cooperative tasks affects survival, gaming may work as a stable strategy for judging and competing for valuable allies. This view helps explain the widespread presence of games, their male-biased participation in many cultures, and the modest social effects often observed in modern lab studies.

It also suggests we may be focusing on the wrong social phase. Rather than helping primarily to form friendships, games may be especially useful for maintaining them, for instance, by providing a controlled, nonviolent outlet for rivalry or conflict resolution. We refer to this speculative possibility as the Conflict Gambling Hypothesis.

Finally, not all games are created equal. NMM is an abstract, one-on-one board game. Many of the strongest sex differences and coalitional dynamics emerge instead in team sports and real-time multiplayer games such as FPSs or MOBAs, which more closely resemble the coordination, competition, and group dynamics of hunting and warfare.

By combining experiments with evolutionary modeling, this research offers a broader way to understand when games matter socially. It suggests that the evolutionary role of gaming is closely tied to risk, honest signaling, and the long process of building and maintaining alliances when stakes are high.

Yago Lukševičius de Moraes, Marco Antonio Correa Varella, Leonardo Cezar Silva Costa, Nayara Teles, and Jaroslava Varella Valentova. 2026. Evolutionary and social functions of gaming: Integrating experimental evidence and mathematical modeling. Evolution and Human Behavior 47(2): 106833.

– by Tobias L. Kordsmeyer

When we form impressions of others, we rarely rely on just one cue. Faces provide one stream of information, bodies provide another, and voices add yet another layer. In everyday life, these cues are usually perceived together – so it is tempting to assume they are redundant. If someone has an attractive face, we might expect an attractive body and an attractive voice as well.

But should we indeed expect that kind of cross-modal consistency, i.e. noteworthy correlations of perceptions of attractiveness and other socially relevant traits between faces, bodies, and voices? Two competing ideas about such correlated perceptions have been debated for decades.

The one ornament hypothesis (also called backup signals hypothesis or redundancy hypothesis) proposes that different body parts and modalities reflect shared underlying qualities such as immunocompetence, genetic quality, or developmental stability. If that were true, then perceptions of attractiveness (and related traits like health or dominance) should correlate across faces, bodies, and voices.

By contrast, the multiple messages hypothesis argues that what different modalities signal is more independent: faces might reveal different information (e.g. based on how average faces are, which in turn is related to perceptions of attractiveness and health), than bodies (e.g. health based on shape and fat distribution) and voices (e.g. dominance and masculinity based on voice pitch). If so, correlations among perceptions across modalities should be weak or absent – because each channel carries partly distinct information.

Our study puts these ideas to a rigorous test by asking a seemingly simple question: When people judge faces, bodies, and voices separately, do those ratings line up? Put differently, are perceptions of attractiveness, dominance, and health correlated positively across faces, bodies, and voices? Additionally, steroid hormones (testosterone, estradiol), which influence the development of facial, bodily, and vocal characteristics, were investigated as partial explanations of purportedly correlated perceptions.

To investigate these questions, we assembled a large stimulus set from 320 men and women, including neutral face photographs, 3D body scans converted into short rotating videos, and brief voice recordings. Each of these three modalities was then judged by separate groups of raters regarding perceived attractiveness, perceived health, and perceived physical dominance (the latter conceptualised as the likelihood of winning a physical fight). That separation matters – if the same raters had judged faces, bodies, and voices, halo effects could have inflated correlations (“I liked his face, so I feel like his voice must be attractive, too”). Using different groups of raters for different modalities is a tougher test of true cross-modal concordances.

A major methodological concern in older work on judgments of bodies was that body stimuli often included skin colour and texture, which themselves convey health-related information and could invoke stereotypes, which could create shared variance with facial appearance. In our study, we deliberately used grey-coloured body scans without heads, removing skin tone cues, to shift the focus to body shape. That means any face–body correlations that remain are harder to explain away as “it’s just skin quality.”

What did we find? Generally, we found consistent and medium-sized correlations between faces and bodies for all three traits: attractiveness, health, and physical dominance. On average, people who were considered attractive in terms of their facial features were also considered attractive in terms of their body shape. Correlations for perceived health and perceived physical dominance were similar, but only slightly smaller. So, at least for faces and bodies, the data look consistent, indicating a partly redundant signal and hence supporting the one ornament hypothesis.

For voices, however, the story shifts. Out of the six correlations of voices with faces and bodies for the three traits, only three were significant, and effect sizes rather small. Thus, regarding voices, the multiple messages hypothesis seems to be better supported, and voices may add information that is not strongly predictable from faces or body shape.

When we split the target sample by sex, the face–voice and body–voice correlations were somewhat stronger for men than for women (especially for perceived health and physical dominance), whereas the body-voice correlations were more comparable across the sexes. Thus, whether the one ornament hypothesis or multiple messages hypothesis is more valid may not only depend on the modality, but may also differ by target sex.

A common idea in evolutionary psychology is that sex steroid hormones contribute to the development of sexually dimorphic traits (including facial masculinity/femininity, body composition including fat distribution, and vocal characteristics), which then influence perceptions of attractiveness, health, and dominance. If that’s right, one might expect that part of the cross–modal correlations exists because the modalities are simultaneously influenced by the same endocrine factors.

We tested this using salivary hormone measures: baseline testosterone and testosterone reactivity (after having engaged in a dyadic competition in the laboratory) in men, and baseline testosterone, baseline estradiol, the baseline testosterone/estradiol ratio, as well as hair testosterone (a measure of long-term aggregated baseline levels) in women.

The insight from these additional analyses was rather simple: We found no evidence that these hormone measures explained the cross-modal correlations. The study did find a few small direct associations between men’s baseline testosterone and their vocal attractiveness and physical dominance, but not in a way that explained the broader cross-modal pattern.

One implication is that if a shared biological mechanism links facial, bodily, and vocal characteristics, it may not be well captured by current adult hormone levels. Instead, prenatal or pubertal hormonal measures with more organisational effects during development, or additional genetic factors and environmental perturbations during development may play a stronger role as explanatory mechanisms.

What is the take-away message? This study offers a nuanced update to a debate that is sometimes framed too starkly as “redundant” versus “independent” signals. For faces and bodies, we demonstrate moderate associations across perceived attractiveness, health, and physical dominance (somewhat stronger in men than women). That supports the idea that these visual channels share underlying cues – despite the body stimuli in our study lacking skin information – in support of the one ornament hypothesis. However, for voices, cross-modal correlations were rather weak, suggesting voices may communicate partly distinct information from faces and bodies – more consistent with the multiple messages hypothesis. Concerning potential explanatory mechanisms for such cross-modally correlated perceptions, at least current adult testosterone and estradiol levels do not seem to play a role here, calling for the investigation of alternative causes.

Tobias L. Kordsmeyer, Philine A.C. Rasokat, Julia Stern, Christoph Schild, and Lars Penke. 2026. Correlated perceptions and underlying hormonal factors of bodily, facial, and vocal attractiveness, health, and physical dominance. Evolution and Human Behavior 47 (2): 106818.

– by Irene Delval and Solimary García Hernández

Traditionally, scientific and popular assumptions about sexual behavior have struggled to decouple it from its reproductive purpose. From primates to nematodes, sexual behaviors that didn’t lead to conception were often dismissed as errors, curiosities, byproducts, or evolutionary leftovers.

Recently, however, this perspective has shifted. In Western societies, as social standards have changed and the visibility of LGBTQIAPN+ communities has grown, science has moved away from restrictive moral ideals to explore the diverse functions of sexuality. The scientific community now recognizes that across many species, including ours, sexual behavior serves a range of social functions like bonding, conflict resolution, reconciliation, or relationship maintenance. This has fueled growing interest in non-conceptive sexual behavior. Much of the research in this area has focused on Same-sex Sexual Behavior (SSB). Once framed as a Darwinian paradox, SSB is now understood as a functional and adaptive component of mammalian social systems. We now know that SSB is a prevalent trait in primates, driven by ecological and social pressures in socially-living species.

Yet, one form of non-conceptive sex remains largely unexplored: the sex-like behaviors that happen before sexual maturity, in prepuberal subjects, long before individuals develop reproductive drives. This begs a major question: how and when do these behaviors first emerge? Understanding this timeline can tell us a lot about their evolved functions.

In our new research published in Evolution and Human Behavior (March, 2026), we explored how sexual behavior emerged during the first year of life in two species of wild robust capuchin monkeys (Sapajus libidinosus and S. xanthosternos). By following 16 wild infants longitudinally in Brazil, our team provided a rare window into the development of sexual behavior under natural conditions. Our findings flip the traditional view that young primates (including humans) engage in sexual behavior merely to practice for adult reproduction, providing strong evidence for the growing consensus among researchers that, across primate species, sexual interactions help animals navigate their social worlds long before they can reproduce.

Starting from Month One

The biggest surprise was just how early these behaviors begin. Both male and female infants exhibited socio-sexual behaviors (i.e., behavior sexual in form but serve but serve social functions), such as mounting, genital inspection, and courtship gestures, within their first month of life. This remarkably early onset parallels observations in humans and shows that sexual behavior doesn’t simply lie dormant until puberty. Instead of appearing suddenly at sexual maturity, it seems to be part of the behavioral repertoire from the very beginning, at least in its early-developing form.

Even though the two capuchin species live in very different environments (one in semi-arid scrubland, the other in tropical rainforest), their developmental trajectories were similar. Among a repertoire of 25 analyzed sexual behaviors, both species and sexes shared the six most frequently performed behaviors. However, although males and females initially performed the same types of behaviors, like eyebrow raising, chest rubbing or mounting, which are usually performed during adult heterosexual courtships, sex differences emerged as the infants grew.

Early and Marked Sex Differences

Male infants showed higher rates of sexual behavior, greater behavioral diversity, and an earlier onset compared to females. The increase in the emergence of sexual behavior over time was also steeper in males, indicating a sex-specific developmental trajectory. These findings align with research in other primates, such as macaques, and suggest that sex differences in sociosexual behavior emerge early in ontogeny, likely shaped by genetic and hormonal effects right from the start.

What makes this particularly fascinating is that in adult robust capuchins, courtship is predominantly female-led. Adult females are the proactive agents, using elaborate facial expressions and even throwing stones to get the attention of often-indifferent males. If the classic “practice hypothesis” were true, which assumes infants perform early forms of these behaviors to rehearse their future adult roles, we would expect infant females to be the ones practicing the most.

However, our data revealed the exact opposite. There were large differences in the directionality of sexual behaviors. Infant males engaged in these interactions far more often, while females were usually the receivers of such behaviors. This points to a male-specific developmental pathway of arousal that clearly doesn’t just mirror adult courtship roles.

Different Partners, Different Goals

We also found a clear divide in who the infants chose to interact with. Infant males acted as sexual “extroverts,” engaging with a wide range of partners. Most notably, their most frequent partners were adult males, making same-sex interactions very common among them. Infant females, on the other hand, had much narrower preferences, mostly interacting sexually with other infants of both sexes. This divergence suggests something important: early sexual behavior likely serves completely different functions depending on the sex.

For males, these interactions might help establish social bonds, navigate dominance hierarchies with older males, or regulate arousal in complex social settings. For females, these behaviors, at this stage, seem more embedded in general peer play and social exploration. We need to further understand when sexual “awakening” occurs in female robust capuchins.

Why This Matters

Our research highlights that primate sexuality is flexible and multifaceted right from birth. Because sex differences emerge so early and don’t simply mirror adult roles, these findings open up new questions about the true origins of sexual behavior in all primates, humans included. If we want to understand the “why” of adult behavior, we first need to look closely at the “how” in the infant’s world.

Just as grooming behavior, which goes far beyond hygiene and removing parasites to act as a social glue, early sexual behavior seems relevant for building and maintaining social bonds, and canalizing arousal, rather than just preparing individuals for reproduction. Our findings also point to a broader issue: studying infant sexuality remains a highly sensitive and often avoided topic, especially in humans. This is where research on non-human primates becomes so valuable, giving us a comparative window to investigate these processes rigorously and without cultural constraints.

Delval, I., García-Hernández, S., Teles, N., Caixeta, J., Cezar, L., Valentova, J. V, Izar, P., & Leca, J-B. 2026. Early sex differences in sociosexual behavior of wild robust capuchin monkeys: Ontogenetic and evolutionary insights. Evolution and Human Behavior, 47 (2), 106834.