– 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.

– 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.

– by Juan Du

Quick question: If something went wrong tonight and you needed help fast—someone to watch a child, lend money, give you a ride—who comes to mind first: a friend, your own family, or your partner’s family? And if you asked your partner, would they choose the same side? People sometimes tease that men have an endless supply of “brother dinners”, while women seem to keep a smaller circle—but hold onto it tightly. It’s easy to shrug and say “that’s just gender” or “that’s just culture”. But our new paper suggests those patterns aren’t fixed and there are gendered differences.

Instead of asking whether men and women are “naturally” different, we asked what happens to social relationships when the rules of everyday life change? In particular, what happens when a community shifts from mainly farming and herding toward deeper involvement in markets—more wage work, more trading, more travel, more time pressure?

We understand that relationships are not just “social”. They are how people get labor help, childcare support, information, and backup when things go wrong. But maintaining relationships takes time. And time is the one resource that becomes painfully scarce as market life expands. The basic idea is that not all social ties cost the same to maintain. In our paper, we focus on two types of relationship: friendship and kinship.

You may able to list a few friends in your daily life, but it is hard to say whether you will keep your friends the same during your lifetime. And, as you grow older, you find that maintaining friendships become even more difficult. Thus friendships become incredibly valuable and expensive. You don’t keep friendships strong by accident—you keep them strong by showing up.

Whereas kin ties can work differently. In many rural settings, cooperation among relatives is woven into everyday routines: shared labor work, shared responsibilities, shared obligations, and the kind of long-term accounting that doesn’t reset every time you miss a meal together. That doesn’t mean kin ties are always easy or conflict-free, but they can be harder to “drop” and sometimes more resilient when time is tight.

So when market participation increases, we expected a simple trade-off to intensify: friendship becomes more expensive; kin cooperation may become a safer bet.

And because market work and mobility often change more for men than for women in many settings, we also expected something else: men’s networks might be reshaped more strongly by market involvement, while women’s networks might remain steadier. Not because women are “born stable,” but because women’s daily cooperative demands—childcare, household coordination, local mutual aid—may stay locally focused even as markets expand.

We worked in a Tibetan community in the Shangri-La region of Yunnan, where economic life has been shifting quickly from traditional farming and herding subsistence to more market involved. Our dataset comes from more than a decade of cumulative research across 14 villages. We collected detailed information on 1,169 married adults, focusing on their core social interactions.

We separated these relationships into four types:

• the person’s overall core network
• biological kin
• in-laws (affinal kin)
• friends (non-kin)

And we used a simple structural measure—network density—to capture whether someone’s close ties form a tightly connected cluster or a looser set. Dense networks can be a sign of coordinated support: people know each other, information travels fast, and cooperation can be reinforced through reputation and mutual monitoring.

We found that among men, higher market participation went with a shift away from friends and toward kin. As men became more engaged in market activities, their kin networks became more cohesive, while friend networks became less cohesive.

This is not the same as saying “men lose friends.” What we see looks more like reallocation. When time and movement are squeezed, men appear to concentrate social effort on relationships that are more durable and more predictably cooperative. If showing up frequently becomes hard, friendship—especially the kind built on frequent contact—may be the first thing to thin out.

Women’s networks, looked comparatively stable across market involvement. Women maintained dense, high-quality core ties regardless of whether they were more or less involved in market activities. The most plausible explanation is not that women are “just better at relationships,” but that women’s roles and cooperative needs remain more locally embedded: if you are coordinating childcare, household work, and everyday mutual help, you cannot afford to let your support network drift.

We also looked at post-marital residence—whether people live near their own family or near their spouse’s family affects their relationships. This matters because residence literally determines who is within reach for daily cooperation. And we found that:

• if you live near your natal kin, your biological-kin network is stronger;
• if you live near your spouse’s side, your in-law network becomes stronger.

That might sound obvious, but it implied that in-laws can operate as real cooperative partners, not merely “secondary” ties. When biological kin are less accessible, affinal kin can step in and support a bilateral cooperative network—a support system sustained by both sides of the family rather than only one lineage line.

So yes, you can still recognize the old pattern—men with lots of social occasions, women with steadier core ties. But what matters is how that pattern moves when work starts pulling people around and evenings disappear. In this community, the changes were clear: as time and mobility constraints grew, men and women reorganized which relationships they kept warm, and which ones were allowed to cool.

Yaming Huang, Gabriel Šaffa, Shiting Zhang, Pengpeng Bai, Liqiong Zhou, Gui He, Ruth Mace, & Juan Du. (2026). Gender differences in social networks under subsistence changes. Evolution & Human Behaviuor, 47, 106814.

– by Marius Stavang

Imagine you’re on a movie date with someone. Everything seems to be going great. The conversation flows easily, you laugh together, you exchange smiles—there’s chemistry. After the movie, you walk them home. On their doorstep, you exchange a lingering look and a shared laugh about how good—or bad—the movie was. The moment feels perfect. There are sparks. Nervous, but summoning what courage you can, you close your eyes, purse your lips, and lean in for a kiss.

Suddenly you’re interrupted with a loud protest, “I thought we were just friends!”  Your heart sinks immediately, you feel devastated, crushed, and also—deeply ashamed. You realize you’ve misinterpreted the relationship between the two of you. You mistook their friendliness for romantic interest. Meanwhile, your friend—confused and saddened by what’s happening—feels their own sting of guilt and embarrassment. How could they not have seen that your invitation might have meant something more?

In evolutionary psychology, these two errors of inference are referred to as sexual overperception and sexual underperception. Sexual overperception involves mistaking someone’s friendliness for romantic interest, whereas sexual underperception involves mistaking romantic interest for mere friendliness.

If people were always completely transparent about their romantic interest—“Hey, I fancy you as a long-term romantic partner. Want to go see a movie together?”—errors in sexual perception wouldn’t occur. But for some reason humans are highly anxious and cautious about revealing their non-platonic interest. Instead, we engage in a subtle dance, gradually signaling that we want “something more,” while trying to infer whether the other person is spending time with us for the same reason. And because humans are imperfect mind readers, errors in sexual inference are inevitable.

It also turns out that men and women are not equally likely to sexually over- or underperceive one another. A large body of research shows that men are more likely to overperceive women’s interest than to underperceive it, whereas women are more likely to underperceive men’s interest than to overperceive it. The magnitude and consistency of these sex differences have led researchers to label them the male sexual overperception bias and the female sexual underperception bias.

Put simply, men tend to overestimate how keen women are on them, while women tend to underestimate how keen men are on them. These sexual misperception biases have been identified consistently in adults across a wide range of methods. But an important question remains: When do these biases emerge in development? At what age do males begin to overperceive female’s interest—19, 17, or perhaps as early as 14? And when do females begin to underperceive male’s interest?

We wanted to find out. Studying topics related to sex among those under 18, however, is—for good reason—highly sensitive and requires great caution. For cultural, practical, and ethical reasons, it is not feasible to ask very young individuals directly about their socio-sexual inference making.

This means there is a lower age limit for what can reasonably be studied. For us, that limit was 16. Using a Norwegian high-school sample of adolescents aged 16 to 19 (N = 1 290), we asked participants to report how often, over the past 12 months, their friendliness had been mistaken for sexual interest—and how often their sexual interest had been mistaken for friendliness.

The results showed that from ages 16 to 19, girls were increasingly—and seemingly linearly—more likely to be overperceived. From age 17 onward, they reported that males exhibited an overperception bias. Boys, on the other hand, were consistently underperceived from age 16, and the frequency of these reports did not change with age. Taken together, these findings suggest that male’s sexual overperception bias strengthens throughout mid-to-late adolescence and may first emerge around age 17. In contrast, female’s sexual underperception bias appears to be activated and relatively stable already by age 16.

In sum, the findings suggest that from age 16 onward, males and females are already heavily engaged in inferring one another’s romantic interest. Like adults, they are frequently exposed both to misinterpreting others and to being misinterpreted themselves—boys more often ending up disappointed and ashamed for overestimating others’ interest, and girls more often confused, embarrassed, and even guilty for feeling they should have noticed that others wanted something more.

Thus, unless someone makes it trendy to disclose romantic interest upfront, the “likes me, likes me not” guessing game will forever remain a classic.

Stavang, Marius, Bendixen, Mons, & Kennair, Leif Edward Ottesen. (2025) Adolescent development of sexual misperception biases: females increasingly overperceived, males consistently underperceived. Evolution and Human Behavior, 46, 106758.