Long-lasting, romantic partnerships are a universal feature of human societies; but almost as ubiquitous is the risk of instability when one partner strays. Jealous response to the threat of infidelity is well-studied, but most empirical work on the topic has focused on a proposed sex difference in the type of jealousy (sexual or emotional) men and women find most upsetting, rather than on how jealous response varies 1,2. This stems in part from the predominance of studies using student samples from industrialized populations, which represent a relatively homogenous group in terms of age, life history stage, and social norms 3,4. To better understand variation in jealous response, we conducted a two-part study in 11 populations (1,048 individuals). In line with previous work, we find a robust sex difference in the classic forced-choice jealousy task. However, we also show substantial variation in jealous response across populations. Using parental investment theory, we derived several predictions about what might trigger such variation. We find that greater paternal investment and lower frequency of extramarital sex are associated with more severe jealous response. Partner jealousy thus appears to be a facultative response, reflective of the variable risks and costs of men's investment across societies. Main One of the essential features of human mating is the prominence of stable, long-lasting partnerships, which in almost every society are socially enforced through the institution of marriage 5,6. A widespread feature of marriage is the custom of sexual exclusivity. Despite the near ubiquity of this expectation, marriages are at risk of disruption by extramarital partnerships. Adultery is the most commonly cited reason for divorce across cultures 7 , and concurrent partnerships are often common 8,9. In response to threats of infidelity, humans, like other species with stable partnerships, have evolved adaptations to protect against mate poaching and defection. Some of these behaviors, like partner concealment, vigilance and sexual coercion, are shared with other species 10-12 , while others like foot binding and purdah are culturally constructed and unique to humans 13,14. Underlying these behaviors is a suite of psychological mechanisms, of which jealousy is one of the most important. While jealousy itself is thought to be a universal human emotion, traditional evolutionary explanations predict that jealous response to cues of infidelity will differ in men and women, reflecting the unique adaptive problems they face 1,13,14. Men face a risk of paternity uncertainty, which results in the loss of a fitness opportunity, but also in the potential misallocation of investment. Women, on the other hand, risk the diversion of critical resources by their partner
