The ‘social distancing’ that occurred in response to the COVID-19 pandemic in humans provides a powerful illustration of the intimate relationship between infectious disease and social behaviour in animals. Indeed, directly transmitted pathogens have long been considered a major cost of group living in humans and other social animals, as well as a driver of the evolution of group size and social behaviour. As the risk and frequency of emerging infectious diseases rise, the ability of social taxa to respond appropriately to changing infectious disease pressures could mean the difference between persistence and extinction. Here, we examine changes in the social behaviour of humans and wildlife in response to infectious diseases and compare these responses to theoretical expectations. We consider constraints on altering social behaviour in the face of emerging diseases, including the lack of behavioural plasticity, environmental limitations and conflicting pressures from the many benefits of group living. We also explore the ways that social animals can minimize the costs of disease-induced changes to sociality and the unique advantages that humans may have in maintaining the benefits of sociality despite social distancing.

Why do race stereotypes take the forms they do? Life history theory posits that features of the ecology shape individuals' behavior. Harsh and unpredictable ("desperate") ecologies induce fast strategy behaviors such as impulsivity, whereas resource-sufficient and predictable ("hopeful") ecologies induce slow strategy behaviors such as future focus. We suggest that individuals possess a lay understanding of ecology's influence on behavior, resulting in ecology-driven stereotypes. Importantly, because race is confounded with ecology in the United States, we propose that Americans' stereotypes about racial groups actually reflect stereotypes about these groups' presumed home ecologies. Study 1 demonstrates that individuals hold ecology stereotypes, stereotyping people from desperate ecologies as possessing faster life history strategies than people from hopeful ecologies. Studies 2-4 rule out alternative explanations for those findings. Study 5, which independently manipulates race and ecology information, demonstrates that when provided with information about a person's race (but not ecology), individuals' inferences about blacks track stereotypes of people from desperate ecologies, and individuals' inferences about whites track stereotypes of people from hopeful ecologies. However, when provided with information about both the race and ecology of others, individuals' inferences reflect the targets' ecology rather than their race: black and white targets from desperate ecologies are stereotyped as equally fast life history strategists, whereas black and white targets from hopeful ecologies are stereotyped as equally slow life history strategists. These findings suggest that the content of several predominant race stereotypes may not reflect race, per se, but rather inferences about how one's ecology influences behavior. Traits such as these have long characterized white Americans' stereotypes of black Americans (1-3). Why do race stereotypes in the United States take these particular forms?Stereotypes are useful to the extent they can rapidly provide perceivers with information about the affordances-threats and opportunities-posed by others (4). Indeed, a major function of the mind is to identify and anticipate affordances and to respond to them in ways that are threat reducing and opportunity enhancing so that we may more successfully achieve our goals (5-10). However, because we cannot directly see others' behavioral intentions, strategies, or capacities, we must infer them (imperfectly) from cues we can perceive. Here, we argue that one such cue is an individual's home ecology, because ecologies shape the behavior of those within them. Thus, by knowing another's home ecology, people possess useful information (in the form of stereotypes) about others' behavioral intentions, strategies, and capacities. To the extent that different races are associated with different home ecologies, an individual's race becomes a secondary cue to his or her ecology, with the implication that race may evoke ecology-driven stereot...

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Friendships can foster happiness, health, and reproductive fitness. But friendships end-even when we might not want them to. A primary reason for this is interference from third parties. Yet little work has explored how people meet the challenge of maintaining friendships in the face of real or perceived threats from third parties, as when our friends inevitably make new friends or form new romantic relationships. In contrast to earlier conceptualizations from developmental research, which viewed friendship jealousy as solely maladaptive, we propose that friendship jealousy is one overlooked tool of friendship maintenance. We derive and test-via a series of 11 studies (N = 2918) using hypothetical scenarios, recalled real-world events, and manipulation of on-line emotional experiences-whether friendship jealousy possesses the features of a tool well-designed to help us retain friends in the face of third-party threats. Consistent with our proposition, findings suggest that friendship jealousy is (1) uniquely evoked by third-party threats to friendships (but not the prospective loss of the friendship alone), (2) sensitive to the value of the threatened friendship, (3) strongly calibrated to cues that one is being replaced, even over more intuitive cues (e.g., the amount of time a friend and interloper spend together), and (4) ultimately motivates behavior aimed at countering third-party threats to friendship ("friend guarding"). Even as friendship jealousy may be negative to experience, it may include features designed for beneficial-and arguably prosocial-ends: to help maintain friendships.

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