Understanding cooperation through fitness interdependence Some acts of human cooperation are not easily explained by traditional models of kinship or reciprocity. Fitness interdependence may provide a unifying conceptual framework, in which cooperation arises from the mutual dependence for survival or reproduction, as occurs among mates, risk-pooling partnerships and brothers-in-arms.

The ability to create lasting, trust-based friendships makes it possible for humans to form large and coherent groups. The recent literature on the evolution of sociality and on the network dynamics of human societies suggests that large human groups have a layered structure generated by emotionally supported social relationships. There are also gender differences in adult social style which may involve different trade-offs between the quantity and quality of friendships. Although many have suggested that females tend to focus on intimate relations with a few other females, while males build larger, more hierarchical coalitions, the existence of such gender differences is disputed and data from adults is scarce. Here, we present cross-cultural evidence for gender differences in the preference for close friendships. We use a sample of ∼112,000 profile pictures from nine world regions posted on a popular social networking site to show that, in self-selected displays of social relationships, women favour dyadic relations, whereas men favour larger, all-male cliques. These apparently different solutions to quality-quantity trade-offs suggest a universal and fundamental difference in the function of close friendships for the two sexes.

Maslow's self-actualization remains a popular notion in academic research as well as popular culture. The notion that life's highest calling is fulfilling one's own unique potential has been widely appealing. But what do people believe they are doing when they pursue the realization of their full, unique potentials? Here, we examine lay perceptions of self-actualization. Self-actualizing, like any drive, is unlikely to operate without regard to biological and social costs and benefits. We examine which functional outcomes (e.g., gaining status, making friends, finding mates, caring for kin) people perceive as central to their individual self-actualizing. Three studies suggest that people most frequently link self-actualization to seeking status, and, concordant with life history theory, what people regard as self-actualizing varies in predictable ways across the life span and across individuals. Contrasting with self-actualization, people do not view other types of well-being-eudaimonic, hedonic, subjective-as furthering status-linked functional outcomes.

What motives do people prioritize in their social lives? Historically, social psychologists, especially those adopting an evolutionary perspective, have devoted a great deal of research attention to sexual attraction and romantic-partner choice (mate seeking). Research on long-term familial bonds (mate retention and kin care) has been less thoroughly connected to relevant comparative and evolutionary work on other species, and in the case of kin care, these bonds have been less well researched. Examining varied sources of data from 27 societies around the world, we found that people generally view familial motives as primary in importance and mate-seeking motives as relatively low in importance. Compared with other groups, college students, single people, and men place relatively higher emphasis on mate seeking, but even those samples rated kin-care motives as more important. Furthermore, motives linked to long-term familial bonds are positively associated with psychological well-being, but mate-seeking motives are associated with anxiety and depression. We address theoretical and empirical reasons why there has been extensive research on mate seeking and why people prioritize goals related to long-term familial bonds over mating goals. Reallocating relatively greater research effort toward long-term familial relationships would likely yield many interesting new findings relevant to everyday people’s highest social priorities.

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