Friend Request Accepted: Fundamental Features of Social Environments Determine Rate of Social Affiliation

Humans start new friendships and social connections throughout their lives and such relationships foster mental and physical well-being. While friendship initiation may depend on alignment of subtle and complex personal variables, here we investigated whether it also depends on basic features of social environments. This would be analogous to other fundamental behaviours like foraging which depend on basic features of the environment such as the density of opportunities and the likelihood of success. In a pre-registered online study (n=783), we found people were more likely to send friend requests as the density of friendship opportunities decreased and frequency of success increased. Further, we found task-related measures, like overall friend requests, were correlated with personality-related factors like social thriving and anhedonia. Next, in an ultra-high-field fMRI study (n=24), we found that both fundamental features of social environments – opportunity density and frequency of success – affected neural activity across a network of regions linked to foraging including dorsal raphe nucleus, substantia nigra, and anterior insula. Finally, in resting-state fMRI data (n=400), we showed that model predicted estimates of anhedonia were related to functional connectivity between components of the same network. Thus, humans consider the background statistics of an environment while making social decisions and these decisions are linked to activity in ancient subcortical circuits mediating the influence of environmental statistics on other aspects of behaviour. Moreover, individual differences in how environmental features influence social behaviour are associated with variation in personality and psychiatric traits, offering new insights into inter-individual variability in social functioning.