# Fundamental features of social environments determine rate of social affiliation

## TL;DR

This study shows how basic features of social environments influence people's decisions to form new friendships and how this relates to mental health and brain activity.

## Contribution

The study identifies how opportunity density and success frequency in social environments affect social affiliation and links these to brain circuits and mental health.

## Key findings

- People send more friend requests when opportunities are sparse and success is frequent.
- Social environment sensitivity correlates with mental health dimensions like social thriving and anhedonia.
- Opportunity density and success frequency affect neural activity in foraging-related brain regions.

## Abstract

Social connection is important for our well-being, but little is known about the environmental factors and brain mechanisms that enable its initiation. In an experimental task, we show that two fundamental features of social environments, how friendly people are in that environment and how frequent opportunities for social connection arise, shape decisions to initiate friendships with others. In addition, people’s sensitivity to the social environment related to their mental health dimensions such as their social thriving. Using ultra-high field MRI, we show that a cortico-subcortical neural circuit responsible for tracking rewards during foraging also tracked features of social environments. This suggests that initiating social connections is linked to evolutionarily conserved brain mechanisms, underscoring the fundamental nature 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. In a preregistered online study (n = 783) using a novel social-affiliation seeking paradigm, 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 mental health dimensions 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. Thus, humans consider the background statistics of an environment while making social decisions and these decisions are linked to activity in cortico-subcortical circuits mediating the influence of environmental statistics on other aspects of behavior. Moreover, individual differences in how environmental features influence social behavior are associated with variation in mental health dimensions, offering key insights into interindividual variability in social functioning.