| Summary: | Humans start new friendships and social connections throughout their lives, and it has been consistently found that such relationships lead to mental and physical well-being. In this thesis, I investigated the behavioural and neural mecha- nisms governing our decisions to initiate friendships with other people. I examined whether such decisions are influenced by the friendliness, i.e., the social reward rate, and the density, i.e., the rate of opportunities, afforded by the environment. In a computer based online task (n=783), I found that people were more likely to send friend requests in friendly and sparse environments in comparison to hostile and dense environments. Further, I found task-related measures like overall friend requests were correlated with personality-related factors like social thriving. Next, in a 7T fMRI study (n=24), I found that the subcortical dorsal raphe nucleus represented density-related effects and the substantia nigra (SN) represented friendliness-related effects. Further, cortical regions like the anterior insula (aI) represented both friend- liness and density related effects. Next, in resting state fMRI data (n=400), I showed that model predicted factor score corresponding to anhedonia was related to functional connectivity between the SN and the aI. Finally, in a social learning task, I found that people took background statistics of an environment into account when deciding between learning from others and foraging for food by themselves. Taken together, these findings suggest that the human brain takes background statistics of an environment into account while making social decisions and that such decisions can be explained by personality or psychiatric factors.
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