One of the deepest problems in the social sciences concerns the causal impact of society, that is, of properties of the group, on the properties of individuals. This problem arises because individuals affect the properties of groups and vice versa such that it is very difficult to get at causality. Here we take advantage of the possibility to affect the properties of internet communities to show that groups with a higher density of social interactions render their members generally more altruistic and trusting towards anonymous strangers. Moreover, a higher density of social interactions also causes a boost in trust towards those who reciprocate favours while it diminishes trust towards those who fail to reciprocate, thus generating a much stronger implicit punishment for untrustworthy individuals. Finally, increased social contact also enhances the strategic sophistication of individuals and raises the prevalence of Machiavellian strategies. These results indicate that the density of social interactions has a deep impact on individuals’ preferences, beliefs, and behaviors, lending support to sociological views of society.