Although unrelated friends are genetically equivalent to strangers, several lines of reasoning suggest that close friendship may sometimes activate processes more relevant to kinship and that this may be especially true for women. We compared responses to strangers, friends, and kin in two studies designed to address distinct domains for which kinship is known to have functional significance: i...

We examined how the distribution of information among team members and accountability pressures affected the quality of negotiated settlements reached among teams of friends negotiating against teams of strangers. The main conclusions of the experiment may be summarized by the following findings: (1) Teams of strangers reaped a greater share of the joint profit than did teams of friends when te...

In mammals, helping is preferentially provided to members of one's own group. Yet, it remains unclear how social experience shapes pro-social motivation. We found that rats helped trapped strangers by releasing them from a restrainer, just as they did cagemates. However, rats did not help strangers of a different strain, unless previously housed with the trapped rat. Moreover, pair-housing with...

The inhibition of 99 children was observed from the start ofpnsschool through Grade 1 in multiple settings: adult strangers, dyadic play with unfamiliar and familiar peers, and regular free play in class. A multisetting-multimcthod-multioccasion analysis revealed (a) a high longitudinal stability of inhibition toward strangers and a medium stability of inhibition in class, (b) a decreasing cons...