Sex differences in close friendships and social style

Friendships play a central role in human sociality, and are a major influence, both directly and indirectly, on our fitness. The two most important forms of friendship are the support clique and the best friend. Although the basis on which we choose friends and romantic partners have been studied in considerable detail, we know a great deal less about how individuals' own psychological traits affect whom they form relationships with. Here, we use an ethnically homogenous UK sample of 757 adults (aged 18–75 years; 56 % female) attending national science festivals to show that there are striking differences between men and women in both the structure of friendship groups and the psychological mechanisms that underpin their capacity to hold and maintain close friendships. Individual differences in the size and structure of women's cliques, and their likelihood of having a best friend, are underpinned mainly by prosocial tendencies, whereas in men they correlate negatively with anti-social, rather than prosocial, tendencies. These findings add to the evidence that male and female social worlds are organised in very different ways. This begs the evolutionary question as to why this is so.