The Mystery of Close Friendships
– by Robin Dunbar
Friends are the single most important resource we have. There is now vast quantities of evidence to show that the single best predictor of our psychological health and wellbeing and our physical health and wellbeing is the number and quality of close friendships we have. The optimal number of friends (including family, by the way) is consistently five, with both smaller and larger numbers being equally disadvantageous (Dunbar 2025).
At the same time, what makes a friendship remains one of the enduring puzzles of the human social world. Somehow it seems to work, but it is an intuitive thing rather than defined by any obvious criterion. We know when we hit it off with someone, but we couldn’t say exactly why or how. “Am I your friend” is one of the unwritten things you just don’t ask. If you aren’t sure, then the answer is: probably not. We are just supposed to know when it happens.
Is my sense of friendship the same as yours? We can never know because our knowledge can only ever be based on our own experiences. And nowhere is this more ambiguous than in cross-sex friendships. Are girls’ friendships the same as boys’ friendships? How would we ever know, since we can only directly experience our own social world?
As part of an attempt to explore this knotty conundrum, we ran a largescale study that sought to understand how human sociality works. We sampled over 1000 people at four UK science festivals. Aside from providing us with DNA samples, they generously completed a large number of questionnaires about their social predispositions and social relationships. In our most recent paper on these data, we looked at sex differences in best friends and the small inner circle of “shoulder-to-cry-on” friends (the close friends and family on whom you would depend for support in moments of great crisis).
One of the most striking differences between the sexes concerned the phenomenon of the best friend (best-friend-forever, or BFF). These turn out to be far more common in women than in men. At any one time, around 85% of women will have an identifiable BFF, 85% of whom will be female. Men can and do have a best friend (equally typically male), but the nature of this relationship is very different: it is more casual, more a partner-in-(social)-crime than an emotional companion for sharing self-disclosures.
Women typically have a BFF in addition to a romantic relationship, whereas in men it’s more a case of one or the other but rarely both together. (I resist the temptation to make any comment on what this tells us about sex differences in social skills and the ability to handle many relationships….) These best friendships are typically established for both sexes in the late teens or early 20s (the college years), and are often lifelong, out-surviving all other friendships. However, there is a tendency for women’s BBF relationships to fracture more easily, perhaps because, like romantic relationships, they are emotionally more intense.
There are parallel differences in the size and structure of the circle of “shoulders-to-cry-on” friends. Over large samples, this group (which includes your BFF and romantic partner) consistently average five individuals (including both family and friends). However, women’s cliques are, on average, significantly larger than men’s (though, in defence of half the world, I should add that the difference is modest even though consistent and significant – about one extra person).
Women’s cliques differ from men’s, however, in that they are less well integrated and less homogenous, mainly because they are a set of dyadic personal friendships. They form more of a hub-and-spokes model. In contrast, men’s cliques are more anonymous and clublike, creating a more interconnected spider’s web of weaker interrelationships. For men, who you are matters less than what you are (the club you belong to). In this context, the club is often very loosely defined – a very common club among older men is the club of “the partners of my wife’s girlfriends”. The women get together and organise social events; their husbands and partners get dragged along (usually reluctantly), and then end up going out for a beer together now and again as a “boys club” merely because they have spent so much time together.
These structural patterns seem to be reflected in marked differences in personal characteristics at the individual level. Women with larger support cliques have more positive, explicitly affiliative, traits (agreeableness, community bonding, attachment style). In contrast, the size of men’s cliques is more likely to be associated with the absence of negative traits: the fewer anti-social traits (poor self-control, sexually promiscuous attitudes and behaviour), the larger the clique. In a previous paper, we showed that women’s cognitive management of their relationships is more complex and involved integrating more sources of information, whereas men’s are more unidimensional. This appears to reflect the fact, as we showed in an analysis of 10,000 neuroimaged brains, that women’s management of relationships involves more brain regions than men’s.
So how on earth do men and women manage to get along if their social styles are so different? At one level, they don’t. Three-quarters of women’s extended social networks (100-200 people) are women, and three-quarters of men’s networks are men (with the other quarter mainly being family). We even see this in casual conversations. Once a conversation exceeds four people, it will invariably subdivide, and when it does it will do so along gender lines. We have documented this in both Europe and in Iran, so it is not a peculiarity of Euro-American culture. These effects might reflect the fact that the fitness gains each sex gets come from different subsets of the community and target different functional benefits. It’s almost as though the “village” consists of two separate networks that overlap briefly in the household.
Read the original paper: Dunbar, R., Pearce, E., Wlodarski, R. & Machin, A. (2024). Sex differences in close friendships and social style: an evolutionary perspective. Evolution & Human Behavior 45: 106631.
See also: Dunbar, R. (in press). Why friendship and loneliness affect health. Annals of the New York Academy of Sciences.
