Taken together, these results confirm previous findings that homophily is an important criterion for close relationships (Curry and Dunbar 2013; Launay and Dunbar 2015). In particular, similarity in dependability was consistently found to be strongly predictive of higher levels of intimacy. For women, this was the case in both best friendships and romantic partnerships, but for men dependability was included in the best-fit model only for intimacy in best friendships. For romantic partnerships, none of the variables measured showed significant partial relationships in men. Aside from these similarities, however, the results suggest that intimacy in males' friendships is underpinned by very different dynamics than intimacy in females' friendships.
Mirroring previous findings with respect to romantic partners (Buss 1989; Pawlowski and Dunbar 1999, 2001), we found that women seemed to be much more demanding in their selection of romantic partners than men were. The intimacy of women’s relationships were homophilous for at least four traits (financial prospects, outgoingness, dependability and kindness), whereas no traits predicted intimacy for males. Similarly, longterm maintenance of women’s romantic relationships were predicted by relationship duration, gift-giving and supportiveness, but for males there was only one significant predictor (the frequency of face-to-face contact). In contrast, best friend relationships exhibited a very different pattern: their intimacy is predicted by similarity on four traits for both women and men, but the traits are very different (education, humour, dependability and happiness for women versus relationship duration, financial prospects, outgoingness and dependability in men).
Longevity in both women’s and men’s friendships was best predicted by provision of mutual support, but differed in the influence of shared histories (negative in the case of women, positive in the case of men). The traits characterizing women’s friendships seem to have more to do with the closeness of the relationship itself, whereas those characterizing men’s friendships seem to have more to do with engaging in social activities. Interestingly, none of the best-fit models included physical attractiveness or athleticism, indicating that personality and resource factors (such as education and financial potential) may be more important for intimacy levels in these close non-kin relationships than traits that might be assumed to correlate more directly with genetic fitness. This likely reflects the fact that relationships are indirect, rather than direct, means of enhancing fitness. In other words, this is a two-step process: we form close relationships not simply to access a direct fitness reward but in order to create coalitions or alliances that in turn allow us to maximise fitness. One possibility, for example, might be to mitigate the fertility costs of group-living (Mesnick 1997; Wilson and Mesnick 1997; Dunbar 2018a, 2019; Dunbar and MacCarron 2019).
The fact that outgoingness was a predictor for the intimacy of men’s friendships might be linked to the fact that males tend to prefer social interaction in groups whereas females have a strong preference for one-to-one interactions (Baumeister and Sommer 1997; Benenson & Heath, 2006; Dávid-Barrett et al. 2015; Gabriel and Gardner 1999; Rustin and Foels 2014). In addition to these homophily effects, we also found that mutual support and shared history are important for intimacy, and are therefore key factors underpinning the successful maintenance of close personal relationships. Mutual support had a much stronger influence on intimacy in female participants for both romantic partners and best friends, but only in respect of best friends for men (Fig. 2). In relation to their romantic partners, the degree to which men considered in-person contact an important mechanism for relationship maintenance was the strongest predictor of intimacy, at least when the sample was considered as a whole, irrespective of the sex of the best friend.
Interestingly, the extent to which shared history were considered an important mechanism of relationship maintenance in best friendships had opposite effects on intimacy in men and women. Whereas this relationship was positive in men, in women it was negative (the greater the emphasis on shared history, the lower the level of intimacy). This might, again, reflect the difference between men’s preference for group-based activities (for which shared history is usually an important component) and women’s preference for more intimate dyadic ones (for which shared history might be less important than, for example, conversation and levels of mutual disclosure).
In women, both the importance placed on gift-giving and mutual support as ways of sustaining romantic partnerships were included in the best-fit model, but these variables had opposite effects on intimacy. The greater the importance placed on gift-giving, the lower the intimacy; in contrast, the greater the importance given to mutual support as a mechanism of relationship maintenance, the greater the reported intimacy. Whereas gift-giving is observed cross-culturally as a means of creating and maintaining social network ties (e.g. Wiessner 1983), it may be that this strategy is only appropriate in the more distal layers of the social network where tokens of affiliation are required; in the inner layers, intimacy and emotional closeness may be more important (see Sutcliffe et al. 2012). It is possible that gift-giving is associated with forms of strict reciprocity in relationships that block the development of deeper emotional ties.
The importance of intimacy in same-sex female friendships may explain why similar humour profiles were found to be important for female but not male best friendships: laughter is thought to be important in the creation of social bonds (Dunbar 2017; Dunbar et al. 2012; Manninen et al. 2017). In contrast, similarity in social characteristics (outgoingness and social connections) were deemed more important for intimacy in male best friendships, perhaps reflecting the fact that men tend to prefer interacting in groups rather than one-to-one (Dávid-Barrett et al. 2015). Why this might be so evolutionarily remains to be answered, but one obvious suggestion relates to men’s near-universal role in communal defence in small scale societies and the demand this imposes for being able to cooperate in groups.
These behavioural differences suggest that best friend relationships are viewed very differently by the two sexes, corroborating and extending previous studies which suggest that the two sexes have very different expectations as regards friendships (Hall 2011, 2012; Machin and Dunbar 2013) and very different social styles (Roberts and Dunbar 2015). This strongly suggests that friendships serve rather different functional roles in the two sexes arising from different evolutionary selection pressures. While romantic relationships are, inevitably, equally common in the two sexes (in both cases, 86% of respondents reported having a romantic partner), a smaller proportion of males reported having a best friend (85%, compared to 98% of females). Moreover, whereas only 2% of females had a romantic partner but no best friend, 15% of males were in this situation suggesting that males, but not females, are more likely to have one or the other but not both. Although a significant proportion of males reported having a best friend, the quality of these relationships seemed to be a great deal less intimate than was the case for females (Fig. 2). This reflects earlier findings suggesting that the male social world is built around half a dozen relatively casual relationships, whereas the female social world is built around one or two much more intimate, and hence more fragile, dyadic relationships (Benenson and Christakos 2003; Roberts and Dunbar 2015; Dávid-Barrett et al. 2015).
In both sexes, only a minority of best friends were opposite-sex (15% for females; 22% in males). The gender homophily is itself striking, and probably reflects the fact that social networks are highly assortative for sex (Block and Grund 2014; Mehta and Strough 2009; Roberts et al. 2008; Rose 1985; Dunbar 2021). Even conversations readily segregate by sex once they contain more than four individuals (Dunbar 2016b; Dahmardeh and Dunbar 2017). Although having male best friends may be advantageous to females in terms of protection against the unwanted attentions of other males (Mesnick’s bodyguard hypothesis: Mesnick 1997; Wilson and Mesnick 1997; Dunbar 2010; see also Snyder et al. 2011; Ryder et al. 2016), it may be that male partners are likely to become jealous if their romantic partners show too much interest in male best friends, fearing either mate theft or cuckoldry. This might make cross-sex best friends less functional for paired females. Alternatively, intimate friendships between women may be more beneficial or easier to maintain (if only because of similar conversational styles: Coates 1996; Grainger and Dunbar 2009), while common interests make cooperation more straightforward (de Waal and Luttrell 1986).
These data are, of course, self-report data and represent the views of only one party in a relationship, and so are inevitably subject to the usual distortions this can involve. Nonetheless, in that respect, they do represent the aspirations and expectations of the person concerned, and it is these as much as anything that we are here interested in. While relationships are necessarily two-way processes, it is nonetheless failure of one individual’s expectations to be met in a relationship that is the usual cause of relationship breakdown (Dunbar and Machin 2014). Relationships break down because one party is dissatisfied with the deal they are getting, not because both parties “agree to disagree”. In this sense, these results provide us with direct insights into how individuals view their relationships, irrespective of whether they are right in their views.
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