sex differences in close friendships

Sex is highly relevant to communication and sociality.  Sex is a biological reality rooted in more than a billion years of Darwinian evolution.  Sex is a salient characteristic of many human groups, e.g. football teams and Women’s Studies Departments.

Define as a person’s “best friend” the other most frequently contacted via mobile phone.  Define as the “second best friend” the second most frequently contacted other.  These definitions capture the important point that friendships require investments in time and attention.  Both time and attention in the real world are limited: you cannot have an unlimited number of close friends.

Like men’s reproductive potential compared to women’s, men’s close friendships vary less sharply across adulthood than do women’s close friendships.  Women on average shift to an opposite-sex best friend at age 18, while men do so about four years later.  The share of women with a male best friend peaks at 73% at age 27, while the share of men with a female best friend peaks at 70% at age 32. These differences are plausibly interpreted as women seeking and establishing strong opposite-sex pair bonds at a younger age on average than men do.

Middle-age men do not shift their best friendships away from women as rapidly as middle-age women shift their best friendship away from men.  The share of men with a female best friend decreases from age 32, but the share remains above 50% for men at all subsequent ages.  For women, in contrast, the share maintaining a male best friend drops sharply about age forty to less than 50% by the mid-fifties.  From this age onward, a larger share of both men and women have women as best friends.  Women’s “second best friend” shifts in middle-age  toward men.  The age pattern of friendships suggest that a child, predominately a daughter, replaces the opposite-sex reproductive partner as the predominate women’s best friend after age 40.

Compared to women’s lifespan pattern of best friendships, men’s friendships place less demands on social skills.  Not surprisingly, men are less active on social networking services than are women.

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Source notes:

The discussion above is based on the following recently published article:  Palchykov, Vasyl, Kaski, Kimmo, Kertész, Janos, Barabási, Albert-László, and Dunbar, Robin I.M. 2012. “Sex differences in intimate relationships.” Scientific Reports. 2. doi:10.1038/srep00370  See in particular Figure 1. The “average gender” is an average calculated with male = 1 and female = -1. Hence an equal sex ratio is a gender average of zero.  The male share is the average gender plus one, divided by two.  The statistics cited are based on analysis of a mobile phone dataset for a single mobile phone operator in a specific European country across seven months.  The dataset includes about 3.2 million subscribers, 1.95 billion calls, and 489 million text messages.

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