I just called…
…to ask about my grandkids
WHEN Stevie Wonder crooned “I just called to say ‘I love you’,” he was bang on when it comes to men and women in their sexual prime. Were the ballad sung by a post-menopausal matron, though, the person at the other end of the line would probably be her daughter—and the conversation would revolve around grandchildren. That, at least, is the picture which emerges from a study published in Scientific Reports by Robin Dunbar, of Oxford University, and his colleagues.
Evolutionary psychologists like Dr Dunbar are interested in how investment in close relationships differs between the sexes. Dr Dunbar, indeed, has just published a book on the phenomenon of sexual love, analysed from a scientific point of view (see article). These differences reflect distinct strategies that have evolved to maximise reproductive success across a lifetime. One prediction that makes intuitive sense, but which has been difficult to nail down empirically, is that when women hit the reproductive wall of the menopause they funnel their remaining energy into bolstering their children’s—especially their daughters’—odds of producing viable offspring. (Sons spend less time minding their progeny than their spouses do.)
The main obstacle to testing the grandmother hypothesis, as it has come to be known, is that most studies have involved small numbers of people, making it hard to draw sweeping conclusions. Dr Dunbar leapt this hurdle by tapping a trove of 2 billion anonymised telephone calls and 500,000 text messages between customers of an unnamed European mobile-phone operator over the course of seven months. After eliminating those for which age and sex data were unavailable, they identified the people each subscriber contacted most often. Frequency of contact is a good proxy for emotional closeness, so this yielded a list of 1.2m “best friends”, and 800,000 “second-best friends”.
For any given age, the researchers then calculated the average sex of men’s and women’s phone pals. They did this by adding 1 every time the friend was a man and subtracting 1 every time it was a woman, and dividing the result by the number of friends for the age/sex group in question. If every person in the sample had a male best friend, the average-sex index would be 1; if all the best friends were women, it would be -1. An equal number of male and female best friends would mean the index came out at precisely zero.
Between the ages of 20 and 40 men and women behaved similarly (see chart). Both tended to have best friends of the opposite sex. The proclivity was slightly more pronounced among ladies, whose best-friend sex index peaked in their late 20s at 0.46 (equivalent to roughly three male best friends for every female one in the sample) and stayed more or less the same throughout their 30s. Men reached a maximum of -0.41 a bit later and remained there for less time. In both cases, these best friends tended also to be of a similar age, suggesting they were actually sexual mates. Second-best friends, meanwhile, were typically of the same sex—chums, in other words.
Things change markedly, though, as people enter middle age. For men, the best-friend sex index falls steadily from its peak until it levels off at the age of 50 or so, while remaining skewed towards females for the rest of their lives. However, any sexual bias for second-best friends more or less disappears when men reach their 40s, consistent with the hypothesis that these “friends” are by then no longer chums, but children (who are about as likely to be male as female) and that fathers do not favour those of any one sex.
Among women, by contrast, the best-friend sex index plummets around the time menopause strikes. By the age of 55, it actually turns negative, in favour of other females who are, tellingly, about half their age. At around that time, women’s second-best friends are increasingly men from their own generation. Older women, it appears, do indeed invest more time in furthering their daughters’ welfare—and reproductive success—and less in nurturing relationships with their husbands, no doubt to the latter’s chagrin. Strong evidence, then, for the grandmother hypothesis. And possibly an explanation for men’s mid-life crisis.