Various studies have shown that the frequency of contact between individuals is a reliable indicator of the emotional link between them. So it should come as no surprise that the data from mobile phone calls is a potential treasure trove of information about the social lives of humans. 

But analyses of this data so far have been distinctly unspectacular. For example, the location data associated with phone calls has revealed various new intricacies in the movements of commuters. Interesting but hardly jaw-dropping.

That is set to change with the work of Vasyl Palchykov at the Aalto University School of Science in Finland and a few buddies including a couple of old hands in the form of Albert-László Barabási at Northeastern University and Robin Dunbar at the University of Oxford (of Dunbar’s number fame). 

These guys have got hold of a corpus of mobile phone data relating to calls between 1.4 million women and 1.8 million men in an unspecified European country. Between them, these phone subscribers made almost 2 billion calls and sent almost half a billion text messages. In addition to the gender of each subscriber, Palchykov and co also managed to get their age as well. 

That’s significant because it allows them to study not allow the pattern of calls between genders but the way this changes with age. 

They began by taking each subscriber and determining the age and gender of the person they werein contact with most frequently, second most frequently and so on. These, they assume, are the ‘best’ friend, second best friend and so on.

Then, they looked at how the ‘best friends’ changed as subscribers age. It turns out in general that between the ages of 18 and 40 or so, men and women have best friends of the opposite sex. Palchykov and co assume this reflects the general pattern of mating in society. Second best friends are generally of the same sex at this age.

But they tease the most interesting phenomena out of the fine detail in their dataset. They conclude for example that women are more focused on opposite-sex relationships than men are during the period of their lives when they are reproductively active. That indicates that women invest  more heavily in creating and maintaining their relationships than men.

As women age, their attention shifts from their spouse to younger females some 25 years or so younger. That’s about equal to a generation gap and Palchykov and co assume these younger females are daughters. This attention shift also seems to equate to the arrival of grandchildren, when the older female again once again begins to invest more heavily.

While older women focus more heavily on younger females, older men maintain an even gender balance in the second best friends, presumably this reflects an equal attention between children of opposite sexes.

What’s striking about this is how strongly female relationships are determined by their reproductive cycle. “Women’s gender-biases thus tend to be stronger than men’s, seemingly because their patterns of social contact are strongly driven by the changes in the patterns of reproductive investment across the lifespan,” say Palchykov and co.

Clearly, female reproductive strategies change more explicitly as they age, switching from mate choice to personal reproduction to parental investment and finally grandparental investment, particularly after they reach 40. 

However, the most dramatic conclusion from this work is about the pattern of social relationships that play the most important role in society. Palchykov and co say the tendency in the past has been to assume that father-son relationships dominate. 

By contrast, “our results tend to support the claim that mother-daughter relationships play a particularly seminal role in structuring human social relationships,” they say. 

This difference on the way the sexes invest in relationships is exactly what evolutionary biologists expect. But although previously suspected, it has proved particularly difficult to test. That’s why this work is something of a landmark.

Clearly, the ability to study human relationships on such a vast scale opens up a host of new avenues for research in social and reproductive strategies.

In particular, this study looks only at the existence of links between people, not the the directional asymmetries in relationships or who initiates contact.  Palchykov and co leave that for another day.

There’s a mountain of data ready to be mined on this. And clearly, there’s gold in them thar hills.

Ref: arxiv.org/abs/1201.5722: Sex differences in intimate relationships