A recent study of over 2 billion cell phone calls and text messages exchanged between people showed that women call most often their boyfriend or husband from the age of 20 and for 15 years, after which they call most often their daughters. Men, instead, call their girlfriend or wife most often in their early 30s and for about seven years, after which they switch to other friends. This suggests that women play a larger role in maintaining heterosexual romantic relationships than men do. A team of researchers from various European countries led by Robin Dunbar, a professor of evolutionary anthropology at Oxford University in the U.K., analyzed a large dataset of cell phone calls provided by a single service provider in an unspecified European country. The dataset covered a seven-month period and included 1.95 billion calls and 489 million text messages. The researchers analyzed call data for over 3.2 million subscribers: 1.8 million males and 1.4 million females. They defined the "best friend" of a given subscriber (“ego”) as the person that a caller was most frequently in contact with, counting both the number of calls and text messages; the "second best friend" was the next most frequently contacted individual, and the "third best friend" was the next most frequently contacted individual. Restricting the analysis to pairs of subscribers for both of whom age and gender information was available, the researchers ended up with 1.19 million ego/best-friend pairs, 0.80 million ego/second-best-friend pairs, and 0.66 million ego/third-best-friend pairs. The researchers did not have information about whether pairs of individuals were married or dating, simply friends, or parents and children. They assumed that opposite-sex individuals of similar ages were in a relationship (married or dating) or in a courtship phase of a relationship, that same-sex young adults and adults were friends, and that adults talking with children were their parents. In interpreting the results, the researchers assumed that cell phone communication represents the most important relationships of subscribers and that the strength of communication reflects the level of emotional closeness. Their results suggested that women are more focused on opposite-sex relationships than men are during the reproductively active period of their lives, suggesting that they invest more heavily in creating and maintaining pairbonds than men do. Second, as they age, women’s attention shift from their spouse to their daughters and this further increases when their daughters become old enough to have children. According to Robin Dunbar, the study shows that pair-bonding is much more important to women than men. "It's the first really strong evidence that romantic relationships are driven by women," he told BBC News. "What seems to happen is that women push the 'old man' out to become their second best friend, and he gets called much less often and all her attention is focused on her daughters just at the point at which you are likely to see grandchildren arriving," he said. Dunbar also argued that human societies are moving away from a patriarchy and back to a matriarchy. "Men's relationships are too casual. They often function at a high level in a political sense, of course; but at the end of the day, the structure of society is driven by women, which is exactly what we see in primates," he told the BBC.