In a romantic relationship, it's the woman who takes the lead by relentlessly pursuing her man with phone calls and text messages, a new study has found.
But, once they reach middle-age their interest switches to a younger woman, presumed to be their daughters, who become old enough to have children, according to the analysis of 1.95 billion cell phone calls and 489 million text messages.
The study, published in the journal Scientific Reports, also shows that men call their spouse most often for the first seven years of their relationship, and then they shift their focus to other friends.
Study co-author Prof Robin Dunbar of Oxford University in the UK said the investigation 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.
But the data shows that women start to switch the preference of their best friend from about the mid-30s, and by the age of 45 a woman of a generation younger becomes the "new best friend", Prof Dunbar said.
"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 added.
Prof Dunbar also claimed that the findings suggested that human societies are moving away from a patriarchy back to a matriarchy.
The aim of the project, according to him, was to find out how close, intimate relationships vary over a lifetime. After looking at an at an extremely large cell phone database, they were able to track these changes extremely accurately.
They found that men tend to choose a woman the same age as themselves -- which the researchers presumed to be their girlfriend or wife -- as a best friend much later in life than women do, and for a much shorter time. This occurs when they are in their early-30s, possibly during courtship, and stops after seven years or so. the researchers found.
Women, however, choose a man of a similar age to be their best friend from the age of 20. He remains for about 15 years, after that he is replaced by a daughter.
According to the researchers, a woman's social world is intensely focused on one individual and will shift as a result of reproductive interests from being the mate to children and grandchildren.
The data, Prof Dunbar said, suggested that "at root the important relationships are those between women and not those between men".
"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 explained.
Many anthropologists argue that most human societies are patriarchal on the basis that in most communities men stay where they are born whereas the wives move.
But Prof Dunbar and his team argue that this only occurs in agriculturally based societies.
"If you look at hunter-gatherers and if you look at modern humans in modern post-industrial societies, we are much more matriarchal," he said.
"It's almost as if the pendulum between the two sexes, power-wise, is swinging (back) as we move away from agriculture toward a knowledge-based economy."