An analysis of 1.95 billion cell phone calls and 489 million text messages reveal how men and women follow different relationship patterns during their lifetimes. The researchers argue that women's friendships in particular drive the process of finding a mate and supporting the next generation.

The data could also undermine traditional notions about how humans like to organize themselves. "There has been a view in anthropology that the ancestral state for humans is a form of patriarchy, and I'm not sure that that's true," says University of Oxford anthropologist Robin Dunbar, an author of the study published April 19 by Nature Scientific Reports. (Scientific American is a part of Nature Publishing Group.)

Dunbar and an interdisciplinary team examined cell phone data from a single provider in an undisclosed European country. (Specific locations were kept anonymous to protect cell phone users' identities.) The researchers worked with data gathered over a seven-month timeframe and restricted themselves to studying communications between cell phone users of a known age and sex, making a data set of about 3.2 million subscribers, or about 20 percent of the nation's cell phone users. Working on the assumption that close friends communicate most frequently, the team analyzed the top three friendships of each cell phone user based on the frequency of communication to spot patterns in the average male or female user at various ages.

The researchers expected to find "homophily," or the tendency for an individual to pick a friend like him or herself. Instead, it seems that romance trumps other forms of friendship: The data revealed that an individual's best friend, particularly in one's 20s and 30s, happens to be someone of the opposite sex and a similar age. In addition, striking differences exist in how men and women communicate with their presumed romantic partner. For one, the man in a woman's life was her very best friend for roughly 15 years, compared with seven years in the case for men. The peak age for partner parlance also differed: 27 years old for women and 32 for men.

After age 50, however, things change. The preference for a romantic partner peters out in both men and women, and toward the oldest age range in the data set, both sexes seek companionship first and foremost. For a woman, friendship with her man was replaced by a strong relationship with another woman, usually about a generation younger. Dunbar and his colleagues interpret this pattern as a mother–daughter relationship.

Putting together the strong preference in women for first a man and then a daughterlike figure, the researchers conclude that biology shapes female behavior, which in turn affects men. Dunbar suggests that women initiate and prioritize the relationship with a romantic partner earlier in life than men, an action that gradually leads men to reciprocate. This relationship remains top priority throughout the average woman's childbearing years. After that, she turns her attention to supporting the next generation of women as they approach childbearing.

"Generally, we have probably underestimated how important these family support networks are," Dunbar says. He speculates that contemporary declines in family size may reflect the mobility of modern women, isolating them from their supportive maternal network. In addition, he believes that the bonds between mother and daughter and the strength of a woman's influence on mating are so strong that they may underlie human society's natural tendencies. "I think the default for humans, if all else is equal, is actually a matrilineal society."

The data set also hints at a disparate and diffuse model in male friendship. The phone records support the narrative that women have intense, one-on-one friendships maintained and shaped through frequent communication. In fact, Dunbar believes that digital communication, with its texts, instant messages and other quick bursts, is generally tailored to a female's friendship style. Men, the data suggest, have a very different approach: other than those romantic years with a woman as their best friend, men have multiple friendships with an equal ratio of men and women. This conclusion supports a popular model of male relationships in which men prefer to bond in groups doing shared activities.

The patterns of male and female friendship follow long-established observations in psychology and other fields, but the study's broader biological interpretations strike some researchers as too speculative. "This is very interesting data," says University of Rochester psychologist Harry Reis. "However, there are innumerable alternative explanations for the patterns they have come up with." Reis studies human social interactions and has written extensively on intimacy and friendship in men and women. Among his concerns are situations in which non-romantic opposite-sex individuals communicate frequently, such as between co-workers or with an employer. Another case is the possibility that a woman's relationship pattern shifts with age because later in life she may have lost her romantic partner through death or divorce.

Anthropologist Daniel Hruschka of Arizona State University in Tempe, who has a written book on the evolution of friendship across cultures, was struck by the similarities rather than the differences in the data on men and women. "In their reproductive prime, both men and women call the opposite sex much more than they do later in life," Hruschka says. Even presumed mother–daughter patterns are weaker than he expected. The data suggest that both men and women split their time between calling their children and their spouses. "These differences seem quite small relative to reigning stereotypes about how frequently women communicate with children."

Dunbar nonetheless suspects that the patterns they have identified are universal; he and his colleagues have a paper in press comparing male–female relationships differences across cultures. That is not to say he believes that these patterns apply to everyone. "Our problem, in a way, is that we're looking at averages," Dunbar says. Individuals who do not conform to the assumptions of the study—for example, childless women—are assumed to be in the minority. "Undoubtedly, they're in there somewhere, but we probably wouldn't be able to pick that up."