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."