Having children is contagious among high school friends during early adulthood

"The study shows the contagion is particularly strong within a short window of time: it increases immediately after a high school friend gives birth, reaches a peak about two years later, and then decreases, becoming negligible in the long-run," said co-author Nicoletta Balbo, a postdoctoral fellow at the Carlo F. Dondena Centre for Research on Social Dynamics at Bocconi University in Italy. "Overall, this research demonstrates that fertility decisions are not only influenced by individual characteristics and preferences, but also by the social network in which individuals are embedded. In addition, it shows that high school friends impact our lives well after graduation."

Titled, "Does Fertility Behavior Spread among Friends?," the study, which appears in the June issue of the American Sociological Review, relies on data from the National Longitudinal Study of Adolescent Health in the United States.

"We know that friends influence each other on many behaviors, such as smoking, drinking, and how much we exercise," Balbo said. "Several sociological theories have proposed social influence as an important factor for shaping fertility choices, and a limited number of studies have demonstrated that such a connection exists among relatives and co-workers. But we believe our study is the first to show this type of connection among friends."

In their study, Balbo and co-author Nicola Barban, a postdoctoral fellow in the department of sociology at the University of Groningen in the Netherlands, focus on more than 1,700 American women who were tracked from when they were at least 15-years-old through approximately age 30. The researchers looked only at the impact of female high school friends on the birth of first children and their findings only held true for planned pregnancies. The median age at first birth for women in the study was just over 27-years-old.

So why does having a high school friend who has a baby increase the likelihood that a woman will have a baby of her own?

"We believe there are three possible explanations," Balbo said. "First, people compare themselves to their friends. Being surrounded by friends who are new parents makes people feel pressure to have kids as well. Second, friends are an important learning source. Becoming a parent is a radical change. By observing their friends, people learn how to fulfill this new role. Lastly, having children at the same time as friends may bring about many advantages -- friends can share the childbearing experience and thus reduce the stresses associated with pregnancy and childrearing. It's also easier for people to remain friends when they are experiencing parenthood at the same time."

The fact that the likelihood of a woman having a child increases after a high school friend gives birth, reaches a peak approximately two years later, and then decreases makes sense, according to Barban. "It takes time to have a child -- because there is a natural period before conception and because the desire to have a child develops over time," he said. "As a result, the effect of a friend giving birth is not immediate. If we had observed an immediate effect, we would have concluded that, rather than being influenced by each other, friends just decide to have children at the same time."