Pregnancy is contagious. That’s the conclusion of a study just published in the American Sociological Review; the decision to have a child is influenced by social networks stretching back to high school. “A friend’s childbearing positively influences an individual’s risk of becoming a parent,” the study concludes, with a phrasing reminiscent of “friends don’t let friends do drugs.”

Furthermore, “an individual’s risk of childbearing starts increasing after a friend’s childbearing,” reaching “a peak around two years later.” Coauthor Nicoletta Balbo summed those results up like this in a press release: “The study shows the contagion is particularly strong within a short window of time.” Like the flu.

Though this isn’t about high school pregnancy pacts and teen moms. (Teen pregnancy rates are actually at their lowest in years.) This study looked at the effects of high school friends 15 years on, following 1,700 women tracked from the age of 15 to around 30. In the group, the median age for birth of the first child was 27.