Behavioral contagion in sibships.

A behavior is "contagious" if one person is more likely to exhibit it when a relevant other person has already done so. In this sense, behavioral contagion is commonly thought to contribute to many social problems, such as drug abuse and teenage promiscuity. In this paper we focus on behavioral contagion in sibships. Borrowing a model from the theory of contagious diseases, we show that contagion will cause prevalence to increase with sibship size. This model also allows us to estimate the magnitude of the contagious factor relative to non-contagious factors. Finally, we develop two statistical tests for the presence of contagion. Results are presented for participation in a skill-development program and four child-psychiatric conditions: neurosis, hyperactivity, somatization, and conduct disorder. Evidence is presented that program participation is probably contagious and conduct disorder possibly so. The other three child-psychiatric conditions are shown not to be contagious. Implications for research and practice are discussed.