Research with 400 pairs of 7-year-old twins assessed the genetic and environmental effects on children’s physical and social aggression (behaviors such as spreading rumors). Specifically, the researchers found that physical aggression in a friend is likely to interact with genetic tendency towards physical aggression; however, a child’s social aggression was not affected by one’s genetic disposition. Further, effects of friends’ aggression on other children’s aggression were only observed within the same type of aggression.