In sociology, a peer group is both a social group and a primary group of people who have similar interests (homophily), age, background, or social status. The members of this group are likely to influence the person's beliefs and behaviour.

The Nurture Assumption: Why Children Turn Out the Way They Do is a 1998 book by the psychologist Judith Rich Harris. Originally published 1998 by the Free Press, which published a revised edition in 2009. The book was a 1999 Pulitzer Prize finalist.