The perception that peers are sexually active increases with age (Alexander & Hickner, 1997). Adolescents who report believing that most of their peers have had sex are more than twice as likely to report having a high intention to initiate sexual intercourse in the coming year (Kinsman et al., 1998). Believing that peers endorse and engage in sexual intercourse was associated with an increased incidence of teen sexual intercourse for male and female adolescents receiving health care at private family practices in Michigan (Alexander & Hickner, 1997). Sexually experienced adolescents under the age of 15 were more likely to report that their peers were also sexually experienced compared to sexually inexperienced adolescents of the same age in samples of male and female African American adolescents in Philadelphia (Jaccard, Dittus & Litardo, 1999) and male and female, white, black, Hispanic, and Asian sixth-grade students in 14 public schools in Philadelphia in 1994 (Kinsman et al., 1998).

In a longitudinal study of middle school students in an urban area of Northern California, adolescents who reported having a greater number of friends who endorse having early sexual intercourse were more likely to have had sex (Marin et al., 2000). Similarly, having a best friend who is sexually experienced increases the likelihood of sexual experience among adolescent females (Lock & Vincent, 1995), and having sexually experienced friends is associated with a younger age at first intercourse among males (Miller, Norton, et al., 1997). Teens who are more likely to be sexually experienced and who report a greater number of sexual partners are also more likely to have a greater percentage of sexually active friends (Whitaker & Miller, 2000).