An exception to the general clear decrease of risk behavior during young adulthood is HIV-related sexual risk behavior, defined as unprotected intercourse with an occasional or a new partner. The late and less pronounced decline might reflect a temporal effect that adolescents and young adults engage in sexual relationships later than they become involved with substances. In a U.S. study, an inverted U-shaped trajectory was found for the number of partners, peaking around the age of 20 and then declining (Kan et al., 2010). Sexual risk behavior as the result of a dyadic interaction might also have a different functionality compared with other risk behaviors.
