Number of Sexual Partners and Relationship Status Are Associated With Unprotected Sex Across Emerging Adulthood.

Sex with multiple partners, consecutively or concurrently, is a risk factor for contracting sexually transmitted infections (STIs) as multiple partner-partner contacts present increased opportunity for transmission. It is unclear, however, if individuals who tend to have more partners also use protection less reliably than those with sexual histories of fewer partners. Longitudinal data can elucidate whether an individual shows a consistent pattern of sex with multiple partners. We used latent class growth analyses to examine emerging adult survey data (N = 2244) spanning 10 waves of assessment across 6 years. We identified three trajectory classes described with respect to number of partners as (a) Multiple, (b) Single, and (c) Rare. Trajectory group, relationship status, and their interactions were tested as predictors of using protection against STIs and pregnancy at each wave. The Multiple Partners class had the greatest odds ratio of reporting sex without protection against STIs and pregnancy, followed by the Single and Rare classes. Exclusive relationship status was a risk factor for unprotected sex at earlier waves, but a protective factor at most later waves. There was no significant interaction between relationship status and trajectory class in predicting use of protection. The Multiple Partners class reported more permissive values on sex and an elevated proportion of homosexual behavior. This group overlaps with an already identified at-risk population, men who have sex with men. Potential mechanisms explaining the increased risk for sex without protection, including communication, risk assessment, and co-occurring risk behaviors are discussed as targets for intervention.