Development of individuals' own and perceptions of peers' substance use from early adolescence to adulthood.

This study evaluated how individuals' own substance use and their perception of peers' substance use predict each other across development from early adolescence to middle adulthood. Participants were from two longitudinal studies: Fast Track (FT; N = 463) and Child Development Project (CDP; N = 585). Participants reported on their own and peers' substance use during early and middle adolescence and early adulthood, and their own substance use in middle adulthood. From adolescence to early adulthood, individuals' reports of their own substance use in a given developmental period predicted reports of their peers' substance use in the next developmental period more than peers' substance use in a given developmental period predicted individuals' own substance use in the next. In the higher-risk FT sample, individuals' own substance use in early adulthood predicted alcohol, cannabis, and other substance use in middle adulthood, and peers' substance use in early adulthood predicted cannabis use in middle adulthood. In the lower-risk CDP sample, participants' own substance use in early adulthood predicted only their own cannabis use in middle adulthood, whereas peers' substance use in early adulthood predicted participants' alcohol, cannabis, opioid, and other substance use in middle adulthood. The findings suggest that peer substance use in early adulthood may indicate a greater propensity for subsequent substance use in lower-risk groups, whereas those in higher-risk groups may remain more stable in substance use, with less variability explained by peer contexts.