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Positive alcohol expectancies (PAE) and associating with drinking peers are reliable predictors of adolescent alcohol use. Knowledge of when and for whom these risk factors are most influential could enhance intervention effectiveness. Reciprocal relations between PAE and adolescent and peer alcohol use were examined between the ages of 13 and 18 in a sample (N = 566; 50% female) from the National Consortium on Alcohol and NeuroDevelopment in Adolescence (NCANDA), as well as sex differences in these associations. Associating with drinking peers prospectively predicted more frequent alcohol use for both sexes, although peer socialization was evident earlier for girls compared with boys. Higher PAE influenced later drinking in mid-adolescence, from age 14 to 16, for boys only. PAE influenced peer group selection for both sexes, although the influence was evident earlier in boys than girls. The relative impact of environmental risk factors for problematic alcohol use may vary over time and across developmental periods. These results suggest that prevention and treatment efforts for adolescent drinking can be improved by targeting age-appropriate risk factors. Early adolescent interventions may be best served by minimizing involvement with drinking peers and correcting normative beliefs of peer use. Among adolescent girls, early interventions focused on reducing peer influence may be most effective. Prevention and treatment programs aimed at addressing PAE would likely prove more effective for boys in mid- to late adolescence.