The Role of Assertiveness and Decision Making in Early Adolescent Substance Initiation: Mediating Processes Linda Trudeau, Catherine Lillehoj, Richard Spoth, and Cleve Redmond Iowa State University This study examined the mediating processes linking assertiveness and decision making to early adolescent substance initiation, along with the moderating effect of gender on those processes. Models specifying negative expectancies and refusal intentions as mediators of individual rights assertiveness and decision-making effects on substance initiation were evaluated across 18 months on a nontreatment cohort of young adolescents participating in a prevention trial (average age 12.3 years at baseline; N 5 357). Results indicated that individual rights assertiveness and decision making had indirect effects on substance initiation through effects on negative outcome expectancies and refusal intentions. Gender differences were found in both the average level and the pattern of relationships among the variables. For girls, refusal intentions were negatively associated with later substance initiation. For boys, early levels of substance initiation were negatively associated with later levels of negative expectancies and refusal intentions. Implications for prevention programming are discussed. A recent report suggests that substance use among young adolescents in the United States remains high (Johnston, O’Malley, & Bachman, 2002). Because substance use during early adolescence has been associated with multiple negative outcomes (Dryfoos, 1990; Riggs & Whitmore, 1999), the past two decades have seen an increase in approaches to delay and JOURNAL OF RESEARCH ON ADOLESCENCE, 13(3), 301–328 Copyright r 2003, Society for Research on Adolescence Requests for reprints should be sent to Linda Trudeau, Institute for Social and Behavioral Research, ISU Research Park, Building 2, Suite 500, 2625 North Loop Drive, Iowa State University, Ames, IA 50010. E-mail: ltrudeau@iastate.edu