The results supported the hypotheses that participation in the SAAF prevention program would increase intervention-targeted parenting regardless of youth genotype at DRD4 and that youths with the DRD4 7-repeat allele would be more responsive to the intervention than would youth with two copies of the DRD4 4-repeat allele. A main effect of intervention on change in past-month substance use over time emerged; main effects for both DRD4 and 5HTT approached significance. As the susceptibility hypothesis (Belsky & Pluess, 2009) and research with toddlers (Bakermans-Kranenburg et al., 2008) suggested, youths with the 7-repeat allele were more responsive than those without it to a parenting-focused intervention. Among youths with the 7-repeat allele, those in the control group increased past-month substance use at a substantially greater rate than did those in SAAF. Conversely, for youths with 4-repeat alleles, no evidence indicated that participation in the intervention reduced age-related increases in past-month substance use. The net result was greater differential change in the predicted direction between intervention and control youth for those with the 7-repeat allele relative to those with two 4-repeat alleles. In
