Comparison of cocaine infusions (0.5mg/kg/20μL inf) earned under (A) FR2 and (B) FR5 reinforcement schedules between GRL and PRL strains. Data are presented as mean ± SEM (FR2: n= 7 BxD31, 17 BxD38, 11 BxD42, 6 BxD68; FR5: n= 7 BxD31, 17 BxD38, 11 BxD42, 6 BxD68). Asterisks indicate significant differences between pooled PRL and GRL strains in all pairwise comparisons: * = at least p<0.05, **= at least p<0.01, ***= p<0.001