Dose-dependent differences in the mean reinforcers obtained (±SEM) across acquisition sessions in Experiment 4 collapsed across Sex and Social Condition (a; all rats), and collapsed across Social Condition for males (b) and females (c). Rats either had access to saline (0.00 mg/kg, Sal, open squares) or nicotine (0.015 mg/kg, Nic, triangles; 0.03 mg/kg, Nic, closed circles). An FR1 schedule of reinforcement was in effect during sessions 1-3 and a progressive ratio schedule was in effect during sessions 4-20 that increased from an FR1 to FR2 to FR 3 depending on individual performance with the criterion for advancement of 5 reinforcers obtained within 60 min. Asterisks (*) represent a difference between 0.015 and 0.00 mg/kg doses, test of simple effects, ps<0.05. Plus signs (+) represent a difference between 0.03 and 0.00 mg/kg doses, test of simple effects, ps<0.05. Pound sign (#) represents a difference between 0.015 and 0.03 mg/kg doses, test of simple effects, p <0.05.