Behavioral Effects of Combining Alcohol and Caffeine: Contribution of Drug-Related Expectancies

This experiment tested the hypothesis that drinkers' expectations about the behavioral effect of combining alcohol and caffeine predicted their psychomotor performance when they expected to receive both drugs. Fifty male social drinkers were assigned to 1 of 4 treatment groups or to a no-treatment control group. Participants in the 4 treatment groups expected and received alcohol (0.56 g/kg). The expectation and the receipt of caffeine (4.4 mg/kg) were manipulated independently. After training on a Pursuit Rotor task, participants rated the expected effect of combining alcohol and caffeine and then received the drug treatments. As predicted, individual differences in expected effects predicted participants' performance when they expected to receive caffeine in combination with alcohol. Regardless of whether caffeine was actually received, those who expected the most impairment from the drug combination performed most poorly. This evidence has implications for understanding factors that contribute to individual differences in behavioral responses to drugs.