Forty-four subjects were assigned to a high- or low-caffeine user group based on self-reported caffeine consumption. Subjects received either 200 mg caffeine or placebo. Testing on extroversion/introversion (E/I), tapping, serial learning, reaction time, writing speed, and reading comprehension indicated no significant effect of drug administration, but revealed a significant positive correlation between E/I and caffeine consumption, r= +0.326. Subsequent analyses revealed that extroversion levels in individuals may differentially influence variables of interest, and also can be influenced by chronic caffeine consumption.