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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.
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Landrum et al. (1988) studied this question.
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