Low levels of caffeine may decrease performance on complex cognitive tasks. To test this hypothesis 63 psychology students participated in a study examining the effects of 45 mg of caffeine on the Stroop task. The participants consumed either a 12-oz (355-ml) can of Coca-Cola or a 12-oz (355-ml) can of caffeine-free Coca-Cola. A short questionnaire was then answered by the participants, after which they completed a computerized version of the Stroop task. We observed that women had slower reaction times than men (p < .001), and that the effects of caffeine were sex specific, having a significant effect only on men (p < .01). We also found a significant main effect of display (p < .001), and an interaction between sex and display (p < .05), which occurred only in the conflict condition of the Stroop task. These sex differences may reflect the lipid-soluble nature of caffeine. The decreased performance in the caffeine group can be explained by the Yerkes-Dodson law.

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