Undergraduates (4 men, 4 women) at a midwestern university participated in a study of the effects of caffeine on lexical decision making. The study also examined the effect of the medium in which caffeine was consumed, that is, water versus cola. Using a within-subjects design counterbalanced for order of presentation, the experimenters compared 4 treatments: caffeinated water, caffeine-free water, caffeinated cola, and caffeine-free cola. Contrary to expectation, caffeine slowed reaction times when consumed in either water or cola. However, caffeine improved lexical decision accuracy when it was consumed in a water-based beverage (Water Joe®); yet, accuracy was also marginally higher relative to control (spring water) after participants drank both caffeinated and decaffeinated cola. These unexpected results may be due to confounds arising from inadequate time to absorb the caffeine. Classical conditioning, whereby participants may come to associate cola taste with caffeine-induced enhancement of mental function, may also explain some of the unexpected results.

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