Numerical Estimation: Caffeine, but Not Introversion-Extraversion, Results in Underestimates of Numbers of Faces

We hypothesized that high arousal (caffeine, introversion) causes over-estimates of numbers of people. Participants ingested caffeine or placebo and estimated numbers of happy, neutral, or threatening faces. All participants accurately estimated low numbers of faces but increasingly underestimated larger sets; caffeine accelerated this trend. Caffeinated introverts actually underestimated the most.