Children and monkeys overestimate the size of high-contrast stimuli.

The letter height superiority effect reveals that human adults judge letters to be taller than identically sized pseudoletters. This effect extends to words, such that words are estimated to be greater in size or lasting longer in duration than pseudowords of the same size or those presented for the same duration. The physical properties of letters and words also impact their perceived size, such that higher contrast between figure-ground stimuli leads to greater size estimates. Specifically, black letters on a white background (high contrast between figure and ground) are judged to be taller than gray letters and gray pseudoletters on a white background (low contrast between figure and ground) for adult humans. In the current study, we assessed whether this effect would extend to nonverbal stimuli (shapes) such that high-contrast shapes would lead to greater size estimates relative to low-contrast shapes for human children and rhesus monkeys in a two-choice discrimination task. We found that children and monkeys tended to overestimate the size of high-contrast shapes relative to low-contrast shapes consistent with results reported among human adults. Implications for perceptual fluency and its impact on subjective size estimates are discussed.