A developmental study of the regular-random numerosity illusion.

Perceived number is influenced by item arrangement. It has been shown previously with adults that a regular pattern of dots appears more numerous than a random pattern with the same physical number. The present study used similar stimuli: two sets of 37 dots arranged within circular areas, one regular and the other random. Forty-eight grade school students were tested; they ranged in age from 5.6 to 14.8 years and included 27 girls and 21 boys. The illusion was found to increase in strength from the second to the eighth grade. The results were discussed in relation to Birnbaum and Veit's theory of contrast with expectancy.