Development of cognitive averaging: When light and light make dark

In two experiments we studied developmental changes in reasoning about inte nsive quantities. In contrast to previous studies, color-an intensive quant ity that is readily perceivable-was chosen as stimulus material. Children b etween 6 and 12 years of age and adults predicted the mixture intensity of pairs of liquids with different intensities of red color, the liquid volume s in the two source glasses either being the same (Experiment 1) or differe nt (Experiment 2). The data show that cognitive averaging in this domain de velops late and slowly. An extensitivity bias, a strong tendency to use rul es (adding in particular) that hold for extensive but not for intensive qua ntities, predominated up to the age of 12 years and was not totally absent in adults. Contrary to common expectation, children's use of the correct av eraging rule became more likely when an extensive quantity, volume, was als o varied and thus the task increased in complexity. (C) 2001 Academic Press .