Development of cognitive averaging: when light and light make dark

In two experiments we studied developmental changes in reasoning about intensive quantities. In contrast to previous studies, color--an intensive quantity that is readily perceivable--was chosen as stimulus material. Children between 6 and 12 years of age and adults predicted the mixture intensity of pairs of liquids with different intensities of red color, the liquid volumes in the two source glasses either being the same (Experiment 1) or different (Experiment 2). The data show that cognitive averaging in this domain develops late and slowly. An extensitivity bias, a strong tendency to use rules (adding in particular) that hold for extensive but not for intensive quantities, 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 averaging rule became more likely when an extensive quantity, volume, was also varied and thus the task increased in complexity.