Summary.This investigation studied the differential effects of the significance of stimuli on educable mentally retarded (EMR) children in a conservation of number experiment. The sample consisted of 30 low and 30 middle SES children attending classes for the EMR. The effect of using candies or paper‐clips was investigated in Piaget's one‐to‐one correspondence task. A significant interaction between SES and the significance of stimuli was found. This was discussed in terms of the possible misdiagnosis of lower class children, multiple criteria diagnostic procedures, and the relevance of