Fifty sets of twins were compared for the relationship between birth weight and WISC (Wechslen Intelligence Scale for Children) IQ. The lighter members within sets were found to have lower IQs. When identical (N = 22) were separated from fraternal twins (N = 28), the finding of lower IQ in the lighter members of sets was seen in the identical but not in the fraternal group. Furthermore, it was found that the performance section of the WISC IQ test bore a relationship to birth weight, but the verbal section did not.