Choices, challenges, and constraints: a pragmatic examination of the limits of mental age matching in empirical research

A common method of experimental control in the study of intellectual disability in children is mental age matching, which allows for meaningful comparisons between intellectually disabled children and typically developing children that consider the inherent differences in developmental rates between these two groups. One's mental age is proportional to the product of one's IQ and one's chronological age. It follows that development on IQ tests is measurably linear with chronological age. We test this implication by first reverse engineering the distribution of raw scores on the subscales of three common kinds of IQ tests--Stanford Binet, Wechsler Abbreviated Intelligence Scale, and the Wechsler Intelligence Scale for Children--and then determining whether these scores are linear using Bayesian Information Criterion comparisons of segmented regressions. We find linearity in only one subscale, imposing limitations on the accuracy of the mental age matching protocol.