Exploring the Flynn effect in mentally retarded adults by using a nonverbal intelligence test for children

Increases in the scores on IQ tests across generations have been called the Flynn effect (FE). One of the unresolved questions is whether the FE affects all subsamples of the intellectual ability distribution equally. The present study was aimed at determining the size of the FE in moderately mentally retarded individuals. A nonverbal intelligence test developed for children, the Snijders-Oomen Nonverbal Intelligence Test (SON), was administered to 32 retarded adults with a mental age of 3-6 years. Sixty-nine children with a biological age in the same range and with normal intelligence served as a comparison group. Both an older and a more recent version of the SON were presented to all participants in a counterbalanced order. The proportion of items answered correctly was taken as a measure of the dependent variable. It was found that a FE existed in both the group of children and in the group of retarded adults, but that the FE was largest in the latter group. The importance of not using obsolete test norms when diagnosing mental retardation was stressed, and possible causes of the Flynn effect were discussed.