Normal IQ Distributions of Early-treated Phenylketonuric Children and their Unaffected Siblings: Failure to Replicate a Trimodal Response or Negative Skew

Abstract

PHENYLKETONURIA, an inherited metabolic disorder associated with mental retardation, has been generally considered a single gene defect. Fuller and Schuman¹, however, have argued for a more complex genetic model, citing a trimodal frequency distribution of IQ scores for a sample of treated phenylketonurics (PKUs) and a negative skew in the IQ distributions of unaffected siblings as evidence for their model. We have analysed data collected by Hudson² to test the basis of Fuller's findings. Evidence was found for neither a trimodal response to treatment of phenylketonuria nor a negative skew in the IQ distribution of their unaffected siblings. Both groups showed IQ distributions which were not significantly different from the expected, normal, curve for that sample.