Genetic divergence in relatives of PKU's: Low IQ correlation among normal siblings

The IQ frequency distribution of 78 unaffected siblings of patients with phenylketonuria (PKU) differed significantly from the normal population (X 2 = 43.42, df = 5, p < .0001). The excessive number of superior IQ' s of this distribution apparently was not a function of superior families. The low intersibling correlation, r = .13, suggests a difference between carriers and noncarriers of the disease PKU. Because the proportion of high IQ's approaches the expected percentage of homozygotes or noncarriers for the disease, heterozygotes for PKU may be suffering a functional disadvantage. A theory is presented on the possible importance of genetic‐recessive diseases whose carriers are at an intellectual disadvantage, thereby requiring compensatory genetic mechanisms.