In a study of 13 families with genetic microcephaly, it was found that 11 of 24 parents (two fathers not ascertained) and 11 of 33 non-microcephalic siblings were of subnormal intelligence. In a rare autosomal recessive condition, all parents and two-thirds of unaffected siblings are presumed carriers of the gene. It so, nearly 50% of all presumed heterozygotes in the present study were mentally retarded. The proportion agrees well with the other major study of microcephaly in North America, but differs from the Dutch study. On the basis of pooled data from the three studies and the estimated incidence of 1:40000 for genetic microcephaly, it is postulated that (1) about 0.34% of the general population is mentally retarded because it is carrying the gene for microcephaly and (2) about one of nine mentally retarded individuals is heterozygous for the gene for microcephaly.