It is a well-known fact that males predominate among the mentally retarded. Various explanations for this have been offered—among others, more exposure of the male fetus to external pathogenic influences in the antenatal period, a lower threshold of appearance of mutant polygenes responsible for mental retardation in males, or a high frequency of X-linked mental retardation [1]. The first two hypotheses have not been confirmed. Although a lower threshold of appearance of mental retardation in males has been demonstrated [2], this cannot explain such a major quantitative divergence between males and females as exists in mental retardation. It has not been easy to confirm the third hypothesis since the forms of X-linked hereditary pathology frequently appear sporadically, i.e., in only one boy in a family.