There are more mentally deficient men than women − Rakyat Biologi

The distributions of general cognitive abilities, as indexed, say, by IQ, are significantly different in men and women. Though the average intellectual abilities are expectedly similar (recall that IQ tests are designed to equalize the two sexes), there are more geniuses and cankers who are male. In other words, the male distribution is somewhat more platykurtic (flat), even though in both sexes, the expected bell-shaped curb is obtained. Though it is rarely mentioned in the voluminous literature on cognitive sex differences, there is a very obvious biological explanation for one aspect of this sex difference. Indeed, a greater incidence of male mental deficiency is wholly to be expected given the way the sex chromosomes interact with each other. The female of the human species, as of the other mammalian species is blessed by a genetic protective device called "lyonization" after Mary Lyon, a British geneticist who first recognized the phenomenon. Recall that women are born from the combination of two X chromosomes, one contributed by the father, and the other by the mother. In female mammals, one of the two X chromosomes is inhibited (inactivated) by the other in anywhere between 15 to 65% of the body’s cells, in a somewhat random manner. When one of these two X chromosomes contains a disease-determining gene, enough cells contain an inactivation of the mutant gene to protect the female from the disease. This inhibition is not complete, but is nevertheless quite effective. Recall that men are born from the combination of a paternal Y chromosome and a maternal X chromosome. The tiny Y chromosome, being so small, and also very different from the X chromosome, is unable to protect the male from bad genes located on the large X chromosome. As a result, all recessive X-linked genetic disorders affect males more often and more severely than they do females. Most X-linked diseases are recessive. In the few cases whre the X-linked disease is determined by a dominant gene, it is females who express the disease more severely. One example of such a disorder is a special form of rickets, which affects women twice as often as men. However, though men are less frequently affected, those who suffer it have a more severe form of the disease. Even though some X-linked diseases are manifest only in girls (such as Rett’s syndrome), they can be of the recessive type. In those cases, the disease is lethal for the male fetus. Are X-related "bad" genes frequent enough to explain the extent of the known sex difference in prevalence of mental deficiency ? Easily ! There are dozens of X-linked hereditary diseases leading to mental deficiency in boys and men only. In fact, as other authors have pointed out, female mammals are «genetic mosaics», a phenomenon reflected very concretely in the calico fur of female cats.

Table 3.

Mental deficiency syndromes due to genes located on the X chromosome

Syndrome	Description
Martin-Bell syndrome	Fragile X syndrome, may present as autism, macro-orchidy, large ears
Golabi syndrome	Microcephaly, excessive growth, eye, heart and gastrointestinal deformities, diaphragmatic hernia, brain tumors
Menkes syndrome	Copper deficiency, anemia, kinky hair, progressive cerebral atrophy, reduced life expectancy
Juberg syndrome	Retarded growth, deafness, microgenitalism, ocular deformities, flat nasal bridge
Rénier syndrome	Epilepsy, spasticity et deafness
Lujan syndrome	Marfanoid habitus, psychosis, cardiopathy
Renpenning syndrome	Microcéphaly and retarded growth, up-slanting palpebral fissures and small testes
Coffin-Lowry syndrome	Severe psychomotor retardation, facial and digital dysmorphisms, and progressive skeletal deformations
Lowe’s syndrome	Major abnormalities of eyes, nervous system, and kidneys. Clinical manifestations include congenital cataract and renal tubular dysfunction
Schimke syndrome	Progressive atrophy of the basal ganglia, growth retardation, external ophthalmoplegia, postnatal microcephaly, deafness, spondyloepiphyseal dysplasia, nephrotic syndrome and defective cellular immunity
X linked hydrocephalus	Spastic paraplegia, adducted thumbs, and agenesis of the corpus callosum, hypoplasia of the pyramidal tract and septum pellucidum, and a thin cerebral mantle with hypoplastic white matter
Norrie disease	Ocular atrophy, deafness, dysmorphic features, cataplexy, abnormal REM sleep, absent platelet MAO-B activity, increased serum serotonin
Seemanova syndrome	Microcephaly with epilepsy, spastic tétraplegia, absence of abdominal reflex, lymphoreticular malignancies
Lesch-Nyhan syndrome	Over-production of uric acid, spasticity, choreoathetosis and a compulsive form of self-mutilation
Opitz FG syndrome	Macrocephaly, imperforate anus, hypotonia, partial agenesis of the corpus callosum, joint hyperlaxity, subtle facial abnormalities, sagittal craniosynostosis and split hand malformation

Other x-linked mental deficiency syndromes include the Davis, Atkin, Garéis, Vasquez, ATR-X, MASA and SPG1 syndromes.

As we shall see in several of the next chapters however, there is also a predominance of mental deficiency in the male sex associated with several causes which have nothing to do with X-linked genetic mutations.