Brain damage in early development

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It has been documented that there are slightly though significantly more mentally deficient persons of the male sex than of the female sex.  Though it certainly is possible for impoverished socio-cultural conditions to cause mental deficiency in a child,  this is by far the least frequent cause in advanced industrialized countries.  The cause of mental deficiency, in the vast majority of cases, is a hereditary or early-acquired insult to the brain.  Many categories of mental deficiency affecting boys more than girls can  readily be explained by their mode of transmission,  which is X-linked.   But there are also numerous mental deficiency syndromes which are male-preponderant, and yet show family recurrence which is not suggestive at all of an X-linked determination.   There are more epileptics of the male sex than there are of the female sex  (1.5:1),  and most epileptic syndromes of childhood are not believed to be X-linked.       A large-scale epidemiological (survey) study concluded that the male to female prevalence ratio of cerebral palsy is 1.5: 1.   Cerebral palsy is caused by a multitude of prenatal stressors such as anoxia (lack of oxygen),  fetal jaundice,  fetal intoxication, and so on.  It consists of brain damage resulting in a spastic incoordination.   Though there are many causes of deafness,  the disease process is often located in the nervous system.   The male/female ratio of deafness is around 1.17:1.

Aside from the obvious importance of X-linked disorders, several explanations of male preponderance of prenatal neurological disorders have been proposed. One version attributes the greater vulnerability of the male fetus to slower development of the brain. Another version proposes an immune mechanism: mothers produce anti-male antigens.  It is believed that they do so increasingly as a function of the number of male progeny, especially consecutive male conceptions. I present this latter model in some detail in another section of this chapter. Edward Miller views these phenomena as exemplars of greater phenotypic variability of the male sex. Miller posits that male reproductive success is more variable than female reproductive success, because of the large female investment in pregnancy, lactation, and child rearing. In a fluctuating environment, he writes, a genotype that produces various phenotypes can out reproduce a genotype that always produces the same phenotypes (Miller 1997).  In many environments, he reasons,  a human with the best phenotype out reproduces one with a phenotype less well suited to that environment. Frequently the highest ranking male enjoys a disproportionate reproductive success.  Having a phenotype suitable for the environment is a major advantage in obtaining this high rank.  In contrast, because females have a maximum limit (due to limited capacity for childbearing) to how much reproductive benefit they can receive from a good environment, the benefits to males from variability are greater than for females.   This explanation has the interesting advantage of accounting not only for more frequent male deficiency (neurobehavioral syndromes,  mental deficiency,  sexually deviant behavior, etc.) but also for more frequent male genius at the very high end of the human distribution.