Brain damage in early development
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.
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