The testosterone hypothesis: Assessment since Geschwind and Behan, 1982

Abstract

The Geschwind hypothesis proposes a causal interaction among non-right-handedness, immune disorders, and learning disabilities, including dyslexia, via the intrauterine action of the male hormone testosterone. Some epidemiologic studies have supported at least a statistical association among the three traits; others have not. The associations between learning disorders and immune disease and between learning disorders and non-righthandedness appear to be better supported than that between immune disorders and non-right-handedness. However, none of data thus far accumulated are conclusive because it is not clear that the samples studied have been truly representative.The neuropathologic evidence, both in autopsy studies in human dyslexics and in animal models of developmental cortical abnormalities, are consistent with but not diagnostic of immunological pathology. Mechanisms are discussed by which an abnormal immune system could thus injure the developing brain, with an emphasis placed on abnormal maternal-fetal interactions, including maternal autoimmune disease and maternal-fetal incompatibility. A genetic origin is also possible in which the maternal role is less significant.

Developmental instability and cerebral lateralization.

“…Annett (1991) proposed that the rs+ allele slows the growth of the right hemisphere at some point in prenatal development, typically leading to permanant right-handedness. Galaburda (1990) suggested that testosterone could produce this same effect. Corballis and Morgan (1977) stressed the increased rate of early development of the left hemisphere.…”

Immune disorders and handedness in dyslexic boys and their relatives

“…This does not seem to be the only plausible pathogenic mechanism of dyslexia when looking at the present results. However, the possibility of abnormal maternal-fetal interactions in the etiology of dyslexia (see Galaburda, 1990) and that of a reading disability plus immune disorder subtype (Hugdahl et al, 1990;Gilger et al, 1992) cannot be completely ruled out. Thus, future research into family histories of dyslexics may reveal a more fine-grained pattern of causality regarding dyslexia.…”

The underachieving child

“…These will be described in detail in this paper, but most auth-orities think these are of less causal signi®cance than the psychological correlates (Shaywitz, 1996;Snowling, 1996). Whether the subtle anatomical abnormalities in the brain that have been identi®ed in dyslexia (Galaburda, 1990) will prove to underlie the various functional correlates of the condition remains to be seen.…”