Non-right-handedness and schizophrenia

Sommer et al (2001: p. 349) found ‘compelling evidence... for decreased cerebral dominance in schizophrenia’, from a review of studies of handedness and other functional and anatomical asymmetries, consistent with the theory that schizophrenia is associated with an anomaly of the mechanisms of cerebral dominance (Crow, 1997), possibly a ‘right-shift factor’. They suggested that reduced asymmetry may help identify risk for schizophrenia. Procopio (2001) welcomed the review but cautioned that the ‘right shift’ is only a hypothesis and that findings for asymmetry in twins demonstrate an important environmental component.

The Sommer et al review puts it beyond doubt, in my opinion, that asymmetries are reduced in schizophrenia but this needs careful interpretation. The right-shift theory (for review see Annett, 2002) suggests that the main agent of asymmetry is environmental, random accidents of early growth in bilaterally symmetrical creatures. Random accidents occur in monozygotic twins as individuals, just as in other individuals. What is interesting about humans is that several chance distributions of asymmetry are shifted in typical directions when the hypothesised RS+ gene is present. The gene may be absent, or when present its expression may be reduced by factors that influence early growth. Among the variables associated with reduction in the shift of the chance distribution for handedness are male gender, twinning, low birth weight, poor phonological processing (occurs in many people with dyslexia) and early brain lesions. These reductions must be detected against a base rate of non-right-handedness in about one-third of the general population. Differences in asymmetry are not causal, but rather the results of changes in the frequency or expression of the RS+ gene. They are not likely to be useful markers for any specific clinical disorder.

In schizophrenia, I have suggested that the gene may lose its directional coding and become ‘agnostic’ for right or left. Symptoms of schizophrenia are hypothesised to occur when speech cortex is impaired on both sides of the brain, as expected in 50% of the relevant genotypes. Until the RS+ gene and its variants are found, however, the theory remains a hypothesis.