The Right Shift Theory of Handedness and Brain Asymmetry in Evolution, Development and Psychopathology.

The right shift (RS) theory suggests that lateral asymmetries arise from accidental differences between the sides of the body. There is an additional specific influence on human brain asymmetry, an RS + gene present in some but not all people, that induces advantage for the left hemisphere by weakening the right hemisphere. The theory explains associations between handedness and cerebral specialisation for speech, and also handedness in families and twin pairs. The gene is expected to be specific to Homo sapiens. It is expressed through processes that influence brain development and it is hindered by factors that impair normal growth processes. Most raised incidences of nonright-handedness in pathological conditions are secondary to processes that disrupt growth. It is possible, however, that the RS + gene is unstable and mutates to a form which is agnosic for left versus right. This could result in the impairment of speech and language skills in both hemispheres, a possible cause of schizophrenia and perhaps also autism. The effects of the RS + gene on human handedness and other asymmetries often appear weak because they must be detected against the background noise of chance. The RS + gene induces typical cerebral specialisations in most of the population but in its' absence there are risks to the efficiency of speech and phonological processing. Questions about costs and benefits associated with the RS locus deserve further research.