Phonology and handedness in primary school: predictions of the right shift theory.

BACKGROUND The right shift (RS) theory of handedness suggests that poor phonology may occur in the general population as a risk associated with absence of an agent of left cerebral speech, the hypothesised RS + gene. The theory predicts that poor phonology is associated with reduced bias to right-handedness. METHODS A representative cohort of primary school children was assessed on tests of phonology, nonverbal ability, literacy, and handedness. There were three types of analysis; for discrete variables, poor phonology and left hand preference; for continuous variables, phonology factor scores and hand skill; for "cases" of specifically poor phonology. RESULTS Reduced bias to dextrality was found in those with poor phonology for all types of analysis. Trends were similar for both sexes but stronger in males than females. Poor phonology was associated with a raised proportion of left-handed brothers. There was a strong association between poor phonology and poor literacy, but not all those with specifically poor phonology were poor readers or spellers. Among children with poor phonology but not poor for other variables, some 23-31% were left-handed writers. CONCLUSIONS Poor phonological processing is associated with reduced bias to the right hand, consistent with absence of an agent of left hemisphere advantage.