Pathological left-handedness and familial sinistrality in relation to degree of mental retardation.

The relationship between personal and familial hand preference and mental retardation was examined. Six unimanual tasks were performed by 232 subjects within four mental retardation severity groups. Family handedness histories were obtained. Right hand preference varied inversely with severity of mental retardation. The handedness of mentally retarded groups at the different functional levels paralleled, but tended much more to the sinistral, than that of the corresponding parent groups. The parent-proband handedness correlations were largely insignificant, especially in lower functioning groups. The findings support both pathological left-handedness theory and an association between brain damage causing mental retardation and familial sinistrality.