Handedness distribution in a residential population with severe or profound mental retardation.

Handedness within a sample of mentally retarded subjects was assessed using a procedure that includes a large number of items appropriate for lower functioning subjects, with multiple presentations within and between sessions one week apart. Results revealed a dramatic shift from the normal right-handed bias primarily due to the presence of a large mixed-handedness subtype, which proved to be ambiguously handed in that these subjects showed inconsistent hand preference within items. Reasons why this subtype has not been reported previously were discussed, and a model was postulated to explain the resultant handedness distribution in terms of probable central nervous system substrate.