This paper is concerned with what abnormal handedness in Pervasive Developmental Disorders (PDD) reveals about the presence, lateralization, and severity of cerebral dysfunction in this population. From previous work, it was predicted that left-handedness would be elevated in the sample and that mixed-handedness subjects should be more impaired than those with established hand dominance. A battery of cognitive and motor tests were administered to a group of PDD children with autistic symptoms, and performance was compared for the left-handed, right-handed, and mixed-preference children. It was found that left-handers tended to do better than right-handers on all cognitive measures, while the mixed-preference children tended to be the lowest on all cognitive measures. No differences were found on motor measures. An extension of the Satz (1972) model, assuming early brain damage, was developed to explain the superiority of the left-handed children; an alternative explanation assuming anomalous lateralization patterns in the natural left-handers was also suggested. (Fein, D., Waterhouse, L. Lucci, D., Pennington, B. & Humes, M. (1985). Handedness and cognitive functions in pervasive developmental disorders. Journal of Autism and Developmental Disorders, 15, 323-33.)
Interesting that mixed handedness performs lower than the left or right handedness. Other studies have concluded that many mixed handers excel in sports such as tennis. Perhaps mixed handedness mixes extremely adept with extremely impaired. Of course, it is also interesting that left handers exceed right handers in dexterity.
Exploring this evolutionarily in the context of cerebral laterailization and Annett’s right shift theory that left handers are random handed, the study above might suggest that lefties are our evolutionary forebears (less cerebral lateralization) and mixed handers are those having trouble making the transition to current right handed neurologies. A problem with this is that Annett suggests both left handers and right handers are random handed, trending toward having no gene for handedness. Why would only mixed handers reveal problems?