Schizophrenia risk and handedness: a mixed picture

Numerous previous studies have reported on handedness differences among schizophrenics, as well as in normal subjects who are high in ''schizotypal'' traits, and hence putatively at risk for schizophrenia. Results have varied, but there is evidence of a shift away from dextrality, especially consistent among schizotypal individuals. Using both a conventional three-category and Annett's seven-category classification of handedness, we re-examined the question in 681 general population subjects assessed for schizotypy. The three-category analysis confirmed previous findings of increased schizotypy in mixed-handers. However, the more fine-grained analysis showed that, although mild degrees of mixed handedness were indeed associated with increased schizotypy, this trend was reversed in the most mixed-handed subjects whose schizotypy scores did not differ significantly from right-handers. Independently of our work, this subgroup of mixed-handers are also reported to show superior intellectual function, especially on some spatial tasks. We concluded that previous studies of schizophrenia and schizotypy have failed to distinguish different reasons for shifts from dextrality. It is argued that the latter's association with schizophrenia might come about through exogenous, neurodevelopmental, influences, whereas some-possibly genetically basedforms of mixed handedness could reflect a variety of cerebral organisation that protects against, rather than enhances, the risk for mental disorder.