Hand-preference and population schizotypy: A meta-analysis

Language functions in schizophrenia patients are represented more bilaterally (i.e., less lateralized) than in healthy subjects. This decreased lateralization is also observed in individuals at increased risk for schizophrenia. Language lateralization is related to handedness; in that left-and mixed-handed individuals more frequently have decreased lateralization in comparison to right-handed subjects. In line with this, population schizotypy has repeatedly, though inconsistently, been associated with left-handedness. In order to define the exact association between handedness and schizotypy, we performed meta-analyses on the available literature. We found that nonright-handed subjects, but not strong left-handers, had higher scores on schizotypy questionnaires than right-handed subjects. Mixed-handers showed a trend towards higher schizotypy in comparison to strong left-handers. It is argued that the higher schizotypy in non-right-handed individuals reflects the higher incidence of bilateral language lateralization in this group. Bilateral brain organization may underlie loosening of association, possibly leading to higher schizotypy scores. It is hypothesized that increased right-hemisphere involvement contributes to unconventional, divergent thought, of which schizotypy can be considered an extreme example. Introduction Non-right-handedness has been associated with schizophrenia at least since the 1950s (Straaten, 1955). Indeed, meta-analysis shows that the frequency of non-right-handedness is almost twice as high in schizophrenia patients in comparison to healthy subjects (Sommer et al., 2001a). Handedness is related to cerebral lateralization, though this association is rather weak; approximately 95% of right-handed subjects have left-cerebral dominance for language function (Szaflarski et al., 2006) and 5% have bilateral or right cerebral dominance. In non-right-handed subjects 70% have left cerebral dominance and 30% show a right dominant or bilateral pattern of language dominance (Pujol et al., 1999). Given the ease with which handedness can be assessed, and the weak but consistent association between handedness and cerebral lateralization, hand preference seems a useful indirect reflection of cerebral dominance in larger groups. The increased frequency of non-right-handedness in schizophrenia patients has been attributed to a higher incidence of bilateral language lateralization as compared to healthy subjects. Functional imaging studies showed that this was indeed the case (Sommer et al., 2001b; Li et al., 2007a; Sommer et al., 2004; 2003). The lower degree of language lateralization is caused by increased language activity in frontal and temporal areas of the right hemisphere (Sommer et al., 2001b; 2003). Increased language activity of the right hemisphere was also present in unaffected monozygotic co-twins of schizophrenia patients (Sommer et al., 2004; Spaniel et al., 2007), indicating that decreased language lateralization is a genetic predisposition for schizophrenia.