A joint occurrence of atypical behavioral lateralization and schizophrenia: coincidental or causative?

45 Human hands are architecturally symmetrical, yet distinctly different in their functional abilities. This seemingly unique human tendency to use the right hand rather than the left for fine motor function is observed in ~90% of the human population. The remaining ~10% are atypical (i.e. left-handers). In addition to clearly visible hand dominance in humans, there are other behavioural lateralities such as foot or eye dominance, but these have not attracted as much scientific scrutiny as hand dominance. Left-handedness is commonly associated with reduction of brain asymmetries, most notably in brain areas related to language. The general finding, irrespective of the technique used (invasive, such as the intracarotid sodium amytal test; non-invasive, such as transcranial Doppler sonography), is that a large proportion of right-handed individuals have language localised in the left hemisphere, with only a small proportion having the same faculty ipsilateral to the dominant hand. Although the majority of left-handers have language localised in the left hemisphere, just as right-handers, a much larger proportion of left-handers display either bilateral or right hemispheric representation of language. This association of handedness and brain asymmetries for language is likely to have genetic aetiology (Geschwind, Miller, DeCarli, & Carmelli, 2002). Although genetic factors, modified by environmental influences, are most likely involved in the inheritance of handedness, the gene for handedness is yet to be identified (Agtmael, Forrest, & Williamson, 2001, 2002). In addition to genetic models, several other theories have been proposed to explain the origin of handedness and the mechanism of inheritance. Roughly, those theories can be divided into two groups: first, those which consider both leftand right-handedness as neutral and benign variations (e.g., Annett’s Right Shift theory, 1985; McManus, 1985), and second, those which consider that right-handedness is the norm, with departure from the norm due to some pathology (e.g. Bakan’s Birth Stress model, 1973). In these two groups of theories, the former usually try to explain the distribution of handedness in families and likely mode of inheritance, whereas the latter aim to describe the risk factors associated with the occurrence of non-right handedness. An implicit assumption contained in all the latter models is that left-handedness is a pathologic deviation from normality. The distribution of hand preferences appears to be truly taxonic (McManus, 1985; Dragovic, Milenkovic, & Hammond, 2008), with a tripartite structure made up of two major classes of hand preference (leftand right-handedness), and a smaller class made up of individuals showing mixed handedness. Milan Dragović, Centre for Clinical Research in Neuropsychiatry; School of Psychiatry and Clinical Neurosciences, The University of Western Australia, Private Mail Bag No. 1, Claremont, WA 6910, Australia. E-mail: [email protected] (the address for correspondence);