Background: Various behavioural indices of brain lateralisation significantly intercorrelate, but current research in this area still focuses on single behavioural asymmetries, such as handedness.
Aims: To describe a novel approach, which simultaneously integrates various laterality indices and delineates complex phenotypes.
Method: Grade of membership analysis was used to describe latent, complex lateralisation phenotypes in patients with schizophrenia (n=157), their siblings (n=74) and controls (n=77). The indices used were asymmetries of eye, foot and hand; hand motor proficiency; and handedness of patient's first-degree relatives.
Results: Three distinct pure types of lateralisation ('right', 'left' and 'mixed') were evident in patients compared with two ('right' and 'left') in siblings and controls. The 'mixed' type in patients featured absence of eye and foot lateralisation and presence of familial sinistrality, despite a right-hand dominance for writing. Patients with schizophrenia expressing the 'left' phenotype had a more severe course of illness, significantly increased scores on two schizotypy factors and poorer neurocognitive performance. The pure types in the siblings were similar to those in healthy controls.
Conclusions: The findings suggest that a leftward reversal, rather than a reduction in lateralisation, is associated with clinical severity and neurocognitive deficits in patients with schizophrenia.