Hand preference in psychotic twins.

In an attempt to replicate the influential study of Boklage (1977), hand preference for writing was examined in a new series of 30 monozygotic (MZ) and 30 dizygotic (DZ) twin pairs in whom the proband had suffered a functional psychosis. In contrast to the original report, no increased rate of left-handedness was found in MZ compared to DZ twins, or psychotic compared with nonpsychotic twins. In particular, no relationship between within-pair left-handedness and discordance for psychosis emerged. Possible reasons for the disagreement between the two studies are examined.