Comparison of patterns of handedness between twins and singletons in Japan.

The handedness questionnaire of thirteen items which was identical to that employed in our previous study on singletons was administered to 62 monozygotic (MZ) and 48 dizygotic (DZ) twin pairs in Japan. Information on forced conversion of hand usage in childhood was also obtained. Results indicated that the incidence of left-handedness was 3.6% and that of non-right-handedness (which includes mixed- and left-handedness) was 5.9%. There was no significant difference in the incidence of left-handedness or of non-right-handedness between MZ and DZ twin groups. The proportion of converted right-handedness in MZ twins was slightly higher than in DZ twins. MZ pairs were somewhat more concordant for handedness than DZ pairs. Item analysis indicated that the incidence of individuals who use the left hand for writing and eating was only 0.9% and 1.8%, respectively. A comparison of the results of the present survey on twins with those of our previous one on singletons revealed that the incidence of left-handedness or non-right-handedness in twins is the same as that in singletons. Twins (especially MZ twins) have experienced a forced conversion to right-hand usage more frequently than singletons.