[Laterality in twins].

The literature on twins and laterality is reviewed: Both monozygotic (MZ) and dizygotic twins (DZ) show a low concordance in all functional and morphological asymmetries. The proportions of R-R, R-L and L-L pairs in MZ twins and in DZ twins are in binomial distribution. The incidence of left-handedness is the same in MZ twins and DZ twins, but higher than in singletons. Other laterality signs do not show this tendency. The whole issue of twinning is as yet unresolved, yet it is clear already that no simple genetic model for the inheritance can be applied. The present review discusses three genetic models and associated problems with each. The overall tendency to a higher rate of left-handedness in twins could be due to such pathological factors (associated with twinning) as intrauterine crowding and perinatal stress, but is not due to so-called "mirror imaging"-phenomena in twins.