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[Arm folding--an overview].

The literature concerning arm folding is reveiwed. This analysis could be based on 121 populations. In addition to this the paper discusses incidence, sex, age differences and genetical problems including twinning. The incidence of left arm folding ranges from 20% to 65% (mean value 54%). The review shows a predominance of the left-type in Europe. Age and sex differences are only small. Between arm folding and handedness no relationship could be found, and only a very weak one between arm folding and hand clasping. 13 authors examined arm folding in families, and 2 in twins. The family data suggest that arm folding may be under a certain genetic control, yet it is clear that no simple genetic model concerning the inheritance can be assumed. Monozygotic as well as dizygotic twins show a low rate of concordance, and the frequencies of R-R, R-L and L-L pairs in monozygotic and dizygotic twins show a binomial distribution.