LEFT-HANDEDNESS IN TWINS - GENES OR ENVIRONMENT

Twin family data can cast light on the longstanding problem about the influences of genes and environment on the etiology of left-handedness . Therefore, hand preference was assessed in 1700 adolescent twin pair s and their parents. Left-handedness (LH) appeared not significantly e nhanced among twins compared to the general population. In addition th e following observations were made: (1) Significant more LH in first b orn twins than in second born twins. (2) Significant higher left-hande dness association in MZmm pairs compared to DZmm pairs and not or may be marginally so in MZff versus DZff pairs. These results, combined wi th the observations that (a) left-handed fathers increase the probabil ity of LH in sons but not in daughters; (b) LH in mothers increases LH prevalence in both sons and daughters to the same degree; and (c) ver y low birth weight, corrected for the effect of gestational age, incre ases LH prevalence in first born twins only, make an environmental exp lanation more likely. The possibility that exposure to prenatal male h ormones - to which low birth weight and high birth stress children are more vulnerable might be a crucial condition for the etiology of LH, is discussed.