Hand motor performance and degree of asymmetry in monozygotic twins.

To determine whether the absolute degree of asymmetry of hand motor performance (irrespective of direction of this asymmetry) may have a heritable component we examined 20 pairs of monozygotic (MZ) twins of whom 10 pairs were concordantly right-handed (MZ-RR) and 10 pairs discordant for handedness (MZ-RL). The tests comprised measurements of the maximum left and right hand tapping rate as well as a paper-and-pencil test of left and right hand motor proficiency. Intraclass correlations within MZ-RR and MZ-RL for absolute degree of hand motor asymmetry were not significant. In contrast, significant intrapair correlations emerged for overall hand motor performance, a measure unrelated to laterality. These results demonstrate that at least in MZ twins the degree of hand motor asymmetry is mainly determined by non-genetic factors, whereas overall hand motor skill is more likely to be influenced by the genome. In addition, the lack of a difference in overall hand motor performance between MZ twins and 40 singletons studied as controls would not support hypotheses explaining behavioural asymmetry in twins, or their discordance for asymmetry, by developmental dysfunction.