The twin method involves analyses of MZ-DZ differences in order to ascertain the proportion of variance in a given trait attributable to genetic and environmental effects. Being genetically identical, MZ twins tend to show high concordance rates for many behavioral attributes, whereas DZs, having about 50% of their genetic configuration in common, should manifest a reduced degree of concordance. However, researchers investigating possible genetic mechanisms underlying the development of cerebral dominance for handedness have not found the twin method to be useful, because in all of the twin-handedness studies since 1924, the discordance rates for left- and right-handedness are similar for MZs and DZs. Furthermore, twins often show a higher prevalence of left-handedness than singletons. Rather than a concern with hereditability, the purpose of this study was to investigate possible differences between right- and left-handed twins (MZs and DZs combined) and singletons on specific phenotypic variables related to handedness, including: footedness (penalty kicking), consistent handedness, gender differences, familial sinistralty (FS+), hand posture, and manual skills measured on three tests: Key tapping, Annett’s Pegboard, and circle-dotting. There were 286 individual twins (255 dextrals, 34 sinistrals), and 251 singletons (141 dextrals and 110 sinistrals). Data analyses revealed a three-fold increase on left-handedness among males than females. Furthermore, twin right- and left-handers were more mixed in their hand dominance and had a higher incidence of FS+ among first-degree relatives than singleton right- and left-handers. In line with previous studies, the frequency of contralateral kicking preference was higher in those with mixed than with consistent handedness, especially among left-handers. With regard to hand skill, no reliable between-group differences emerged on the three tests of manual skill. As per expectations, right-handers and left-handers were more proficient when manipulating the stimuli with their dominant hands. On all three measures, in contrast with right-handers, left-handers (twin and singleton) showed reduced between-hand asymmetries. By extension, the increased prevalence of left-handedness among twins could not be attributed to pathological influences, asymmetry reversal or birth order. It is possible, taking into the account the robust differences in mixed-handedness that the pattern of neuromotor lateralization might be more diffusely organized across the hemispheres in twins than in singletons, but this suggestion requires direct evidence, at least from neuroimaging experiments.
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