Language lateralization and handedness in twins; an argument against a genetic basis?

There is ample evidence that both handedness and language lateralization have a genetic basis. A first argument is that left-handedness tends to run in families. A second piece of evidence is derived from adoption studies. Handedness of a child is strongly related to handedness of the biological parents, while handedness of the adoption parents is not correlated to the child’s handedness. Twin studies, however, appear to provide evidence against a genetic basis for handedness. Concordance rates for handedness in monozygotic (identical) twins are low and hardly exceed those in dizygotic (fraternal) twins. Similar concordance rates in monozygotic and dizygotic twins would suggest that handedness is almost completely determined by environmental factors. However, this conclusion is not in line with family and adoption studies. This chapter discusses several twin-specific factors that could affect handedness in twins and thereby decrease concordance rates in twins. Handedness and language lateralization in twins appear to be different from those in singletons. Both monozygotic and dizygotic twins have a higher prevalence of left-handedness than singletons. First-degree relatives of twins are also more frequently left-handed than subjects without twin relatives, which suggests a genetic predisposition for left-handedness in twins. The mode of inheritance for handedness probably includes a random factor. In the absence of a dominant allele, hand preference and cerebral lateralization probably results from enlargement of stochastic variance. Concordance for left-handedness will therefore never be higher than 50% in monozygotic twins, even when the phenotype would completely be defined by the genotype. This could explain the relatively low concordance rate for handedness in twins. However, this genetic model would still predict higher concordance for handedness in monozygotic than in dizygotic twins. A possible explanation for the low concordance in monozygotic twins, which hardly exceeds that of dizygotic twins, is the monozygotic twinning process itself, which may also affect the phenotype. Early embryonic division to form twins may disrupt the developmental pathways that normally induce lateralization. This phenomenon is called “mirrorimaging”, and may produce twin pairs with opposite handedness and lateralization. In addition, twin birth carries an increased risk for perinatal brain damage as compared to singleton birth. Twins with perinatal damage to the left hemisphere may develop right cerebral dominance for language and left-handedness, while their unaffected co-twins develop the genetically predisposed right-handedness. This mechanism may act to increase the prevalence of left-handedness in dizygotic and monozygotic twins and decrease concordance for handedness in both types of twins. The exact contribution of mirror-imaging and perinatal brain damage to the distribution of handedness in twins is yet to be determined. It is therefore premature to conclude that handedness and lateralization are not genetically determined on the basis of concordance rates in twin studies.