Handedness in twins: a meta-analysis

A School-based Twin Study of Handedness among Adolescents in Taiwan

This study aimed to evaluate whether twinning might influence handedness and the relative contribution of genetic and environmental factors to handedness in a total of 321 pairs of twins, 36 same-sex sib-pairs, and 1020 singletons, aged 12-16 and systematically recruited from the junior high schools in Taipei. Twins' zygosity was determined by a combination of DNA typing and physical similarity. The direction and consistency of handedness in twins did not differ from that seen in singletons. Compared with the full model containing additive genes (A), shared (C), and non-shared (E) environment, both AE and CE models had equivalently acceptable fit. The contribution from additive genes in the AE model was estimated to be 16% (directional) to 13% (consistent) for the continuous handedness and 34-10% for the categorical one, whereas the corresponding contribution from shared environment in the CE model was 14-14% and 32-11%, respectively. Handedness in adolescents appears to be not influenced by twinning and not substantially heritable, whereas environmental factors, especially those not shared between siblings, are the most important ones for explaining individual variations.

Functional magnetic resonance imaging in adult craniopagus for presurgical evaluation

✓ Cranially conjoined twins are rare and pose unique challenges in the preoperative evaluation of cerebral language function. The authors report on their experience in the functional magnetic resonance (fMR) imaging evaluation of adult craniopagus (temporoparietooccipital fusion) to evaluate hemispheric language dominance and the eloquent language areas in the preoperative planning stages. Conventional clinical imaging hardware originally designed for individuals was adapted and tailored for use in the twins. They were assigned a selection of language tasks while undergoing fMR imaging. Significant blood oxygen level—dependent activations were detected in the main language regions in each twin, that is, the inferior frontal gyrus (around the Broca area), the middle and superior temporal lobes (around the Wernicke area) together with the inferior parietal lobe, and the middle and superior frontal gyri. Overall, the right-handed twin was strongly left lateralized for language, whereas the left-handed twin showed more bilateral activation during language tasks. Noninvasive language mapping with the aid of fMR imaging has been demonstrated for the first time in total craniopagus.

Handedness, homicide and negative frequency-dependent selection

Humans exhibit hand preference for most manual activities in which they are specialized. Right- and left-handers have coexisted at least since the Upper Palaeolithic, and left-handers are in the minority in all human populations. The persistence of the polymorphism of handedness is a puzzle because this trait is substantially heritable and several fitness costs are associated with left-handedness. Some countervailing benefit is required to maintain the polymorphism. Left-handers may have a frequency-dependent advantage in fights--the advantage being greater when their frequency is lower. Sports data from Western societies are consistent with this prediction. Here, we show that the frequency of left-handers is strongly and positively correlated with the rate of homicides across traditional societies. It ranges from 3% in the most pacifistic societies, to 27% in the most violent and warlike. This finding is consistent with a frequency-dependent selection mechanism maintaining left-handedness in these societies.

Genes for left-handedness: how to search for the needle in the haystack?

Although several genes that determine left-right asymmetry for structural syndromes such as situs inversus have been characterised in recent years (Supp, Witte, Potter, & Brueckner, 1997), there has been little progress in determining which genes or loci predispose to left-right handedness in humans. Linkage analysis has been used widely for the localisation of genes followed by their positional cloning. The complex genetics of handedness is one of the greatest problems for standard linkage analysis. Several genetic models have been proposed for the inheritance of handedness in humans. On the basis of these models, left-handedness can be considered a common single gene trait with a high gene frequency and a non-mendelian inheritance pattern. We report here a possible strategy, using these genetic models, that can be applied for the identification for genes determining handedness in humans.

Evaluation of empirical methods and methodological foundations of human left-handedness

The methodological and empirical foundations underlying the theories on human left-handedness are examined, including the definition and classification of left-handedness, its relation to pathology, and the results on family, adoption, and twin research. It is argued that major research findings on left-handedness are based on inadequate empirical methods, and the concrete definitions and assessment procedures do not meet validity and reliability criteria. This also casts doubt on the relationship between left-handedness and pathology reported in the literature. Furthermore, findings from family, adoption, and twin studies do not allow interpretation of possible direct genetic contributions to explaining left-handedness. It is concluded that the methodological limitations of a causal explanation should encourage an examination of the epistemological and methodological premises of theory formulation. The functional method and a theoretical framework based on early childhood experiences and the neural plasticity of the brain may be more appropriate for the selection of empirical methods in research of human left-handedness.

Handedness frequency over more than ten thousand years

Although there are quite important geographical variations in the frequency of left-handers around the world, nothing is known about its temporal evolution. During the upper Palaeolithic (ca. 35,000-10,000 YBP), humans painted 'negative hands' by blowing pigments with a tube onto one hand applied on the rock in caves in Western Europe, by blowing pigments on their own hand through a tube held in the other hand. The frequency of left-handers prevailing during this period could thus be assessed. For comparison, the handedness of French university students has been observed for the same task. No difference was detected between the two proportions of left-handers, separated by more than 10,000 years. Implications for the evolution of the polymorphism of handedness are discussed.

Parametric and nonparametric genome scan analyses for human handedness

We have performed a genome scan using 25 nuclear families consisting of right-handed parents with at least two left-handed children. Handedness was assessed as a qualitative trait using a laterality quotient. Laterality quotients indicate the direction of handedness, which is hand preference for performing unimanual tasks. Both parametric and nonparametric linkage analyses were applied. The parametric analysis using the single-locus genetic model of Klar resulted in four different regions with LOD scores higher than 1. The region on chromosome 10q26 gave a suggestive LOD score of 2.02 at a recombination fraction of 0.05. Nonparametric analysis gave an NPL score for this region of 2.16. However, further fine mapping of the region on chromosome 10q26 failed to obtain a higher LOD score. These results suggest that handedness is a human quantitative trait locus and that the proposed non-Mendelian monogenic models are incorrect.

Incidence of nondextrality in the NAS/NRC Twin Registry

Genetic theories still flounder on the fact that similarity of hand preference is the same in monozygotic (MZ) and dizygotic (DZ) twins. Lateral preference on a well-designed set of 5 activities was obtained from 2,131 male pairs. On item analysis, only "throw" discriminated zygosity, attributable to "excess" nondextral MZ pairs. This item is remarkably free of the intense cultural bias against sinistrality.

Heritability of lobar brain volumes in twins supports genetic models of cerebral laterality and handedness

Although the left and right human cerebral hemispheres differ both functionally and anatomically, little is known about the environmental or genetic factors that govern central nervous system asymmetry. Nevertheless, cerebral asymmetry is strongly correlated with handedness, and handedness does have a significant genetic component. To explore the relative contribution of environmental and genetic influences on cerebral asymmetry, we examined the volumes of left and right cerebral cortex in a large cohort of aging identical and fraternal twins and explored their relationship to handedness. Cerebral lobar volumes had a major genetic component, indicating that genes play a large role in changes in brain volume that occur with aging. Shared environment, which likely represents in utero events, had about twice the effect on the left hemisphere as on the right, consistent with less genetic control over the left hemisphere. To test the major genetic models of handedness and cerebral asymmetry, twin pairs were divided into those with two right handers and those with at least one left hander (nonright handers). Genetic factors contributed twice the influence to left and right cerebral hemispheric volumes in right-handed twin pairs, suggesting a large decrement in genetic control of cerebral volumes in the nonright-handed twin pairs. This loss of genetic determination of the left and right cerebral hemispheres in the nonright-handed twin pairs is consistent with models postulating a right-hand/left-hemisphere-biasing genetic influence, a "right-shift" genotype that is lost in nonright handers, resulting in decreased cerebral asymmetry.

A genomewide linkage screen for relative hand skill in sibling pairs

Genomewide quantitative-trait locus (QTL) linkage analysis was performed using a continuous measure of relative hand skill (PegQ) in a sample of 195 reading-disabled sibling pairs from the United Kingdom. This was the first genomewide screen for any measure related to handedness. The mean PegQ in the sample was equivalent to that of normative data, and PegQ was not correlated with tests of reading ability (correlations between minus sign0.13 and 0.05). Relative hand skill could therefore be considered normal within the sample. A QTL on chromosome 2p11.2-12 yielded strong evidence for linkage to PegQ (empirical P=.00007), and another suggestive QTL on 17p11-q23 was also identified (empirical P=.002). The 2p11.2-12 locus was further analyzed in an independent sample of 143 reading-disabled sibling pairs, and this analysis yielded an empirical P=.13. Relative hand skill therefore is probably a complex multifactorial phenotype with a heterogeneous background, but nevertheless is amenable to QTL-based gene-mapping approaches.