Handedness in twins: a meta-analysis

Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology

It has been estimated that 25% of monozygotic ("identical") twin pairs exhibit reverse asymmetry (RA) or "mirroring" of minor anatomical features as a result of delayed zygote division. Here, we examine whether identical twin mirroring accounts for patterns of dental asymmetry in a sample of monozygotic and dizygotic ("fraternal") twins. We focus on crown morphology to approach the following question: is there an association between dental RA frequency and twin type suggestive of the presence of mirror image twins in our sample? Data were collected from 208 deciduous and 196 permanent dentitions of participants of the University of Adelaide Twin Study using Arizona State University Dental Anthropology System standards. RA frequencies were compared across morphological complexes (deciduous, permanent), twin types (monozygotic, dizygotic), and traits. Fisher's exact tests were performed to formally evaluate the association between twin type and dental RA. Across the entire dataset, RA rates failed to exceed 8% for any twin type. In monozygotic twins, deciduous mirroring totaled 5.3% of observed cases, while permanent mirroring totaled 7.8% of observed cases. We found no statistically significant association between RA and twin type for any morphological character (p-value range: 0.07-1.00). Our results suggest the timing of monozygotic twin division does not explain the structure of asymmetry for our morphology dataset and that published estimates of identical twin mirroring rates may be inflated or contingent upon phenotype. Instead, rates reported for this sample more closely align with the proposed etiology of this condition.

Handedness and 23 Early Life Characteristics in 37,495 Dutch Twins

In studies of singletons, a range of early-life characteristics have been reported to be associated with handedness, but some of these associations have failed to replicate. We examined associations between 23 early life characteristics with handedness in a large sample of 37,495 5-year-old twins. We considered three definitions of handedness: left-handedness (LH), mixed-handedness (MH), and non-right-handedness (NRH). Our main aim was to test whether the associations with sex, birth weight, gestational age, and season of birth - as reported in singletons - replicate in twins, and to examine twin-specific variables, including zygosity, chorionicity, birth order, and intertwin delivery time. Compared to previously published data from adults born as singletons (7.23%), the prevalence of NRH was higher in both twins (16.19%) and their parents (15.09%). In the twins, LH and NRH were associated with parents' LH. Male sex and lower gestational age were associated with NRH, and LH was associated with not being breastfed. MH was related to neurodevelopmental delays and higher externalizing problems later in childhood. Other previously reported associations were not replicated, and no twin-specific characteristics were related to handedness. These results emphasize the importance of considering multiple definitions of handedness and indicate a small number of replicated associations across studies.

Cerebral Polymorphisms for Lateralisation: Modelling the Genetic and Phenotypic Architectures of Multiple Functional Modules

Recent fMRI and fTCD studies have found that functional modules for aspects of language, praxis, and visuo-spatial functioning, while typically left, left and right hemispheric respectively, frequently show atypical lateralisation. Studies with increasing numbers of modules and participants are finding increasing numbers of module combinations, which here are termed cerebral polymorphisms—qualitatively different lateral organisations of cognitive functions. Polymorphisms are more frequent in left-handers than right-handers, but it is far from the case that right-handers all show the lateral organisation of modules described in introductory textbooks. In computational terms, this paper extends the original, monogenic McManus DC (dextral-chance) model of handedness and language dominance to multiple functional modules, and to a polygenic DC model compatible with the molecular genetics of handedness, and with the biology of visceral asymmetries found in primary ciliary dyskinesia. Distributions of cerebral polymorphisms are calculated for families and twins, and consequences and implications of cerebral polymorphisms are explored for explaining aphasia due to cerebral damage, as well as possible talents and deficits arising from atypical inter- and intra-hemispheric modular connections. The model is set in the broader context of the testing of psychological theories, of issues of laterality measurement, of mutation-selection balance, and the evolution of brain and visceral asymmetries.

Quantitative multidimensional phenotypes improve genetic analysis of laterality traits

Handedness is the most commonly investigated lateralised phenotype and is usually measured as a binary left/right category. Its links with psychiatric and neurodevelopmental disorders prompted studies aimed at understanding the underlying genetics, while other measures and side preferences have been less studied. We investigated the heritability of hand, as well as foot, and eye preference by assessing parental effects (n ≤ 5028 family trios) and SNP-based heritability (SNP-h2, n ≤ 5931 children) in the Avon Longitudinal Study of Parents and Children (ALSPAC). An independent twin cohort from Hong Kong (n = 358) was used to replicate results from structural equation modelling (SEM). Parental left-side preference increased the chance of an individual to be left-sided for the same trait, with stronger maternal than paternal effects for footedness. By regressing out the effects of sex, age, and ancestry, we transformed laterality categories into quantitative measures. The SNP-h2 for quantitative handedness and footedness was 0.21 and 0.23, respectively, which is higher than the SNP-h2 reported in larger genetic studies using binary handedness measures. The heritability of the quantitative measure of handedness increased (0.45) compared to a binary measure for writing hand (0.27) in the Hong Kong twins. Genomic and behavioural SEM identified a shared genetic factor contributing to handedness, footedness, and eyedness, but no independent effects on individual phenotypes. Our analysis demonstrates how quantitative multidimensional laterality phenotypes are better suited to capture the underlying genetics than binary traits.

Handedness in twins: meta-analyses

BACKGROUND

In the general population, 10.6% of people favor their left hand over the right for motor tasks. Previous research suggests higher prevalence of atypical (left-, mixed-, or non-right-) handedness in (i) twins compared to singletons, and in (ii) monozygotic compared to dizygotic twins. Moreover, (iii) studies have shown a higher rate of handedness concordance in monozygotic compared to dizygotic twins, in line with genetic factors playing a role for handedness.

METHODS

By means of a systematic review, we identified 59 studies from previous literature and performed three sets of random effects meta-analyses on (i) twin-to-singleton Odds Ratios (21 studies, n = 189,422 individuals) and (ii) monozygotic-to-dizygotic twin Odds Ratios (48 studies, n = 63,295 individuals), both times for prevalence of left-, mixed-, and non-right-handedness. For monozygotic and dizygotic twin pairs we compared (iii) handedness concordance Odds Ratios (44 studies, n = 36,217 twin pairs). We also tested for potential effects of moderating variables, such as sex, age, the method used to assess handedness, and the twins' zygosity.

RESULTS

We found (i) evidence for higher prevalence of left- (Odds Ratio = 1.40, 95% Confidence Interval = [1.26, 1.57]) and non-right- (Odds Ratio = 1.36, 95% Confidence Interval = [1.22, 1.52]), but not mixed-handedness (Odds Ratio = 1.08, 95% Confidence Interval = [0.52, 2.27]) among twins compared to singletons. We further showed a decrease in Odds Ratios in more recent studies (post-1975: Odds Ratio = 1.30, 95% Confidence Interval = [1.17, 1.45]) compared to earlier studies (pre-1975: Odds Ratio = 1.90, 95% Confidence Interval = [1.59-2.27]). While there was (ii) no difference between monozygotic and dizygotic twins regarding prevalence of left- (Odds Ratio = 0.98, 95% Confidence Interval = [0.89, 1.07]), mixed- (Odds Ratio = 0.96, 95% Confidence Interval = [0.46, 1.99]), or non-right-handedness (Odds Ratio = 1.01, 95% Confidence Interval = [0.91, 1.12]), we found that (iii) handedness concordance was elevated among monozygotic compared to dizygotic twin pairs (Odds Ratio = 1.11, 95% Confidence Interval = [1.06, 1.18]). By means of moderator analyses, we did not find evidence for effects of potentially confounding variables.

CONCLUSION

We provide the largest and most comprehensive meta-analysis on handedness in twins. Although a raw, unadjusted analysis found a higher prevalence of left- and non-right-, but not mixed-handedness among twins compared to singletons, left-handedness was substantially more prevalent in earlier than in more recent studies. The single large, recent study which included birth weight, Apgar score and gestational age as covariates found no twin-singleton difference in handedness rate, but these covariates could not be included in the present meta-analysis. Together, the secular shift and the influence of covariates probably make it unsafe to conclude that twinning has a genuine relationship to handedness.

Effect of multiple pregnancy and laterality on infant neurodevelopment

OBJECTIVE

The objective of this study is to investigate the effect of type of pregnancy (singleton, dizygotic, and monozygotic) on infant neurodevelopment and to explore how laterality operates in this relationship.

METHODS

We constructed a prospective cohort of low-risk women with singleton, monozygotic, and dizygotic pregnancies. Laterality was evaluated using the Edinburgh's scale of laterality, while neurodevelopment was assessed using the Ages & Stages Questionnaire (ASQ). The confounding, modifying, and mediating effect of laterality on the relationship between multiple pregnancy and neurodevelopment was explored by linear regression.

RESULTS

We included 207 singletons, 77 dizygotic, and 75 monozygotic pregnancies. The mean age (SD) at assessment of neurodevelopment was 48.5 (7.5) months. There was a significant trend to poorer neurodevelopment across singleton, dizygotic and monozygotic pregnancies in communication (52.2, 47.6 and 42.3; p < .001) and fine movements (49.6, 44.5 and 35.2; p < .001) even after adjusting for confounders. As compared to singletons, both dizygotic (39.6% vs. 22.7%; p < .001) and monozygotic (39.3% vs. 22.7%; p < .001) pregnancies had a higher frequency of non-right laterality. Laterality was not associated with neurodevelopment, nor confounded, mediated or modified the effect of multiple pregnancy on neurodevelopment.

CONCLUSION

The association of multiple pregnancy to poorer neurodevelopment was independent of children laterality.

Prevalence and heritability of handedness in a Hong Kong Chinese twin and singleton sample

BACKGROUND

Left-handedness prevalence has been consistently reported at around 10% with heritability estimates at around 25%. Higher left-handedness prevalence has been reported in males and in twins. Lower prevalence has been reported in Asia, but it remains unclear whether this is due to biological or cultural factors. Most studies are based on samples with European ethnicities and using the preferred hand for writing as key assessment. Here, we investigated handedness in a sample of Chinese school children in Hong Kong, including 426 singletons and 205 pairs of twins, using both the Edinburgh Handedness Inventory and Pegboard Task.

RESULTS

Based on a binary definition of writing hand, we found a higher prevalence of left-handedness (8%) than what was previously reported in Asian datasets. We found no evidence of increased left-handedness in twins, but our results were in line with previous findings showing that males have a higher tendency to be left-handed than females. Heritability was similar for both hand preference (21%) and laterality indexes (22%). However, these two handedness measures present only a moderate correlation (.42) and appear to be underpinned by different genetic factors.

CONCLUSION

In summary, we report new reference data for an ethnic group usually underrepresented in the literature. Our heritability analysis supports the idea that different measures will capture different components of handedness and, as a consequence, datasets assessed with heterogeneous criteria are not easily combined or compared.

On the Neurocircuitry of Grasping: The influence of action intent on kinematic asymmetries in reach-to-grasp actions

Evidence from electrophysiology suggests that nonhuman primates produce reach-to-grasp movements based on their functional end goal rather than on the biomechanical requirements of the movement. However, the invasiveness of direct-electrical stimulation and single-neuron recording largely precludes analogous investigations in humans. In this review, we present behavioural evidence in the form of kinematic analyses suggesting that the cortical circuits responsible for reach-to-grasp actions in humans are organized in a similar fashion. Grasp-to-eat movements are produced with significantly smaller and more precise maximum grip apertures (MGAs) than are grasp-to-place movements directed toward the same objects, despite near identical mechanical requirements of the two subsequent (i.e., grasp-to-eat and grasp-to-place) movements. Furthermore, the fact that this distinction is limited to right-handed movements suggests that the system governing reach-to-grasp movements is asymmetric. We contend that this asymmetry may be responsible, at least in part, for the preponderance of right-hand dominance among the global population.

Handedness heritability in industrialized and nonindustrialized societies

In modern societies, there is a decreased usage of traditional weapons to settle interpersonal or inter-group disputes compared to usage in traditional societies, possibly affecting the frequency-dependent selection on the handedness polymorphism. Another societal difference is the extensive automation of hard manual labour (including agriculture) in industrialized societies, relaxing the selection for hand specialization. Thus, selection of handedness is likely to differ between traditional and modern societies. As heritability determines the relative speed of evolutionary dynamics, handedness heritability was compared between industrialized and non-industrialized societies. First, individuals were sampled from a non-industrialized area in Indonesia, where violent conflicts are relatively frequent and tribal wars have been prevalent recently. Handedness was recorded directly or indirectly for 11,490 individuals belonging to 650 independent pedigrees, and handedness heritability was estimated using a pedigree-based animal model. Second, estimates of handedness heritability derived from published sources were collected to compare heritability estimates, accounting for various confounding variables. Non-industrialized countries displayed a significantly higher heritability value (h2 = 0.56) than that of industrialized countries (h2 = 0.20). Heritability decreased with time along the twentieth century in industrialized countries, independently of the frequency of left-handedness, and independently of the method used to measure handedness. In conclusion, the data are consistent with a decrease in handedness heritability following the industrialization process and/or the associated decrease in violence using traditional weapons. The difference in heritability between industrialized and non-industrialized countries suggests that selection of handedness is thus likely to differ between traditional and modern societies.

Further evidence of a left hemisphere specialization and genetic basis for tool use skill in chimpanzees (Pan troglodytes): Reproducibility in two genetically isolated populations of apes

It has been hypothesized that the evolution of tool use may have served as a preadaptation for the emergence of left hemispheric specialization in motor skill in humans. Here, we tested for intermanual differences in performance on a tool use task in a sample of 206 captive chimpanzees in relation to their sex, age, and hand preference. In addition, we examined heritability in tool use skill for the entire sample, as well as within 2 genetically isolated populations of captive chimpanzees. This was done to determine the degree of reproducibility in heritability on motor performance. The results revealed a significant effect of hand preference on intermanual differences in performance. Right-handed chimpanzees performed the task more quickly with their right compared with left hand. In contrast, no significant intermanual differences in performance were found in left- and ambiguous-handed apes. Tool use performance was found to be significantly heritable for overall performance, as well as separately for the left and right hands. Further, significant heritability in tool use performance was found in both populations of apes, suggesting these results were reproducible. The results are discussed in the context of evolutionary theories of handedness and hemispheric specialization and the genetic mechanisms that underlie their expression in primates, including humans. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

A large-scale population study of early life factors influencing left-handedness

Hand preference is a conspicuous variation in human behaviour, with a worldwide proportion of around 90% of people preferring to use the right hand for many tasks, and 10% the left hand. We used the large cohort of the UK biobank (~500,000 participants) to study possible relations between early life factors and adult hand preference. The probability of being left-handed was affected by the year and location of birth, likely due to cultural effects. In addition, hand preference was affected by birthweight, being part of a multiple birth, season of birth, breastfeeding, and sex, with each effect remaining significant after accounting for all others. Analysis of genome-wide genotype data showed that left-handedness was very weakly heritable, but shared no genetic basis with birthweight. Although on average left-handers and right-handers differed for a number of early life factors, all together these factors had only a minimal predictive value for individual hand preference.

Half a century of handedness research: Myths, truths; fictions, facts; backwards, but mostly forwards

Although most people are right-handed and have language in their left cerebral hemisphere, why that is so, and in particular why about ten per cent of people are left-handed, is far from clear. Multiple theories have been proposed, often with little in the way of empirical support, and sometimes indeed with strong evidence against them, and yet despite that have become modern urban myths, probably due to the symbolic power of right and left. One thinks in particular of ideas of being right-brained or left-brained, of suggestions that left-handedness is due to perinatal brain damage, of claims that left-handers die seven years earlier than right-handers, and of the unfalsifiable ramifications of the byzantine Geschwind-Behan-Galaburda theory. This article looks back over the past fifty years of research on brain asymmetries, exploring the different themes and approaches, sometimes in relation to the author's own work. Taking all of the work together it is probable that cerebral asymmetries are under genetic control, probably with multiple genetic loci, only a few of which are now beginning to be found thanks to very large databases that are becoming available. Other progress is also seen in proper meta-analyses, the use of fMRI for studying multiple functional lateralisations in large number of individuals, fetal ultra-sound for assessing handedness before birth, and fascinating studies of lateralisation in an ever widening range of animal species. With luck the next fifty years will make more progress and show fewer false directions than had much of the work in the previous fifty years.

Behavioral and brain asymmetries in primates: a preliminary evaluation of two evolutionary hypotheses

Contrary to many historical views, recent evidence suggests that species-level behavioral and brain asymmetries are evident in nonhuman species. Here, we briefly present evidence of behavioral, perceptual, cognitive, functional, and neuroanatomical asymmetries in nonhuman primates. In addition, we describe two historical accounts of the evolutionary origins of hemispheric specialization and present data from nonhuman primates that address these specific theories. Specifically, we first discuss the evidence that genes play specific roles in determining left-right differences in anatomical and functional asymmetries in primates. We next consider and present data on the hypothesis that hemispheric specialization evolved as a by-product of increasing brain size relative to the surface area of the corpus callosum in different primate species. Last, we discuss some of the challenges in the study of hemispheric specialization in primates and offer some suggestions on how to advance the field.

Left brain, right brain: facts and fantasies

Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the "norm" of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.

Genome-wide association study of handedness excludes simple genetic models

Handedness is a human behavioural phenotype that appears to be congenital, and is often assumed to be inherited, but for which the developmental origin and underlying causation(s) have been elusive. Models of the genetic basis of variation in handedness have been proposed that fit different features of the observed resemblance between relatives, but none has been decisively tested or a corresponding causative locus identified. In this study, we applied data from well-characterised individuals studied at the London Twin Research Unit. Analysis of genome-wide SNP data from 3940 twins failed to identify any locus associated with handedness at a genome-wide level of significance. The most straightforward interpretation of our analyses is that they exclude the simplest formulations of the 'right-shift' model of Annett and the 'dextral/chance' model of McManus, although more complex modifications of those models are still compatible with our observations. For polygenic effects, our study is inadequately powered to reliably detect alleles with effect sizes corresponding to an odds ratio of 1.2, but should have good power to detect effects at an odds ratio of 2 or more.

The fighting hypothesis in combat: how well does the fighting hypothesis explain human left-handed minorities?

The strong population bias in hand preference in favor of right-handedness seems to be a typical human trait. An elegant evolutionary hypothesis explaining this trait is the so-called fighting hypothesis that postulates that left-handedness is under frequency-dependent selection. The fighting hypothesis assumes that left-handers, being in the minority because of health issues, are still maintained in the population since they would have a greater chance of winning in fights than right-handers due to a surprise effect. This review critically evaluates the assumptions and evidence for this hypothesis and concludes that some evidence, although consistent with the fighting hypothesis, does not directly support it and may also be interpreted differently. Other supportive data are ambiguous or open for both statistical and theoretical criticism. We conclude that, presently, evidence for the fighting hypothesis is not particularly strong, but that there is little evidence to reject it either. The hypothesis thus remains an intuitively plausible explanation for the persistent left-hand preference in the population. We suggest alternative explanations and several ways forward for obtaining more crucial data for testing this frequently cited hypothesis.

Genetic and environmental contributions to the expression of handedness in chimpanzees (Pan troglodytes)

Most humans are right-handed and, like many behavioral traits, there is good evidence that genetic factors play a role in handedness. Many researchers have argued that non-human animal limb or hand preferences are not under genetic control but instead are determined by random, non-genetic factors. We used quantitative genetic analyses to estimate the genetic and environmental contributions to three measures of chimpanzee handedness. Results revealed significant population-level handedness for two of the three measures-the tube task and manual gestures. Furthermore, significant additive genetic effects for the direction and strength of handedness were found for all three measures, with some modulation due to early social rearing experiences. These findings challenge historical and contemporary views of the mechanisms underlying handedness in non-human animals.

Genetic and environmental structure of individual differences in hand, foot, and ear preferences: a twin study

Recent studies with large sample size have reported moderate heritability of hand preference. However, little is known about genetic and environmental factors for lateral preference. We examined the genetic and environmental factors for hand, foot, and ear preferences using a twin design study. A lateral preference questionnaire was administered to twin participants (N=956). Phenotypic correlation matrices of lateral preferences were computed for monozygotic and dizygotic twin pairs, and were subjected to Cholesky decomposition to compute additive genetic and unique environmental correlation matrices. Promax rotation factor analysis of each genetic and environmental correlation matrix yielded six genetic and four environmental factors. Factor-loading patterns for these factors indicated that hand and foot lateral activities were affected by different genetic factors. By contrast, each of the four environmental factors was mainly associated with hand, foot, or ear preference. These results suggest that the genetic structure for lateral preference may be more complex than the environmental structure. In particular, hand preference may be multidimensional in terms of genetic factors.

Asymmetries of the arcuate fasciculus in monozygotic twins: genetic and nongenetic influences

We assessed cerebral asymmetry for language in 35 monozygotic twin pairs. Using DTI, we reconstructed the arcuate fasciculus in each twin. Among the male twins, right-handed pairs showed greater left-sided asymmetry of connectivity in the arcuate fasciculus than did those with discordant handedness, and within the discordant group the right-handers had greater left-sided volume asymmetry of the arcuate fasciculus than did their left-handed co-twins. There were no such effects in the female twins. Cerebral asymmetry for language showed more consistent results, with the more left-cerebrally dominant twins also showing more leftward asymmetry of high anisotropic fibers in the arcuate fasciculus, a result applying equally to female as to male twins. Reversals of arcuate fasciculus asymmetry were restricted to pairs discordant for language dominance, with the left-cerebrally dominant twins showing leftward and the right-cerebrally dominant twins rightward asymmetry of anisotropic diffusion in the arcuate fasciculus. Because monozygotic twin pairs share the same genotype, our results indicate a strong nongenetic component in arcuate fasciculus asymmetry, particularly in those discordant for cerebral asymmetry.

Multiple trajectories in the developmental psychobiology of human handedness

We show that handedness is a product of a multifaceted biosocial developmental process that begins prenatally and continues into adulthood. Although right-handedness predominates, handedness varies continuously across the population. Therefore, our phrase "multiple trajectories"refers to both differences in developmental pathways that can lead to similarities in handedness and similarities in pathways that can lead to differences in handedness. The task for the researcher is to identify how, when, and for what actions the trajectory of handedness development can be maintained or changed for an individual. Given the complexity of these developmental pathways, it is likely that the asymmetric sensorimotor activity that occurs during the development of handedness influences other hemispheric variations in neural processing. Indeed, researchers have investigated how handedness relates to cognitive, social, and emotional functioning because handedness represents different patterns of hemispheric specialization. Although the story of handedness development is not complete, it is well worth pursuing because it makes the development of brain-behavior relations more transparent, especially for hemispheric differences in function.

Nongenetic factors associated with human handedness and footedness in Japanese twin children

OBJECTIVE

The aim of this study is to clarify the factors related to the handedness and footedness of twins using two of the largest databases on Japanese twins available.

METHODS

The first group consisted of 1,131 twin pairs, all school children either 11 or 12 years old (S group), and the second group consisted of 951 twin pairs of different ages (1-15 years) in several maternal associations (M group). All data were gathered using a questionnaire. Factors associated with the handedness or footedness of twin individuals were analyzed by univariate and multivariate logistic analyses.

RESULTS

Multivariate logistic analysis showed that for handedness, birth year (OR=1.02) and neonatal asphyxia (OR=1.62) were selected in the S group, and sex (OR=1.34), the age of twins (OR=1.56), parity (OR=1.31), gestational age (OR=1.58), and family history (OR=1.82) were selected in the M group. for footedness, birth complications (OR=1.37) were selected in the S group, and sex (OR=1.33), the age of twins (OR=1.69), gestational age (OR=1.83), and family history (OR=2.49) were selected in the M group. Factors associated with handedness and footedness specific to twins, such as zygosity, placentation, birth order within twin pairs and the sex of the cotwin, were not found, although being a twin might have some effects.

CONCLUSION

It was concluded that factors that affect handedness or footedness in general, such as sex, birth year, age, parity, neonatal asphyxia, gestational age, birth complications, and family history, seem to have stronger effects on handedness and footedness than being a twin.

The concept of homology in the development of handedness

The construct of homology in the development of behavior is examined using human handedness. Research on the development of hand-use preferences in infants provides some evidence that it may be homologous with adult handedness. Nevertheless, I propose that the construct of homology may not be relevant to the study of human behavioral development. However, other constructs from developmental biology may prove useful in improving developmental psychological research.

Left-handedness is statistically linked to lifetime experimentation with illicit drugs

Handedness has been linked to an enhanced risk of alcohol abuse, while less is known about other drugs. A convenience sample of 1004 male and female Italian participants (females=58%) from the general community (18 to 65 years old: average age = 30; standard deviation = 10, median = 25) was asked about: handedness (preference in writing); lifetime use of alcohol, tobacco, and illicit drugs; levels of psychological distress, as measured by the General Health Questionnaire (GHQ); and levels of delusion proneness, as measured by the Peters et al. Delusions Inventory (PDI). Overall, 92 individuals (9.2%) were classified as left-handed, with no significant difference reported among genders. Lifetime use of illicit drugs, primarily cannabis, was reported by 20% of the sample. In a multiple logistic regression analysis, after taking into account sex, age, and caseness on GHQ and PDI, left-handed people in the sample were statistically more likely to report lifetime experimentation with heroin, ecstasy/amphetamine, and, marginally, hallucinogens, but not alcohol or tobacco. Different mechanisms might contribute to an explanation of greater lifetime experimentation with some illicit drugs among left-handed people as compared to right-handed people. However, replications with clinical samples are necessary before any definitive statements can be made.

Understanding left-handedness

BACKGROUND

The human cerebrum is asymmetrical, consisting of two hemispheres with differing functions. Recent epidemiological and neurobiological research has shed new light on the development of the cerebral lateralization of motor processes, including handedness. In this article, we present these findings from a medical perspective.

METHOD

We selectively searched the PubMed online database for articles including the terms "handedness," "left handedness," "right handedness," and "cerebral lateralization." Highly ranked and commonly cited articles were included in our analysis.

RESULTS

The emergence of handedness has been explained by physiological and pathological models. Handedness arose early in evolution and has probably been constitutive for the development of higher cognitive functions. For instance, handedness may have provided the basis for the development of speech and fine motor skills, both of which have played a critical role in the evolution of mankind. The disadvantages of certain types of handedness are discussed, as some cases seem to be associated with disease.

CONCLUSION

The consideration of handedness from the epidemiological, neurobiological, and medical points of view provides insight into cerebral lateralization.

Right hand, left brain: genetic and evolutionary bases of cerebral asymmetries for language and manual action

Most people are right-handed and left-cerebrally dominant for language. This pattern of asymmetry, as well as departures from it, have been reasonably accommodated in terms of a postulated gene with two alleles, one disposing to this common pattern and the other leaving the direction of handedness and language asymmetry to chance. There are some leads as to the location of the gene or genes concerned, but no clear resolution; one possibility is that the chance factor is achieved by epigenetic cancelling of the lateralizing gene rather than through a chance allele. Neurological evidence suggests that the neural basis of manual praxis, including pantomime and tool use, is more closely associated with cerebral asymmetry for language than with handedness, and is homologous with the so-called "mirror system" in the primate brain, which is specialized for manual grasping. The evidence reviewed supports the theory that language itself evolved within the praxic system, and became lateralized in humans, and perhaps to a lesser extent in our common ancestry with the great apes. WIREs Cogn Sci 2012, 3:1-17. doi: 10.1002/wcs.158 For further resources related to this article, please visit the WIREs website.

Isolation and genetic characterization of mother-of-snow-white, a maternal effect allele affecting laterality and lateralized behaviors in zebrafish

In the present work we report evidence compatible with a maternal effect allele affecting left-right development and functional lateralization in vertebrates. Our study demonstrates that the increased frequency of reversed brain asymmetries in a zebrafish line isolated through a behavioral assay is due to selection of mother-of-snow-white (msw), a maternal effect allele involved in early stages of left-right development in zebrafish. msw homozygous females could be identified by screening of their progeny for the position of the parapineal organ because in about 50% of their offspring we found an altered, either bilateral or right-sided, expression of lefty1 and spaw. Deeper investigations at earlier stages of development revealed that msw is involved in the specification and differentiation of precursors of the Kupffer's vesicle, a structure homologous to the mammalian node. To test the hypothesis that msw, by controlling Kupffer's vesicle morphogenesis, controls lateralized behaviors related to diencephalic asymmetries, we analyzed left- and right-parapineal offspring in a "viewing test". As a result, left- and right-parapineal individuals showed opposite and complementary eye preference when scrutinizing a model predator, and a different degree of lateralization when scrutinizing a virtual companion. As maternal effect genes are expected to evolve more rapidly when compared to zygotic ones, our results highlight the driving force of maternal effect alleles in the evolution of vertebrates behaviors.

Left-Handedness and Male-Male Competition: Insights from Fighting and Hormonal Data

Male-male competition can shape some behavioral or morphological traits of males. Here we investigate if this competition could play a role in the persistence of the polymorphism of handedness in human populations. A negative frequency-dependent selection mechanism has been hypothesized, based on the fact that left-handed men may benefit from a “surprise” advantage during fighting interactions because they are rare in human populations. This advantage may thereby enhance the probability of survival of left-handed men and/or their reproductive success through an increase in social status. In this study, we first explored the association between hand preference and lifetime fighting behavior in a population of 1,161 French men. No effect of hand preference on the probability of fighting was detected, suggesting that the innate propensity to fight does not differ between left- and right-handers. However, among men who had been involved in at least one fight during their lifetime, left-handers reported significantly more fights than right-handers. To explore the biological basis of this behavior, we also investigated the testosterone concentration in saliva samples from 64 French university students. Consistent with frequencies of fights, we found a significantly higher average testosterone concentration in left-handers than in right-handers. We suggest that these behavioral and hormonal differences may be acquired throughout life due to previous experiences in a social context and may favor the persistence of left-handers in humans.

Determinants of handedness in twins

It is well established that the distribution of the combinations of handedness (LL, LR, and RR) in twin pairs is roughly binomial regardless of zygosity or sex. This fact has challenged current genetic models of handedness. We show here that: (a) there must be at least one environmental determinant of handedness; and (b) the binomial distribution does not in itself invalidate genetic models of handedness. In singletons, left-handedness is strongly associated with low birth weight. In twins, this association is weaker (apparently as a consequence of a stronger association with birth order--i.e., with being first-born--within the pair). We speculate that the hazards associated with being first-born in twin pairs (e.g., trauma) are more closely associated with left-handedness than are the hazards associated with being second-born (e.g., hypoxia).

Epigenesis of behavioural lateralization in humans and other animals

Despite several decades of research, the epigenesis of behavioural and brain lateralization is still elusive, although its knowledge is important in understanding developmental plasticity, function and evolution of lateralization, and its relationship with developmental disorders. Over the last decades, it has become clear that behavioural lateralization is not restricted to humans, but a fundamental principle in the organization of behaviour in vertebrates. This has opened the possibility of extending descriptive studies on human lateralization with descriptive and experimental studies on other vertebrate species. In this review, we therefore explore the evidence for the role of genes and environment on behavioural lateralization in humans and other animals. First, we discuss the predominant genetic models for human handedness, and conclude that their explanatory power alone is not sufficient, leaving, together with ambiguous results from adoption studies and selection experiments in animals, ample opportunity for a role of environmental factors. Next, we discuss the potential influence of such factors, including perinatal asymmetrical perception induced by asymmetrical head position or parental care, and social modulation, both in humans and other vertebrates, presenting some evidence from our own work on the domestic chick. We conclude that both perinatal asymmetrical perception and later social modulation are likely candidates in influencing the degree or strength of lateralization in both humans and other vertebrates. However, in most cases unequivocal evidence for this is lacking and we will point out further avenues for research.

Why are some people left-handed? An evolutionary perspective

Since prehistoric times, left-handed individuals have been ubiquitous in human populations, exhibiting geographical frequency variations. Evolutionary explanations have been proposed for the persistence of the handedness polymorphism. Left-handedness could be favoured by negative frequency-dependent selection. Data have suggested that left-handedness, as the rare hand preference, could represent an important strategic advantage in fighting interactions. However, the fact that left-handedness occurs at a low frequency indicates that some evolutionary costs could be associated with left-handedness. Overall, the evolutionary dynamics of this polymorphism are not fully understood. Here, we review the abundant literature available regarding the possible mechanisms and consequences of left-handedness. We point out that hand preference is heritable, and report how hand preference is influenced by genetic, hormonal, developmental and cultural factors. We review the available information on potential fitness costs and benefits acting as selective forces on the proportion of left-handers. Thus, evolutionary perspectives on the persistence of this polymorphism in humans are gathered for the first time, highlighting the necessity for an assessment of fitness differences between right- and left-handers.

Origins of handedness: a nationwide study of 30,161 adults

The origins of human handedness remain unknown. Genetic theories of handedness have received much attention, but some twin studies suggest modest, perhaps negligible genetic effects on handedness. A related question concerning handedness is whether twins have higher rates of left-handedness than do singletons. We studied handedness, with information on forced right-handedness, in a sample of 30,161 subjects aged 18-69 from a questionnaire survey of the older Finnish Twin Cohort. Left-handedness was found to be more common in twins (8.1%) and triplets (7.1%) than in singletons (5.8%), whereas ambidextrousness was more common in triplets (6.4%) than in twins (3.4%) and singletons (3.5%). As in many other studies, males were more likely to be left-handed. Ambidextrous subjects were more likely to become right-handed writers even if not forced to use their right hand. We fit maximum likelihood models to our twin data to estimate the contribution of additive genetic, common environment and unique environmental effects to hand preference. Results, depending on the model, indicate that unique environmental effects account for most observed variance in handedness, both in childhood (92-100%) and adulthood (74-86%). When forced right-handedness was taken into account, estimates of familial effects increased. Concordance for left-handedness in twins is rare, and accordingly, very large samples are needed to detect the familial effects. Our results show that forced-handedness can have an effect on estimates of genetic effects.

Genetic influences on handedness: data from 25,732 Australian and Dutch twin families

Handedness refers to a consistent asymmetry in skill or preferential use between the hands and is related to lateralization within the brain of other functions such as language. Previous twin studies of handedness have yielded inconsistent results resulting from a general lack of statistical power to find significant effects. Here we present analyses from a large international collaborative study of handedness (assessed by writing/drawing or self report) in Australian and Dutch twins and their siblings (54,270 individuals from 25,732 families). Maximum likelihood analyses incorporating the effects of known covariates (sex, year of birth and birth weight) revealed no evidence of hormonal transfer, mirror imaging or twin specific effects. There were also no differences in prevalence between zygosity groups or between twins and their singleton siblings. Consistent with previous meta-analyses, additive genetic effects accounted for about a quarter (23.64%) of the variance (95%CI 20.17, 27.09%) with the remainder accounted for by non-shared environmental influences. The implications of these findings for handedness both as a primary phenotype and as a covariate in linkage and association analyses are discussed.

Crossed cerebral lateralization for verbal and visuo-spatial function in a pair of handedness discordant monozygotic twins: MRI and fMRI brain imaging

To examine the nature of hemispheric lateralization for neural processes underlying verbal fluency and visuo-spatial attention, we investigated a single pair of handedness discordant monozygotic (MzHd) twins. Imaging of the brain was undertaken using magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) in combination with manual performance tasks. The twins were discordant for MRI anatomical asymmetries of the pars triangularis and planum temporale, whose asymmetry was consistent with verbal laterality on fMRI. Thus, the right-handed twin had left lateralized verbal with right lateralized visuo-spatial attention, while the left-handed twin had right lateralized verbal with left lateralized visuo-spatial activation; these data lend further support for to the conclusions of Sommer et al.

Season of birth and handedness in Serbian high school students

BACKGROUND

Although behavioural dominance of the right hand in humans is likely to be under genetic control, departures from this population norm, i.e. left- or non-right-handedness, are believed to be influenced by environmental factors. Among many such environmental factors including, for example, low birth weight, testosterone level, and maternal age at birth, season of birth has occasionally been investigated. The overall empirical evidence for the season of birth effect is mixed.

METHODS

We have investigated the effect of season of birth in an epidemiologically robust sample of randomly selected young people (n = 977), all born in the same year. A Kolmogorov-Smirnov type statistical test was used to determine season of birth.

RESULTS

Neither the right-handed nor the non-right-handed groups demonstrated birth asymmetry relative to the normal population birth distribution. There was no between-group difference in the seasonal distribution of birth when comparing the right-handed to the non-right-handed groups.

CONCLUSION

The present study failed to provide support for a season of birth effect on atypical lateralisation of handedness in humans.

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.