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PubMed1994

An international study of human handedness: the data.

Human handedness has been the subject of systematic study since 1646, but there is no agreement among researchers as to who can be considered a left-hander, what is the etiology of left-handedness, or what the proportion of left-handedness is in the world's population. This article reports the results of a handedness survey administered to 12,000 subjects in 17 countries, the largest handedness survey attempted. The paper discusses methods for determining handedness, the probability of a genetic component for handedness, and the relationship of sex, birth order, multiple birth, and first-degree relative's handedness on subject's handedness. A hypothesis for the etiology of left-handedness is presented.

Adult↗

PubMed1983

Comparison of patterns of handedness between twins and singletons in Japan.

The handedness questionnaire of thirteen items which was identical to that employed in our previous study on singletons was administered to 62 monozygotic (MZ) and 48 dizygotic (DZ) twin pairs in Japan. Information on forced conversion of hand usage in childhood was also obtained. Results indicated that the incidence of left-handedness was 3.6% and that of non-right-handedness (which includes mixed- and left-handedness) was 5.9%. There was no significant difference in the incidence of left-handedness or of non-right-handedness between MZ and DZ twin groups. The proportion of converted right-handedness in MZ twins was slightly higher than in DZ twins. MZ pairs were somewhat more concordant for handedness than DZ pairs. Item analysis indicated that the incidence of individuals who use the left hand for writing and eating was only 0.9% and 1.8%, respectively. A comparison of the results of the present survey on twins with those of our previous one on singletons revealed that the incidence of left-handedness or non-right-handedness in twins is the same as that in singletons. Twins (especially MZ twins) have experienced a forced conversion to right-hand usage more frequently than singletons.

Adolescent↗

PubMed2001

Individual differences in handedness and specific speech and language impairment: evidence against a genetic link.

Data from two twin studies were used to address two related questions. First, is there any association between handedness and specific speech and language impairment (SSLI) in children? Second, is there genetic influence on individual differences in handedness and, if so, are the same genes implicated in the cause of SSLI? The first study used data from 58 MZ and 26 DZ pairs previously recruited for an investigation into the genetic origins of SSLI. All pairs contained at least one child with SSLI. Handedness was assessed using a preference inventory and a tapping task from which a laterality quotient (LQ) was derived. There were no handedness differences between these twins and 172 singleborn controls, and neither measure revealed any association between handedness and SSLI. The data were equally well-fitted by a CE model (no genetic influence) and an AE model (no effect of shared environment) for both hand preference and tapping LQ. Nonshared environment was the largest influence on handedness for both measures. Bivariate analysis indicated no overlapping genetic influences on SSLI and handedness. In the second study, handedness was assessed in a general population sample of 48 MZ and 44 DZ twin pairs, aged 7 to 13 years, using a preference inventory and a peg-moving task. A subset of children was also given a test that assessed persistence of hand preference when reaching across the midline. The latter was the only measure to relate to children's language status, with language-impaired children showing less midline crossing. This appears to reflect neurodevelopmental immaturity, rather than a stable trait. To investigate familial transmission of handedness, inventory data for parents and their twins were combined for both samples. The most parsimonious model was one that accounted for parent-child resemblance solely in terms of cultural transmission. Overall, there was no evidence that genes play a role in determining stable individual differences in hand preference. Insofar as there are links between handedness and speech and language difficulties, these reflect delayed neuromotor maturation.

Environment↗

PubMed2002

Language lateralization in monozygotic twin pairs concordant and discordant for handedness.

An unexpectedly high percentage of monozygotic twin pairs is discordant for handedness. Some of these twins show mirror-imaging of several ectodermally derived features. Both features of discordant left-right asymmetry may be caused by relatively late monozygotic twinning, when the original embryo has already lost its bilateral symmetry. Language lateralization is related to handedness and may therefore also be altered during the development of embryological asymmetry in some monozygotic twins. Language lateralization was measured with functional MRI in 12 monozygotic twin pairs who were concordant for handedness and in 13 monozygotic twin pairs discordant for handedness. Lateralization indices were calculated from individual language activation patterns. Correlations were calculated to test intra-pair resemblance for language lateralization. The intra-pair correlation for language lateralization was significant in the handedness-concordant group, but not in the handedness-discordant group. In the handedness-discordant group, five twin pairs were also discordant for cerebral dominance; the other twin pairs of discordant handedness exhibited remarkable similarity in language lateralization. The high intra-pair correlation for language lateralization in the handedness-concordant twins suggests a genetic basis for language lateralization. However, in monozygotic twin pairs of discordant handedness, discordance for language dominance occurs in a significant number of twins. Discordant language dominance may be caused by a relatively late time of splitting of the original embryo, which disrupts the normal development of left-right asymmetry.

Female↗

PubMed1994

Ambilaterality: definition from handedness preference questionnaires and potential significance.

Defining handedness according to the results of preference questionnaires is operational. Most often, handedness is defined according to the extremes of the handedness range or equal divisions of the entire handedness range. These definitions may exclude ambilateral (ambidextrous) subjects from analysis or may fail to separate out subjects with intermediate handedness scores. Examples from the literature are presented to show that subjects with handedness scores in the weak right-handedness range may have characteristics that distinguish them from subjects in other segments of the handedness spectrum. Handedness studies should assess the middle handedness range using a scoring method which identifies subjects in this range.

Diethylstilbestrol↗

PubMed1996

[Handedness and cognitive abilities: findings in a representative sample of adolescents and young adults].

Handedness and cognitive abilities in a representative sample of adolescents and young adults. The relationship between laterality and cognitive ability was examined in a representative sample of adolescents and young adults between 16 and 30 years of age. The study was designed as a possible replication of Annett's data supporting her right-shift theory (rst), but included other measures of laterality as well. We found, as Annett did, that strong right-handedness was associated with a weak left hand rather than a strong right hand. However, we could not confirm two other predictions of the rst with our data: The nonverbal IQ was significantly lower in both extreme groups of handedness than in the two middle groups. Because of the sample size this rather small effect, explaining only 1.9% of the variance, is significant, but it disappears with other kinds of grouping for handedness. We could not find the linear decline in nonverbal IQ from the left to the right extreme of handedness as predicted by the rst. In spelling, too, there was a rather small, but significant effect of handedness, explaining 2.4% of the variance. Spelling in the first (left most) quartile of handedness was worse than in quartiles two and four. The predicted poorer spelling in the extreme groups than in the middle groups (inverted U) was not found in our sample. In multivariate analysis with variables explaining larger proportions of the variance in spelling (education, nonverbal IQ and sex) the small effect of handedness on spelling completely disappeared. A weak relationship between left-handedness and dyslexia was evident only with the strictest definition of dyslexia, regardless of how handedness was defined. The implications of these data for the rst are discussed.

Adolescent↗

PubMed1987

Associations of handedness with hair color and learning disabilities.

Forms containing the Edinburgh handedness inventory and questions about learning disabilities, hair color, self-described handedness, age, gender, parental handedness and twinning were received from 1117 randomly selected professionals. Laterality scores (LS, range -100 to +100) were calculated for each respondent based on the handedness inventory and were correlated with the above variables. Among blonds, the frequency of non right-handedness (NRH, LS less than or equal to 70) was 44% compared to 24% of non-blonds (chi 2 = 23.5, P less than 0.0001). Learning disabilities (LD) were present in 9% of NRH (LS less than or equal to 70) as against 3% of those with LS greater than 70 (chi 2 = 22.1, P less than 0.0001). Associations between LS and self-described handedness, parental handedness, gender, and age are also presented. Possible explanations for the association of hair color and handedness are discussed in light of recent data on altered visual system pathways in albinos. Problems in the measurement of handedness are discussed.

Adult↗

PubMed1985

Hand preference in schizophrenics and handedness conversion in their childhood.

A handedness questionnaire which was given to 1774 schizophrenic inpatients was identical to that employed in our previous study on healthy students. Information was obtained on forced conversion of hand usage in childhood and the occurrence of left-handedness in their families. Family history of schizophrenia was also investigated. There were no significant differences in the prevalence of left-handedness or non-right-handedness (i.e., left-handedness and ambidexterity combined) between schizophrenic patients and normal subjects. However, the rate of converted right-handedness in schizophrenics was higher than that in normal people. The incidence of original non-right-handedness (i.e., present non-right-handedness and converted right-handedness combined) in schizophrenics was greater than that in normal controls.

Adolescent↗

PubMed2005

Handedness in schizophrenia: a quantitative review of evidence.

OBJECTIVE: The prevalence of various anomalous handedness subtypes in schizophrenia patients remains ambiguous. Although current literature favours the notion that the shift in lateral preferences seen is because of an increase of mixed-handedness, several studies suggest that exclusive left handedness is more prevalent than in the general population. METHOD: Over 40 studies with reported prevalence data on various handedness subtypes in a schizophrenia population were evaluated by meta-analysis. Combined odds ratios for the three common handedness subtypes (left, mixed, and right) were separately calculated. RESULTS: Each of the three atypical hand dominance patterns were significantly greater in schizophrenia patients than in control subjects, showing that the leftward shift in handedness distribution is not entirely because of an increase in mixed-handedness alone. CONCLUSION: An increase of exclusive left-handedness is at variance with the prevailing assertion that the handedness shift in schizophrenia patients is because of a diffuse and bilateral hemispheric insult.

Functional Laterality↗

PubMed2006

Relationship between language lateralization and handedness in left-hemispheric partial epilepsy.

OBJECTIVE: To investigate the relationship between language lateralization and handedness in patients with epilepsy and a left-sided seizure focus and in healthy control subjects. METHODS: We recruited a consecutive series of 74 patients and 70 control subjects. Functional MRI, using a noun-verb generation task, was performed to establish the language laterality index (LI). Handedness was quantified using the Edinburgh Handedness Inventory. RESULTS: Patients showed a shift toward atypical language lateralization (0.43 +/- 0.47; controls 0.57 +/- 034; p < or = 0.05) and left-handedness (55 +/- 57; controls 74 +/- 39; p < or = 0.05). The LI and handedness were correlated in patients (r = 0.54; F = 25.9; p < 0.001) but not in control subjects (r = 0.1; F = 0.64; NS). A combination of left-handedness and atypical LI was more frequent in patients (12%) than control subjects (0%; p < or = 0.05). Crossed hemispheric specialization (e.g., right-handedness associated with atypical LI) was equally frequent in patients (20%) and control subjects (16%; NS). CONCLUSION: In epilepsy patients with a left-sided seizure focus, language lateralization is correlated to handedness. The increased frequency of left-handedness and associated atypical language lateralization is most likely related to the left-hemispheric seizure focus, influencing hemispheric specialization for both domains.

Adolescent↗

PubMed2005

Genetic and environmental influences on the handedness and footedness in Japanese twin children.

The purpose of this study was to examine the genetic contribution to handedness and footedness in childhood using one of the largest available databases of Japanese twins. The participants were 1131 twin pairs, 1057 males and 1205 females, of 11 or 12 years of age (6th grade of secondary school in the Japanese education system). All data were gathered by questionnaire. The prevalence of left (nonright) handedness was 15% in males and 13% in females. The prevalence of left (nonright) footedness was 13% in males and 11% in females. The similarities between twin pairs, estimated by concordance rates and tetrachoric correlations, suggested a slight genetic effect on male handedness, no genetic effect on female handedness, and no genetic effect on footedness in either sex. Structural equation modeling showed small genetic factors (11%) in male handedness and no genetic factors in female handedness. As to footedness, no genetic factors were observed in either sex. The effects of nonshared environmental factors were large (85%) in males and moderate (44%) in females. Moreover, handedness and footedness tended to be concordant irrespective of sex, with polychoric correlations over r = .70. The results of bivariate genetic analyses were not necessarily satisfactory. For males, no model fit. For females, shared and nonshared environmental factors explained the concordance of handedness and footedness. It was concluded that the genetic effects on handedness and footedness are relatively small, as is their association; moreover, considerably large twin samples are needed to obtain stable and appropriate results.

Child↗

PubMed1999

[Current aspects of handedness].

Handedness is one example of many forms of behavioural lateralization seen in humans. Left-handedness has existed in a small subset of the human population, approximately 8%, since the origin of man. The incidence of left-handedness is usually reported to be consistent among human populations. Sinistrality is more common in males than in females. A vast range of testing techniques have been used to assess handedness. There are preference and performance tests. Writing hand and self-report are two of the most popular techniques. There is a strong evidence that the dominance of language functions in the cerebral cortex is different in left-handed from that in right-handed people. An understanding of handedness may provide valuables clues as to how the brain becomes organized the way it is. Several theories have been advanced over the years to explain the genesis of handedness and, in particular, left-handedness. Theories have ranged from genetic models to socio-cultural theories. Other authors suggested a pathological origin of left-handedness in man. Left-handedness runs in families and adoption studies suggests a genetic rather than an environmental origin. However, monozygotic twins appear to be substantially discordant. A polygenetic explanation which takes environmental influences into consideration is probably called for.

Adult↗

PubMed1991

The inheritance of left-handedness.

Left-handedness occurs in about 8% of the human population. It runs in families and an adoption study suggests a genetic rather than an environmental origin; however, monozygotic twins show substantial discordance. The only genetic models that successfully explain the family and twin data are those of McManus and Annett, which share the feature of incorporating a random component reflecting the biological phenomenon of 'fluctuating asymmetry'. The models have each been modified to explain the greater incidence of left-handedness in males. The McManus model is more successful at explaining the maternal effect--left-handed mothers have more left-handed offspring than do left-handed fathers. Both models explain the association of handedness with cerebral language dominance. The models differ principally in their conception of the phenotypes of handedness: Annett proposes a unimodal continuum, McManus proposes two discrete categories of handedness. Finding the gene for handedness and hence for language dominance would unlock the neurobiology of language. Two ways of finding the gene for handedness are proposed: searching the pseudoautosomal region of the X chromosome or invoking a specific evolutionary model of lateralization in which the handedness gene has evolved from the situs gene then searching the human genome for homologues to the mouse situs gene.

Animals↗

PubMed2002

Chimpanzee right-handedness reconsidered: Evaluating the evidence with funnel plots.

Evidence for population-level right-handedness in nonhuman primates seems inconsistent and contradictory, and many hypotheses have been advanced to account for this volatility. Funnel plots (scatter plots of percent right-hand use vs. sample size) offer a straightforward graphical technique for assessing: 1) the strength and consistency of handedness, 2) whether variability is consistent with normal sampling variation, and 3) how likely reports of statistically significant handedness might have arisen due to chance (i.e., type I error). They are informative for both within- and among-population variation. Reexamination of within-population variation from a detailed and widely cited study reporting significant population-level right-handedness in 140 individual captive chimpanzees (Hopkins [1994] Dev. Psychobiol. 27:395-407) revealed several puzzling patterns: 1) funnel plots showed higher percent right-hand use among individuals for which fewer observations were recorded, 2) when individuals with fewer than 25 observations were excluded, statistical support for population-level right-handedness either became marginal (P = 0.043, when computed as average percent use of the right hand) or disappeared (P = 0.62, when computed as proportion of individuals using the right hand more than the left, whether they did so significantly or not), and 3) the proportion of statistically ambilateral chimpanzees actually increased with increasing number of observations per individual, rather than decreased as would be expected for true population-level right-handedness. In addition, funnel plots of among-population variation from an earlier meta-analysis (McGrew and Marchant [1997] Yrbk. Phys. Anthropol. 40:201-232) suggested that the four reports of significant right-handedness, out of 37 estimates from 14 studies, were likely those that achieved statistical significance simply due to chance. Funnel plots, and the more refined statistical tests they suggest, confirm that the current evidence for population-level right-handedness in chimpanzees remains equivocal.

Animals↗

PubMed1995

Prevalence and correlates of mixed-handedness in schizophrenia.

Increased rates of nonright-handedness have been reported in schizophrenia, but a clear distinction has not been made between left- and mixed-handedness. Handedness preferences in 96 patients fulfilling DSM-III criteria for schizophrenia and 43 normal comparison subjects were assessed with the Edinburgh Handedness Inventory. A 100% criterion was used to establish left- and right-handedness. Results were analyzed with mantel-Haenszel odds ratios adjusted for age and sex. The schizophrenic group showed a significant increase in the proportion of mixed-handers compared with the normal group. There was no increase in pure left-handedness in the schizophrenic relative to the normal group. Mixed-handedness in the schizophrenic patients was significantly associated with chronicity of illness. Mixed-handed patients were less likely to have a family history of psychotic illness than patients with strong right- or left-handedness. The results indicate a neurodevelopmental rather than a genetic origin for anomalous lateralization in schizophrenia.

Adult↗

PubMedDate not supplied

Reduced incidence of left-handedness in clinically diagnosed dementia of the Alzheimer type.

Although it is generally held that about 10% of the population is left-handed, reported figures vary widely due to differences in handedness classification criteria and subject characteristics. Among those population studies that have used the same handedness classification criteria, a consistent relationship between increasing right-hand preference and increasing age has been reported. One recent study of Alzheimer's disease (AD) reported a higher incidence of left-handedness in early onset relative to late onset cases. In the present study we examined handedness patterns in three elderly groups; normal (N = 217), depression (N = 73), and AD (N = 114). Our results indicated a reduced frequency of left handedness in AD (2.6%) relative to control (11.1%) and depression (13.7%) groups. Within the limited age range we studied (60-80 years), no relationships were found between age and handedness preference either within or across the groups. Furthermore, for the AD group there was no relationship between severity of global impairment and strength of handedness. Our results suggest that compared to right handers, left handers are less vulnerable to the cognitive changes associated with AD. Nevertheless it is also possible that left handers are overrepresented among early onset dementia patients and die before entering the pool of senile dementia patients. Further work is required to determine if early and late onset AD are associated with different incidences of left handedness.

Aged↗

PubMed2001

Schizotypy and the shift from dextrality: a study of handedness in a large non-clinical sample.

Numerous previous studies have reported associations between schizophrenia/schizotypy and atypical handedness. While there is broad agreement that schizophrenia/schizotypy is associated with a shift away from typical (strong right) handedness, it is not entirely clear what aspect of atypical handedness is implicated: 'ambiguous' handedness (the absence of hand preference for given actions) or 'mixed handedness' (different hand preferences for different actions). The present study used several indices of handedness (derived from the Annett scale) to address these questions in 3000 + (mostly) University student subjects recruited by electronic mail. It was found that positive schizotypy (indexed by a scale of 'unusual experiences') was associated with both aspects of atypical handedness. These findings are discussed in the light of various possible explanations of the association between handedness and schizophrenia/schizotypy.

Adolescent↗

PubMed1999

Schizophrenic patients and their first-degree relatives show an excess of mixed-handedness.

An excess of mixed-handedness in schizophrenia has been reported. However, it is not established whether this excess is manifest in non-schizophrenic psychoses, nor whether the underlying etiology is genetic or environmental. We investigated these issues in a group of patients with schizophrenia (n=94), affective psychosis (n=63), other psychosis (n=26); their respective first-degree relatives (total n=183) and a control group (n=85). A narrow definition of mixed-handedness was used corresponding to groups 5 and 6 as defined by the Annett Handedness Questionnaire. We found an excess of mixed-handedness in the schizophrenic group compared with controls (OR=5.2, 1.4-18.6, p<0.006). There was no difference between the other psychotic groups and controls. There was a trend for an excess of mixed-handedness in the first-degree relatives (n=99) of schizophrenic patients (p=0.055), but not in the relatives of affective or other psychotic patients. There was a striking linear trend in the proportion of mixed-handedness between controls, the relatives and the schizophrenic patients (chi2=7.0, p=0.008). There was no association between mixed-handedness and a history of pregnancy or birth complications in the schizophrenic group. There was some evidence for impaired sociability in the mixed-handed schizophrenic patients. Our results indicate that the excess of mixed-handedness in schizophrenia may have a genetic basis.

Adult↗