Is the handedness gene on the X chromosome? Comment on Jones and Martin (2000).

G. V. Jones and M. Martin (2000) argued, contrary to M. C. Corballis (1997), that a gene for handedness might plausibly be located in homologous, noncombining regions of the X and Y chromosomes. The specific model they proposed is unlikely to be correct, but a case can be made for an X-linked gene that has no homologue on the Y chromosome and that is subjected to X-inactivation in females. An X-linked gene predicts no overall sex difference in the incidence of left-handedness; the slight preponderance of left-handers among males might then be attributed to a higher incidence of pathologically induced left-handedness.     

Handedness patterns in deaf persons

The handedness patterns of 226 deaf high-school and college students were compared to those of 210 college students with normal hearing. Both groups evidenced many more right-handed than left-handed members, as determined by responses to a hand preference questionnaire and performance on an activity test battery. There was, however, a significantly higher incidence of left-handedness among the deaf subjects than among the hearing. Moreover, the left-handed deaf students were found to be less likely to have deaf relatives, and to have been introduced to sign language later in their development than the deaf student population as a whole. These findings were interpreted as showing that age of acquisition of language was related to the development of handedness patterns, whereas auditory processing experience probably was not.     

Evaluating the Empirical Support for the Geschwind-Behan-Galaburda Model of Cerebral Lateralization

The Geschwind-Behan-Galaburda (GBG) model of cerebral lateralization provides a complex but testable theory of the origins and associates of cerebral lateralization. An overall evaluation of the model suggests that it is not well supported by empirical evidence and that in the case of several key theoretical areas, the evidence that does exist is inconsistent with the theory. In particular: the concept of "anomalous dominance" is shown to be theoretically and methodologically flawed; a meta-analysis of the relationship between handedness and immune disorders finds a marginal overall association, and while three conditions (allergies, asthma, and ulcerative colitis) do show significant associations with left-handedness, two other conditions (myasthenia gravis and arthritis) show significant negative associations with left-handedness. Finally, a review of the origins of the neural crest, and its associations, suggests there is almost no empirical support for the GBG theoretical model in this area.     

Spatial ability and handedness

The relationship between preferred handedness and spatial visualization was examined for 124 female and 229 male undergraduate students. Handedness and spatial ability were used in the design as continuous variables. Other variables examined were sex of subject and family history of sinistrality. Spatial ability was found to be systematically related to measured handedness, familial left-handedness, and sex of subject. Males outperformed females at all levels of handedness. The lowest performance was obtained by subjects who were either extremely left-handed or extremely right-handed. The highest scores were obtained by individuals with left-handed relatives and whose handedness scores were in the range considered mixed or slightly right-handed. The results suggest that decreased hemispheric specialization is associated with increased spatial ability.     

Cohort Effects in the Lateral Preference of Japanese People

Handedness data of the Japanese were collected twice with an interval of 20 years, using an identical handedness inventory. In the second cohort, non-right-handers among male and female students numbered 11.5% and 5.9%, respectively. The results showed that the incidence of non-right-handedness among Japanese females has increased from 20 years ago.     

Cognitive Performance as a Function of Sex and Handedness: An Evaluation of the Levy Hypothesis

Investigated incidence of left-handedness and non-right-handedness in a sample of 9th-grade students (n=313). In comparison, incidence among undergraduates is underestimated. Gitld were not significantly more right-handed than boys in the 9th grade sample but were significantly (p<01) more right-handed than boys in a sample of math students, grades 9-11 (n = 1097). Handedness did not affect math achievement (Test of Academic Progress), verbal (Quick Word Test) or spatial (Space Relations Test of the Differential Aptitude Test) performance in the 9th (n = 108) or 10th (n = 75) grade students, for 11th grade students (n = 98) a significant sex-by-handedness-by-measures (verbal-spatial) interaction was found (p<05). Results did not support the hypothesis of Levy's that left-handedness decreases spatial performance and benefits verbal performance, nor did the results support the further hypothesis that the cognitive pattern of higher verbal than spatial performance often considered characteristic of females can be attributed to more bilateral cerebral verbal function in females as in left-handers.     

Pathological left-handedness: Evaluation of a model

This study investigated the relationship between manifest handedness and the localization of EEG abnormalities in a large group of mentally retarded persons. Persons who had abnormal EEG recordings, regardless of whether the dysfunction was bilateral or asymmetric, exhibited approximately twice the incidence of left-handedness than persons who had normal EEG's. Data for persons having asymmetric damage support a model of pathological left-handedness, with left-handers showing a substantially higher probability of having left brain dysfunction than right-handers.     

Handedness and immune function

Geschwind, Galaburda, and Behan (GBG) have suggested that in utero levels of testosterone influence both cerebral and immune system developments (Geschwind and Behan, 1982; Geschwind and Galaburda, 1984; Geschwind and Galaburda, 1985). According to this theory, high levels of testosterone result in greater incidences of left-handedness, deviations from standard distribution of cerebral functions (known as anomalous dominance), and increased autoimmune dysfunction. While the original data supported these assertions, more recent tests of the hypothesis have been equivocal. One criticism of these studies is that the definition of both handedness and anomalous dominance are too vague. It was one of the aims of this project to investigate and clarify the GBG model by examining four different aspects of handedness as well as a more direct measure of anomalous dominance. In order to extend the GBG model, degree of left-handedness, general immune system functioning, and current testosterone levels were also examined. First, it was predicted and found that left handers had a higher incidence of autoimmune diseases in their immediate families than did right handers. Second, those left handers with an incidence of at least one autoimmune disease were more strongly left-handed than were those with no incidence of autoimmunity. Finally, it was observed that higher testos terone levels were supportive of general immunity. The present findings both support and expand the GBG model.     

Delinquency, sidedness, and sex

One hundred and thirty male and 213 female college students responded to a self-report delinquency and crime measure and to a scale of sidedness preferences. Consistent with most of the literature on official delinquency and crime, left handedness was positively associated with involvement in several forms of delinquency (including some degree of criminality) among males. Other measures of sidedness among males largely failed to correlate with delinquency, however. Among females, sidedness was much less systematically related to delinquency than among males.     

Lateralization of squamous cell carcinomas in the head-neck region

Earlier studies have shown the lateralization of several immune disorders, including herpes zoster infection and breast cancer. We investigated whether there is lateralization of squamous cell carcinomas in the head-neck region and a relation of handedness with this cancer. Analysis showed right-sided lateralization of head-heck cancers in right-handed patients and vice versa in left-handed ones and a higher rate of left-handedness in patients with squamous cell cancer. Associations among left-handedness, squamous cell carcinoma, and its lateralization may result from a genetic togetherness.     

Sex and eyedness in a sample of Turkish high school students

The relationship of sex with eyedness was examined in 176 boys and 129 girls ages 15 to 21 years (M = 16.9 yr., SD = 1.8) in Turkish high schools. Sex was significantly associated with handedness; the rate of left-handedness was 10.8% in boys and 2.3% in girls and was not related to eyedness. Rate of left-eyedness in the sample was 7.9% for boys and 8.5% for girls. One may wonder whether eyedness shows a reverse sex effect, less than handedness, but methods of measurement must be compared.     

Correlation between handedness and birth order: compilation of five studies

Bakan has suggested that left-handedness is the result of left hemishperic pyramidal motor dysfunction following perinatal hypoxia. To a degree support for the validity of this hypothesis rests on Bakan's (1971, 1977a) findings that left-handed college students were more likely the progeny of birth orders designated as "high-risk" than right-handed students. Attempts by others to replicate Bakan's data have been unsuccessful. To achieve a more powerful test of this relationship than has been provided by any single study, the data from the five studies which have considered it were pooled and tested. The resulting correlation between birth order and handedness was near zero.     

Hand preference for writing and associations with selected demographic and behavioral variables in 255,100 subjects: the BBC internet study

In an Internet study unrelated to handedness, 134,317 female and 120,783 male participants answered a graded question as to which hand they preferred for writing. This allowed determination of hand preference patterns across 7 ethnic groups. Sex differences in left-handedness were found in 4 ethnic groups, favoring males, while no significant sex differences were found in three of the groups. Prevalence of left-handedness in the largest of the ethnic groups (self-labelled as "White") was comparable to contemporary hand preference data for this group [Gilbert, A. N., & Wysocki, C. J. (1992). Hand preference and age in the United states. Neuropsychologia, 30, 601-608] but the prevalence of left-handedness in individuals >70 years of age was considerably higher in the present study. Individuals who indicated "either" hand for writing preference had significantly lower spatial performance (mental rotation task) and significantly higher prevalence of hyperactivity, dyslexia, asthma than individuals who had clear left or right hand preferences, in support of Crow et al. [Crow, T., Crow, L., Done, D., & Leask, S. (1998). Relative hand skill predicts academic ability: global deficits at the point of hemispheric indecision. Neuropsychologia, 36, 1275-1282]. Similarly, an association of writing hand preference and non-heterosexual orientation was clearest for individuals with "either" writing hand responses. We conclude that contradictions in the literature as to whether or not these variables are linked to handedness stem largely from different definitions of hand preference. Due to a lack of statistical power in most studies in the literature, the "either" hand writing preference group that yielded the most salient results in this study is not normally available for analysis.     

The relationship of perinatal birth status to handedness: A prospective study

As part of a longitudinal study of the consequences of high risk birth, 66 infants born to right-handed parents were given two unimanual tasks at 24 months of age to assess their hand preference. The 66 infants had been assigned to one of four diagnostic categories at birth: Premature with no postnatal medical complications; premature and having experienced respiratory distress; term and having experienced birth asphyxia during labor and delivery; term with normal delivery and postnatal course. Results of the handedness tasks revealed an increased incidence of use of the left hand among the term infants who underwent birth asphyxia and among the healthy preterm infants. The data indicate that timing of birth stress may play an important role in the ontogeny of pathological left-handedness.     

Handedness on Tristan da Cunha: The Genetic Consequences of Social Isolation

Provins (1990) has suggested that the apparently low incidence of left-handedness on the island of Tristan da Cunha is the result of social pressures such as deference to authority and group conformity, and that “any genetic factor determining left-handedness must be very weak”. Here we use a large number of Monte Carlo simulations (n = 10,825) of the growth of the population of Tristan da Cunha from 1827 to 1961, to show that if the original population was randomly selected from a large population with a typical gene-frequency, then a combination of founder effects, genetic drift and evolutionary bottlenecks can readily explain the fairly low incidence of left-handedness that was found. Indeed in 15% of the simulations the c-allele, which is postulated to be responsible for left-handedness, has disappeared entirely, resulting in a population with no left-handers at all. The data from Tristan da Cunha are therefore entirely compatible with a genetic model of left-handedness.     

Temperamental Differences Between Healthy,Asthmatic, and Allergic Children Before Onset of Illness: A Longitudinal Prospective Study of Asthma Development

The authors investigated the link between children's temperament and the development of asthma and allergies. Prospective longitudinal data on children at the ages of 3-5 months, 3-5 years, and 7-9 years were collected. At age 7-9 years, analyses were performed on data for 3 groups of children (n = 42): those with asthma (no allergies), those with allergies (no asthma), and those with neither asthma nor allergies (the control group). Data for children who developed asthma or allergies prior to age 7-9 years were not analyzed. Differences were found in the premorbid period between the control group and the children who later developed asthma or allergies as well as between the asthma and allergy groups. After onset of illness, no temperamental differences were observed between the 3 groups. The study shows the importance of longitudinal design for asthma research.     

Temperamental differences between healthy,asthmatic, and allergic children before onset of illness: a longitudinal prospective study of asthma development

The authors investigated the link between children's temperament and the development of asthma and allergies. Prospective longitudinal data on children at the ages of 3-5 months, 3-5 years, and 7-9 years were collected. At age 7-9 years, analyses were performed on data for 3 groups of children (n = 42): those with asthma (no allergies), those with allergies (no asthma), and those with neither asthma nor allergies (the control group). Data for children who developed asthma or allergies prior to age 7-9 years were not analyzed. Differences were found in the premorbid period between the control group and the children who later developed asthma or allergies as well as between the asthma and allergy groups. After onset of illness, no temperamental differences were observed between the 3 groups. The study shows the importance of longitudinal design for asthma research.     

Sex differences in left-handedness: a meta-analysis of 144 studies

Human handedness, a marker for language lateralization in the brain, continues to attract great research interest. A widely reported but not universal finding is a greater male tendency toward left-handedness. Here the authors present a meta-analysis of k = 144 studies, totaling N = 1,787,629 participants, the results of which demonstrate that the sex difference is both significant and robust. The overall best estimate for the male to female odds ratio was 1.23 (95% confidence interval = 1.19, 1.27). The widespread observation of this sex difference is consistent with it being related to innate characteristics of sexual differentiation, and its observed magnitude places an important constraint on current theories of handedness. In addition, the size of the sex difference was significantly moderated by the way in which handedness was assessed (by writing hand or by other means), the location of testing, and the year of publication of the study, implicating additional influences on its development.     

Handedness as a function of test complexity.

Handedness as a function of test complexity was investigated with two tests, Dotting and Tapping-on-squares. For each one three levels of complexity were established by varying radius (Dotting) and square sides (Tapping-on-squares). Administration of the different complexity levels to a total of 310 third and fourth grade boys and girls showed that the rate of left-handedness increased from 6% to 16% with decreasing level of test complexity. It is concluded that optimal classification of handedness is only made possible by maximal levels of test complexity, which have to be established empirically for different populations.