HANDEDNESS AND CONFORMITY IN A SMALL ISOLATED COMMUNITY*

The inhabitants of Tristan da Cunha are a socially and geographically isolated community, highly inter-related by blood or marriage with a large average family size. The handedness characteristics of a sample of 76 people out of the total Tristan population of 264 were assessed on eight criteria by personal interview. All subjects wrote with the right hand and only two could be classified as otherwise left-handed (i.e. 2.6%). Seven out of eight of those interviewed who claimed a left-handed parent were themselves right-handed and had only right-handed brothers or sisters. It is concluded that any genetic factor determining left-handedness must be very weak and a preferred explanation of the findings is offered in terms of social pressures such as (i) deference to authority and (ii) group conformity.     

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.     

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.     

Left-handedness: association with college major, familial sinistrality, allergies, and asthma

366 juniors, seniors, and graduate students from seven departments at The Ohio State University were administered the 10-item Edinburgh Handedness Inventory and asked questions concerning familial handedness and personal incidence of allergies or asthma. The rate of left-handedness was 15.6% among the male students and 12.9% among the females. Maternal handedness was significantly related to female offspring handedness and to the incidence of allergies and asthma in both males and females. Departmental incidence of left-handedness ranged from a low of 7.5% (Microbiology) to a high of 21.7% (Law).     

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

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

A note on Corballis (1997) and the genetics and evolution of handedness: developing a unified distributional model from the sex-chromosomes gene hypothesis

A unified, quantitative model for sex, twin, parent, and grandparent influences on handedness is presented. Recent research modeling the evolutionary development of genetic mechanisms for the transmission of handedness on the basis of genotype fitness has appeared to lead to the conclusion that a handedness gene cannot be located on the sex chromosomes. It is shown in this article, however, that this conclusion is not of general validity. The sex-chromosomes hypothesis is developed further, and it is demonstrated that a wide-ranging, detailed, and parsimonious account of the distribution of handedness is obtained when left-handedness is assumed to be associated recessively, and with low penetrance, with genetic variation located on the X chromosome.     

Why do High IQ Societies Differ in Intellectual Achievement? The Role of Schizophrenia and Left-Handedness in Per Capita Scientific Publications and Nobel Prizes

Previous research has attempted to understand why countries with relatively favorable conditions and high estimated average IQs (such as Finland and Japan) have a relatively low per capita number of scientific Nobel prizes. In the present study, we examine whether there is a relationship between national schizophrenia and left-handedness prevalence, on the one hand, and per capita scientific and literary achievement, on the other hand, in countries with IQ estimates of at least 90. We found that per capita science and literature Nobel prizes and scientific publications are strongly negatively associated with schizophrenia and strongly positively correlated with left-handedness. There also was a very pronounced negative correlation between schizophrenia rate and left-handedness rate. These results suggest that genius can be regarded as a combination of very high IQ, aspects of high-functioning autism (specifically low empathy) plus relatively low impulse control, consistent with observations of intellectually outstanding individuals, and the fact that schizophrenia appears to constitute the opposite pole of these aspects of autism spectrum. We posit differences in androgen levels as a possible underlying explanation for these findings.     

Left-handedness: a marker for decreased survival fitness

Life span studies have shown that the population percentage of left-handers diminishes steadily, so that they are drastically underrepresented in the oldest age groups. Data are reviewed that indicate that this population trend is due to the reduced longevity of left-handers. Some of the elevated risk for sinistrals is apparently due to environmental factors that elevate their accident susceptibility. Further evidence suggests that left-handedness may be a marker for birth stress related neuropathy, developmental delays and irregularities, and deficiencies in the immune system due to the intrauterine hormonal environment. Some statistical and physiological factors that may cause left-handedness to be selectively associated with earlier mortality are also presented.     

The Geschwind and Galaburda theory of cerebral lateralisation: An empirical evaluation of its assumptions

The relationship between male gender, left-handedness, high visuo-spatial and mathematical skill, speech problems and increased vulnerability to immune disorders was assessed in a large sample of university undergraduates. The only significant finding was a two-way interaction between hand preference and incidence of speech problems, with “pure” left-handers reporting a three-fold increase in the incidence of speech problems compared to mixed left-handers, mixed right-handers and “pure” right-handers. There was no evidence of an interaction among three or more of the variables. These findings are discussed in relation to the influential theory of cerebral lateralisation proposed by Geschwind and Galaburda (1987).     

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.     

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.     

Molecular Lamarckism: On the Evolution of Human Intelligence

In modern times, Lamarck's view of evolution, based on inheritance of acquired traits has been superseded by neo-Darwinism, based on random DNA mutations. This article begins with a series of observations suggesting that Lamarckian inheritance is in fact operative throughout Nature. I then launch into a discussion of human intelligence that is the most important feature of human evolution that cannot be easily explained by mutational selection. Thus, we are smarter than demanded by our evolutionary experience as hunter-gatherers. The difficulty lies in the inability of neo-Darwinism to satisfactorily answer the following question: How can a large energy-costly set of genes, each member of which has little apparent benefit when first created individually, all gather into a permanent existence within a short time period in each and every member of a small population (that was dispersed and geographically isolated over a huge planet) who had a low reproductive output, a low rate of beneficial mutations, and a low level of genetic contact? The article concludes with a speculative but far-reaching epigenetic theory of intelligence that does not require DNA mutation as the exclusive source of evolutionary change. Instead, cranial feedback relating brain chemistry, as affected by brain activity including education, with the genome. When it comes to the fast rate of evolution, and the dissemination of the intelligence trait worldwide, cranial feedback could make all the difference.     

Genetic Testing and Parkinson Disease: Assessment of Patient Knowledge,Attitudes, and Interest

The most common genetic contributor to late-onset Parkinson disease (PD) is the LRRK2 gene. In order to effectively integrate LRRK2 genetic testing into clinical practice, a strategy tailored to the PD population must be developed. We assessed 168 individuals with PD for baseline knowledge of genetics, perceived risk, and interest and opinions regarding genetic counseling and testing. Most participants felt that they were familiar with general genetics terms but overall knowledge levels were low, with an average score of 55%. The majority of participants thought it was likely they inherited a PD gene (72%), believed genetic testing for PD would be useful (86%), and were interested in genetic testing (59%) and genetic counseling (56%). However, only a few participants had heard of any genetic tests for PD (29%) or LRRK2 (10%). There appears to be a significant level of interest in genetics and genetic testing within the PD population, but a considerable deficit in genetics knowledge and an over-estimation of risk. Genetic education and counseling tools to address these needs were developed to provide patients with the ability to make informed and knowledgeable genetic testing decisions.     

The Use of Family History Questionnaires: An Examination of Genetic Risk Estimates and Genetic Testing Eligibility in the Non-responder Population

The use of mailed family history questionnaires (FHQs) has previously been established to be an effective method for obtaining family history information for the triage of patients for genetic counseling and genetic testing of hereditary breast and ovarian cancer syndrome; yet only 53% of patients complete their FHQ within 6 months from the date of mailing (Armel et al. Journal of Genetic Counseling, 18(4):366–378, 2009). Although literature exists evaluating why women may not attend genetic counseling, no data are currently available examining genetic risk or genetic testing eligibility in the population of patients not returning their FHQ (non-responders). Concern exists that if non-responders are not followed-up for the purpose of triage for genetic counseling, individuals at high-risk for a hereditary cancer syndrome may be missed. This article explores the demographics of the non-responder population to assess genetic risk estimates for mutations in the BRCA1 and BRCA2 genes and genetic testing eligibility as compared to a responder population of patients who completed a mailed FHQ. A total of 430 pedigrees were obtained, 215 from non-responders and 215 from responders. Results of this study indicate that 69% of non-responders were either unreachable by telephone (42%), declined an appointment (19%), or were previously seen in another center for a genetic counseling visit (8%). Additionally, results indicate that non-responders are less likely to be eligible for genetic testing (40%) as compared to responders (57%) (p = 0.0004). Together these data shed light on a population of patients for which limited information exists and suggest that we question how and to what extent clinics should pursue non-responders, particularly in light of global reductions in health care funding.     

Is There a Link between Irritable Bowel Syndrome and Left-Handedness?: An Exploratory Study

Recent research has found that people with Irritable Bowel Syndrome (IBS) and those with Inflammatory Bowel Disease (IBD) show a relative deficit in verbal IQ, compared to their own performance IQ, and the IQ of a healthy comparison group. It has been suggested that the elevated prevalence of left-handedness shown in previous studies investigating people with IBD is due to compromised left-hemisphere development. This may be associated with a decrement in certain cognitive functions such as verbal IQ. A shift in brain dominance from left to right has been found in many atopic and immunological diseases as well as in autonomic dysfunctions. It was uncertain whether this would apply to people with IBS, a functional disorder in which the cause or causes remain unclear. Therefore, the aim of this preliminary study was to investigate whether people with IBS have an increased prevalence of left-handedness. It was found that there was an association between IBS and left-handedness, showing that further research on this association both in IBS and other functional disorders (as well as on the similarities between IBS and IBD) is warranted.     

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.     

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.     

Is there a link between irritable bowel syndrome and left-handedness?: An exploratory study

Recent research has found that people with Irritable Bowel Syndrome (IBS) and those with Inflammatory Bowel Disease (IBD) show a relative deficit in verbal IQ, compared to their own performance IQ, and the IQ of a healthy comparison group. It has been suggested that the elevated prevalence of left-handedness shown in previous studies investigating people with IBD is due to compromised left-hemisphere development. This may be associated with a decrement in certain cognitive functions such as verbal IQ. A shift in brain dominance from left to right has been found in many atopic and immunological diseases as well as in autonomic dysfunctions. It was uncertain whether this would apply to people with IBS, a functional disorder in which the cause or causes remain unclear. Therefore, the aim of this preliminary study was to investigate whether people with IBS have an increased prevalence of left-handedness. It was found that there was an association between IBS and left-handedness, showing that further research on this association both in IBS and other functional disorders (as well as on the similarities between IBS and IBD) is warranted.     

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.     

Using a Sharp Instrument to Parse Apart Strategy and Consistency: An Evaluation of PPT and Its Assumptions

Potential Performance Theory (PPT) is a general theory for parsing observed performance into the underlying strategy and the consistency with which it is used. Although empirical research has supported that PPT is useful, it is desirable to have more information about the bias and standard errors of PPT findings. It also is beneficial to know the effects of violations of PPT assumptions. The authors present computer simulations that evaluate bias and standard errors at varying levels of strategy, consistency, and number of trials per participant. The simulations show that, when the assumptions are true, there is very little bias and the standard errors are low when there are moderate or large numbers of trials per participant (e.g., N = 50 or N = 100). But when the independence assumption is violated, PPT provides biased findings, although the bias is quite small unless the violations are large.