Handedness and birth risk

Does the Fraternal Birth Order Effect Influence Handedness?

The fraternal birth order effect (FBOE) is the phenomenon whereby the probability that a man has a same-sex sexual orientation in adulthood increases with each biological older brother. Several studies have found evidence that the FBOE is limited to right-handed men, and left-handed men do not show an FBOE. Recent debates about the appropriate methods for quantifying the FBOE center on distinguishing the FBOE from other effects, such as the female fecundity effect (FFE), whereby mothers more prone to bearing gay sons are also more fecund. The FBOE and FFE are confounded in that a real FFE will result in data consistent with the FBOE under some analyses. Here, we applied some recent proposed analytic methods for the FBOE to the property of handedness. A straightforward application of Khovanova's technique to the binary trait of handedness yielded support for a fraternal birth order effect consistent with the maternal immune hypothesis, in that the ratios of handedness differed between men with one older brother only, and men with one younger brother only, while no such effect was seen in women. This effect was not seen, however, when the confounding effects of parental age were controlled for. Models including factors to simultaneously test multiple posited effects find significant female fecundity effects, as well as paternal age and birth order effects on handedness in men, but no FBOE. The effects seen in women were different, with no fecundity or parental age effects, but birth order and sex of older siblings had effects. We conclude, based on this evidence, that many of the factors thought to contribute to sexual orientation in men may also have an influence on handedness, and further note that parental age is a potential confound which may be overlooked by some analyses of the FBOE.

Sex differences in human brain asymmetry: a critical survey

Dual functional brain asymmetry refers to the notion that in most individuals the left cerebral hemisphere is specialized for language functions, whereas the right cerebral hemisphere is more important than the left for the perception, construction, and recall of stimuli that are difficult to verbalize. In the last twenty years there have been scattered reports of sex differences in degree of hemispheric specialization. This review provides a critical framework within which two related topics are discussed: Do meaningful sex differences in verbal or spatial cerebral lateralization exist? and, if so, Is the brain of one sex more symmetrically organized than the other? Data gathered on right-handed adults are examined from clinical studies of patients with unilateral brain lesions; from dichotic listening, tachistoscopic, and sensorimotor studies of functional asymmetries in non-brain-damaged subjects; from anatomical and electrophysiological investigations, as well as from the developmental literature. Retrospective and descriptive findings predominate over prospective and experimental methodologies. Nevertheless, there is an impressive accummulation of evidence suggesting that the male brain may be more asymmetrically organized than the female brain, both for verbal and nonverbal functions. These trends are rarely found in childhood but are often significant in the mature organism.

HAND PREFERENCE IN TURKISH POPULATION

The distribution of hand preference (self report) was studied in siblings and their parents originating from all parts of Turkey (N = 22,461). In total sample and siblings, there were significantly more right-handed women than men, and significantly more left-handed men than women--no significant sex difference for parents' handedness. The relative number for the right-handed parents significantly exceeded that for the right-handed siblings; the relative number for the left-handed siblings significantly exceeded that for the left-handed parents. It was concluded that there may be a sex difference in hand preference, but being only about 1% more left-handed men, and only about 1% more right-handed women; the right-handedness in new generation (siblings) is less than that in old generation (parents), due to freeing from cultural pressures against the left-hand use in everyday activities.

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.

Maternal age and parity correlates of handedness: gender, but no parental handedness modulation of effects

Mothers supplied information on the handedness of a total of 1079 children (238 left handed, 841 right handed), and also reported information on the handedness of the biological parents, parental ages when each child was born, parity, and birth complications. A MANOVA, employing offspring handedness and parental handedness (presence versus absence of at least one left handed) as the bases of classification, and parity, maternal age, paternal age, and a birth stress composite score as dependent measures, was applied to the data of female and male offspring separately. For females, there was a significant multivariate effect for handedness, but no effect of parental handedness nor any interaction of the handedness and parental handedness factors. Univariate ANOVAs, following the multivariate analysis, showed significant effects of handedness for the maternal age and parity measures, but not for paternal age or birth stress composite score. The analysis for males showed no significant multivariate or univariate effects. The data suggest, that in the absence of high risk parity and maternal age over 32, only about 7.8% of females are left handed. This implies that as many as 29% or so of female left handers may owe their sinistrality to factors associated with high risk parity and maternal ages over 32.

Handedness, prenatal stress and pregnancy complications

Two hundred and ninety children aged 2 yr were assessed for laterality on a number of preference and dexterity tasks. The mothers were interviewed about their child's pre- and perinatal history. Information was also obtained from hospital records about pregnancy and delivery stress and complications. High risk pregnancies or pregnancy complications were not associated with an increase in the proportion of sinistrality. Only one of the stress factors (low Apgar) was associated with a decrease in the strength of dexterity. Since maternal and hospital reports were not in agreement in reporting complications, the validity of data generated from maternal reports is questioned.

An analysis of the naming deficit of left-handers

Left-handers have been found to have a naming deficit when confronted with briefly flashed words. Three alternative explanations of this deficit were tested: whether it was due to deviant interhemispheric cooperation, relatively diffuse neural organization, or left hemispheric dysfunction. In Experiment I, four words were presented for 190 ms, unilaterally (to a single hemisphere), or bilaterally (between the hemispheres). Although left-handers (N = 48) named significantly less than right-handers (N = 30), performance was parallel across hemispheric conditions. Experiment II required semantic categorization, but not naming. The performance of the left-handers (N = 27) was indistinguishable from that of right-handers (N = 27) both in terms of overall performance and interhemispheric collaboration. It is concluded that the linguistic deficit of left-handers is specific to oral naming and that it is not caused by deviant interhemispheric cooperation.

Birth stress, autoimmune diseases, and handedness

A large nonclinical sample of left-handed and right-handed subjects completed laterality and medical health questionnaires. The group of left-handed subjects showed a raised incidence of birth complications and learning disorders whereas no differences were found for birth order and maternal age. Left-handed and right-handed subjects showed similar incidences of autoimmune diseases, allergies, migraine, and stuttering. These results suggest that an association between early pathology and left-handedness may be found in a subset of the nonclinical left-handed population. The underlying pathological influence seems to manifest itself in pregnancy and birth complications rather than in immune disorders.

Birth stress and lateral preferences

Conflicting evidence exists concerning the possible role of birth stress in the etiology of left-sided lateral preferences. In order to clarify this issue, associations among lateral preferences of hand, eye, and foot and eight indices of prenatal and perinatal stress were examined in the present study on a sample of 987 boys and girls who participated in the Philadelphia Collaborative Perinatal Project. Controls were instituted for some of the methodological and measurement problems encountered in past birth stress and laterality research. Results showed that subjects with different lateral preferences did not differ significantly in their distributions of all but one birth stress items. Hence, there was no substantial evidence for a link between birth stress and left-sided preferences. Alternative hypotheses for the etiology of left-sidedness should therefore be explored.

Birth order and lateral preferences

Associations between birth order and lateral preferences of hand, eye and foot were examined in a sample of 6436 black seven year old boys and girls whose mothers participated in the Collaborative Perinatal Project (CPP) in Philadelphia. Overall, most of the subjects (87%) showed right hand preference, and the majority of subjects showed right eye (55%) and foot (63%) preferences. Analysis of cross preferences indicated some tendency for a consistent right side orientation. However, patterns of lateral preferences were similar for both boys and girls across seven birth order groups. The data were interpreted as showing that birth order and lateral preferences are not interrelated.

Pathological left-handedness and ambiguous handedness: a new explanatory model

A new model is presented which attempts to explain three pathological handedness phenotypes: pathological left-handedness (PLH), pathological right-handedness (PRH) and ambiguous handedness (AH). Revisions in the original model of PLH were prompted in order to account for reports of a raised incidence of both manifest left-handedness and AH in more severely retarded and/or brain-injured populations--namely, autistic. The new model generates specific predictions on the likelihood of different etiological subgroups within these disorders.

Birth order, maternal age, season of birth, and handedness

The handedness of 1,186 children about to enter kindergarten was tested. The sample represented approximately 98 percent of all children of this age in a school system during 8 years. Handedness was examined in relation to birth order, maternal age at birth, and sex, for 1,097 subjects (92.5%), and in relation to season of birth for the total sample. The finding that there were significantly more nonright-handed boys for birth orders one and four-or-later, than for two and three, provides support for Bakan (1971). There was a nonsignificant increase in nonright-handedness for boys born to the youngest (less than 20) and oldest (less than 30) mothers. A significant seasonal effect for the birth of nonright-handed boys was observed. For each of the fall and winter months (September - February) the proportion of nonright-handed male births was higher than that for any of the spring and summer months. For girls, no significant effects on handedness were observed, for birth order, maternal age, or season of birth.

Pathologic left-handedness: does it exist?

The concept of pathologic left-handedness is reviewed, both from a historical and an empirical point of view. It is suggested that there is no adequate evidence to justify its continued use. The fact that the concept is still much used may be the result of a desire to restore to the brain its lost symmetry, by allowing Dax's Law once more to be true.

Incidence of left-handed writing in a college population with reference to family patterns of hand preference

Hand preference data were obtained for 1816 university students, 4793 siblings and 3632 parents. Results support the following conclusions. (1) There is currently a 13.8% incidence of left handedness among young adults, representing a dramatic increase over past generations. (2) Left and right-handed respondents do not differ in terms of familial sinistrality. (3) Mother's left-handedness is associated with an increase in the incidence of sinistrality for sons and daughters, while father's left handedness is related only to sons.

Perinatal factors in hand and eye preference: data from the Collaborative Perinatal Project

Data from 5,899 girls and boys who participated in the NIH Collaborative Perinatal Project were analyzed for relationships between birth order and birth stress and hand and eye preference as measured at age seven. None of the birth variables were significantly related to hand preference at the 0.05 level for either boys or girls. However, left eye preference was related to birth stress in boys. When both sexes were combined, left eye preference was also associated with low and high birthweight, and also with right lateral position of the head in the maternal pelvis.

The relationship between birth stress and writing hand posture

Variations in handwriting posture (inverted versus noninverted) were measured in 1203 individuals for whom birth histories, based upon retrospective maternal reports, were available. A history of birth complications was associated with an increased incidence of inverted handwriting posture. Effects interacted with sex and hand preference; the largest difference in handwriting posture between the birth-stressed and non-birth-stressed groups occurred in left-handers and in males.

Handedness and birth stress

The relationship between left-handedness and birth complications was studied. No evidence of any association was found in either 2 retrospective studies, or 1 large prospective study.

Not all sinistrality is pathological

Information regarding age, sex, birth stress, and handedness was obtained form 762 university faculty members and 1869 undergraduate and medical students, who also provided age, sex, and handedness information for their first-degree relatives. In addition, students reported the occupations and educational levels of their parents. Analyses of the effects of birth order, reported birth stress, and maternal, paternal, and joint parental age showed that an increased incidence of sinistrality was only rarely associated with high birth risk; in all cases, the effects were confined to male subjects, most frequently male faculty members. Sinistrality was not associated with low socioeconomic status; on the contrary, there were significantly more sinistrals among parents of high than of low educational and occupational levels. The pathogenic hypothesis has other implications which fail to find support in the current literature, thus casting further doubt on the proposition that all sinistrality is pathological in origin.

Left handedness, birth order and birth stress

The handedness of 942 subjects (305 tertiary students, 591 of their siblings and 46 of their children) was ascertained by a 14-item questionnaire. The mothers of the subjects supplied information about maternal age at birth, birth weight and the presence or absence of twelve conditions likely to be associated with birth stress for each subject. No increase in left handedness was found among fourth or later born children. A significant decrease occurred in first-borns of both sexes, although these had more stressful births than the other subjects. No relationship between maternal age, birth weight or reported birth stress and left handedness was found. Thus the hypothesis that birth stress is a major cause of left handedness in normal subjects was not supported. Acknowledgements. We are particularly grateful to the students from the Institute for Early Childhood Development and their families, who provided the data for this study. We also wish to thank Dr John L. Bradshaw for his very constructive comments during the preparation of this paper.

Handedness, clumsiness and cognitive ability

An unselected group of 170 children aged between eight and nine years was given tests of intelligence, reading ability and manual dexterity. A 'target' group of children was selected on the basis of very poor performance with the non-preferred hand. Using an extension of Satz's pathological left-handedness model, it is argued that there should be a higher proportion of left-handers in the target group than in the remainder of the sample: this prediction was confirmed. It was also predicted that the target group should have a higher incidence of neurological disorder, and that left-handers in the target group should have a lower incidence of familial sinistrality than other left-handers. Partial confirmation of these predictions was obtained. The target group was impaired on cognitive tasks. It is argued that skill with the non-preferred hand may be more meaningful than direction of hand preference as a basis for understanding relationships between handedness and ability.

The handedness distributions of gifted and non-gifted children

The handedness distribution of a group of 578 gifted elementary school children (I.Q. = 132) was compared to handedness distribution of 391 non-gifted children (I.Q. less than 132), with the result that the gifted group was significantly less right-handed than their non-gifted peers. These data were discussed relative to Bakan's hypothesis that left-handedness results from stressful prenatal and birth conditions. In addition attention was paid to the possibility of a non-linear relationship between intelligence and degree of right-handedness.