Correlation between handedness and birth order: compilation of five studies

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.

Handedness in diplegic cerebral palsy

OBJECTIVE

To determine if the frequency of left-handedness is high in children with spastic cerebral diplegia.

DESIGN

Case-control study.

METHODS

One hundred and eleven children with spastic diplegic cerebral palsy and 444 age- and gender-matched controls were studied. The handedness of each child was assigned on the basis of responses to questions on the hand preference for writing and drawing, feeding and throwing a ball. The data were analysed by conditional logistic regression and computing the odds ratio and 95% confidence intervals for left handedness.

RESULTS

Of the 111 children with spastic diplegic cerebral palsy, 45 were left-handed, while 13 of 444 normal children were left-handed. The odds ratio for left-handedness in children with diplegic cerebral palsy as compared to normal children was 27.33 (95% CI = 11.63, 64.25).

CONCLUSION

The study shows that left-handedness is very frequently encountered in children with spastic diplegic cerebral palsy.

Birth order and left-handedness revisited: some recent findings in chimpanzees (Pan troglodytes) and their implications for developmental and evolutionary models of human handedness

This paper examines the relationship between parity, pregnancy outcome, and handedness in a sample of captive chimpanzees (Pan troglodytes). The relation between parity, maternal age and positive or negative pregnancy outcome was assessed from life history data for 536 chimpanzees housed at the Yerkes Regional Primate Research Center. The incidences of negative pregnancy outcome (notably spontaneous abortions and stillbirths) were significantly higher in parities of 8 or higher compared to all other parities. In a sub-sample of 165 chimpanzees, the relation between parity, maternal age and handedness was assessed to determine whether left handedness may serve as a marker of prenatal pathology. These analyses indicated that left-handedness was more prevalent in 1st and 8 or higher parities compared to parities between 2 and 7, respectively. Possible prenatal hormonal and periparturitional factors are discussed as possible mechanisms for the observed findings.

Birth order and hand preference in chimpanzees (Pan troglodytes): implications for pathological models of handedness in humans

The effect of birth order on hand preference was assessed in a sample of 154 captive-born chimpanzees. Subjects were classified as first, middle, or latter born using 2 classification criteria based on their birth order. Hand preference was measured using a task that elicited coordinated bimanual actions. Significant birth-order effects were found for both classification criteria, with first- and latter-born subjects exhibiting a lesser degree of right-handedness compared with middle-born subjects. These data suggest that biological rather than sociological factors play a greater role in explaining the observed birth-order effects on hand preference in humans.

Left-handedness in twins: genes or environment?

Twin family data can cast light on the longstanding problem about the influences of genes and environment on the etiology of left-handedness. Therefore, hand preference was assessed in 1700 adolescent twin pairs and their parents. Left-handedness (LH) appeared not significantly enhanced among twins compared to the general population. In addition the following observations were made: (1) Significant more LH in first born twins than in second born twins. (2) Significant higher left-handedness association in MZmm pairs compared to DZmm pairs and not or may be marginally so in MZff versus DZff pairs. These results, combined with the observations that (a) left-handed fathers increase the probability of LH in sons but not in daughters; (b) LH in mothers increases LH prevalence in both sons and daughters to the same degree; and (c) very low birth weight, corrected for the effect of gestational age, increases LH prevalence in first born twins only, make an environmental explanation more likely. The possibility that exposure to prenatal male hormones - to which low birth weight and high birth stress children are more vulnerable - might be a crucial condition for the etiology of LH, is discussed.

Does motor cerebral dominance develop secondary to sensory dominance?

Current research and theoretical frameworks for understanding motor dominance assume that motor dominance is primary. Various developmental clues, however, suggest that the maturation of proprioceptive sensory processing predates that of motor control. We hypothesized that the observed adult pattern of lateralized motor dominance may develop as a consequence of preexisting lateralized "sensory" or "proprioceptive dominance." To test whether motor preference could develop in response to sensory dominance, we investigated sighting eye dominance, eye lid winking, and handedness in 164 individuals. Subjects winked the nondominant eye significantly more frequently than the dominant eye and independently of their handedness, lending partial support to the idea that motor function can develop in response to sensory function. Specific investigation of the development of the proprioceptive system would be needed to evaluate whether lateralization of motor handedness develops as a consequence of "proprioceptive" lateralization.

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.

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.

Handedness in two generations

A 14-item questionnaire was administered to tertiary students and their families. The responses of the 508 subjects in the parental generation were compared with those of the 917 younger subjects. For every item significantly fewer older subjects gave left responses. The questionnaire identified more left-handers among subjects than writing hand did, but as only 5.9% of these were classed as left-handed, compared with 11.8% of the younger generation, it seems unlikely that ascertainment was complete in the parental group. Using data from an additional 69 non-right-handers, item analyses indicated that writing and drawing were unsuitable items for older subjects and that, of the items tested, toothbrush and hammer use and pouring would be the best indicators of left-handedness.

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.

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.

Multiple birth-risk factors and handedness in elementary school children

The relationships between a set of birth-risk factors, i.e., sex, parity, and maternal age, and handedness were computed for 600 elementary school children. None of these birth-risk variables (or relevant combinations of them) were significantly related to handedness. These data further question the validity of the hypothesis that left-handedness results from stressful prenatal and birth conditions.

Birth factors and laterality: effects of birth order, parental age, and birth stress on four indices of lateral preference

As an alternative to genetic theories of handedness, some theorists have offered an environmental mechanism, associated with birth stress, for the appearance of left-handedness. They suggest that brain damage as a result of birth difficulties can lead to a switch in hand preference from the right side to the left side. Consequently, one should find more left-handers in groups where the probability of the occurrence of birth stress is greater. Three studies are presented which explore the laterality of not only hand but also foot, eye, and ear, in a total of 5161 individuals, in an attempt to assess any relationship to birth stress. Maternal age seems to predict deviations from dextrality, dependent on the sex of the offspring, while paternal age and birth order do not. The use of a direct measure of conditions predisposing toward birth stress suggests that these results depend on prenatal or perinatal environmental trauma rather than chromosomal factors.