Handedness and season of birth: a gender-invariant relation

A longitudinal examination of perinatal testosterone, estradiol and vitamin D as predictors of handedness outcomes in childhood and adolescence

The developmental origins of handedness remain elusive, though very early emergence suggests individual differences manifesting in utero could play an important role. Prenatal testosterone and Vitamin D exposure are considered, yet findings and interpretations remain equivocal. We examined n = 767 offspring from a population-based pregnancy cohort (The Raine Study) for whom early biological data and childhood/adolescent handedness data were available. We tested whether 18-week maternal circulatory Vitamin D (25[OH]D), and testosterone and estradiol from umbilical cord blood sampled at birth predicted variance in direction of hand preference (right/left), along with right- and left-hand speed, and the strength and direction of relative hand skill as measured by a finger-tapping task completed at 10 (Y10) and/or 16 (Y16) years. Although higher concentrations of Vitamin D predicted more leftward and less lateralized (regardless of direction) relative hand skill profiles, taken as a whole, statistically significant findings typically did not replicate across time-point (Y10/Y16) or sex (male/female) and were rarely detected across different (bivariate/multivariate) levels of analysis. Considering the number of statistical tests and generally inconsistent findings, our results suggest that perinatal testosterone and estradiol contribute minimally, if at all, to subsequent variance in handedness. Vitamin D, however, may be of interest in future 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.

Brighter children? The association between seasonality of birth and child IQ in a population-based birth cohort

OBJECTIVE

Season of birth has repeatedly been found to be a risk indicator for adverse neurodevelopmental outcomes. Several explanations for this finding have been put forward but no conclusion has been reached. In the current study, we explored the role of sociodemographic and biological factors in the association between season of birth and child IQ.

DESIGN

In a prenatally recruited birth cohort (born in 2002-2006), we examined the association between season of birth and non-verbal IQ at age 6 years among 6034 children. We explored how adjusting for socioeconomic status and maternal IQ, childbirth outcomes, pregnancy vitamin D status, nutritional intake, exposure to infections, and child age relative to peers in class changed the relation between season of birth and child IQ.

RESULTS

We found that spring birth was associated with lower non-verbal IQ (estimate: more than 1 point; β-1.24 (95% CI -2.31 to -0.17), p=0.02; seasonal trend β-0.40 (95% CI -0.74 to -0.07), p=0.02) than birth in summer. Adjustment for different covariates led to a substantial reduction (-65.0% change, in a seasonal trend analysis) of this association. In particular, sociodemographic factors and maternal IQ (-10.0% and -22.5% change, respectively) contributed.

CONCLUSIONS

Season of birth is an indicator of many underlying factors related to child IQ. The observed effects on IQ were small and therefore not of clinical significance.

Season of Birth and Later Outcomes: Old Questions, New Answers

Season of birth is associated with later outcomes; what drives this association remains unclear. We consider a new explanation: variation in maternal characteristics. We document large changes in maternal characteristics for births throughout the year; winter births are disproportionally realized by teenagers and the unmarried. Family background controls explain nearly half of season-of-birth's relation to adult outcomes. Seasonality in maternal characteristics is driven by women trying to conceive; we find no seasonality among unwanted births. Prior seasonality-in-fertility research focuses on conditions at conception; here expected conditions at birth drive variation in maternal characteristics while conditions at conception are unimportant.

Female gender but not season of birth is associated with mood seasonality in a near-equatorial Brazilian city

OBJECTIVE: Studies conducted mainly in countries located in the Northern Hemisphere have shown that season of birth influences mood seasonality. Greater mood seasonality has been observed for individuals born during spring/summer months than those born during autumn/winter months. Expanding past research to the Southern Hemisphere, in this study we examine the influence of season of birth on mood seasonality in a sample of 1,247 healthy young Brazilians. METHOD: The Seasonal Pattern Assessment Questionnaire was used to compute a global seasonality score as a measure of mood seasonality in a cross-sectional study. RESULTS: Analysis of covariance was conducted to examine the effects of month of birth and gender on mood seasonality, with age entered as a covariate. A main effect of gender was observed, F (1, 1197) = 17.86, p < .01; partial Eta-squared = .02, with mood seasonality being higher for females (M = 8) than for males (M = 7). Contradicting previous findings, no significant main effect for month of birth was observed, F (1, 1197) = 0.65, p > .05. CONCLUSION: The unexpected finding is tentatively explained by differences in geographic location and weather fluctuations between the sampling location in Brazil and other countries where season of birth has been found to influence mood seasonality. Additional studies with larger samples from the Southern Hemisphere are necessary to shed additional light on the possible significant influence of season of birth on mood.

Season of birth and handedness in young adults

This study explored the relationship between season of birth and handedness, taking into account gender and nationality variables. Young adults from three countries (2120 females and 1353 males; 877 Spanish, 2184 Italian, and 412 French), aged from 18 to 30 years, filled the Edinburgh Handedness Inventory (EHI). According to the literature, the percentage of left-handed participants in the whole sample was 9.30%. The distribution of left- and right-handed participants was not significantly different among the seasons of birth. The present data do not support a significant season of birth effect on handedness in young adults.

Association between affective temperaments and season of birth in a general student population

BACKGROUND

Several studies indicate a significant association between birth season and personality and neuropsychiatric disorders. The aim of our present study was to investigate the association between affective temperaments and season of birth in a nonclinical sample.

METHODS

366 university students completed the standardized Hungarian version of the Temperament Evaluation of Memphis, Pisa, Paris and San Diego-Auto-questionnaire (TEMPS-A). Ordinary Least Squares regression was applied to explain the relationship between TEMPS-A subscale and birth season of the respondents.

RESULTS

We found a significant association between temperament scores and birth season in the case of the Hyperthymic, Cyclothymic, Irritable and Depressive temperaments, while no significant results emerged for the Anxious temperament.

LIMITATIONS

The relatively small sample size, especially in the case of seasonal and monthly subsamples limits generalization of our results.

CONCLUSIONS

Our results support the evidence that there is a strong association between season of birth and personality, extending the results to affective temperaments as well. Furthermore, our results are in line with clinical observations concerning the seasonal variation of onset and hospitalization due to affective episodes. This is especially important, since affective temperaments are conceived as the subaffective and subclinical manifestations of major and minor affective disorders indicating a risk for the development of these disorders and also exerting a possible pathoplastic effect, thus our results also have clinical significance.

Season of birth, Geschwind and Galaburda hypothesis, and handedness

Geschwind and Galaburda (1985a, 1985b) suggested that the season of conception is a non-genetic random variable that may affect laterality and handedness, probably due to seasonal variations in the hormonal influences on the foetal brain. According to this suggestion it is logical to expect seasonal anisotropy in the incidence of birth of right- and non-right-handers. The present study analysed data obtained from a sample of 3182 young Bulgarians: 2825 right-handers and 357 non-right-handers. Significant difference in the seasonal distribution of births of right- and non-right-handers was found in males only. The incidence of non-right-handedness was highest among participants born in winter. In right-handers increased birth rate was observed in spring and summer. The results in males seem to support the Geschwind and Galaburda suggestion. One can speculate that the high levels of mother's androgens during spring and early summer play the role of a "left shift" factor for the male foetuses with genetic potential for non-right-handedness.

Season of birth and risk for adult onset glioma

Adult onset glioma is a rare cancer which occurs more frequently in Caucasians than African Americans, and in men than women. The etiology of this disease is largely unknown. Exposure to ionizing radiation is the only well established environmental risk factor, and this factor explains only a small percentage of cases. Several recent studies have reported an association between season of birth and glioma risk. This paper reviews the plausibility of evidence focusing on the seasonal interrelation of farming, allergies, viruses, vitamin D, diet, birth weight, and handedness. To date, a convincing explanation for the occurrence of adult gliomas decades after a seasonal exposure at birth remains elusive.

Origins of handedness: a nationwide study of 30,161 adults

The origins of human handedness remain unknown. Genetic theories of handedness have received much attention, but some twin studies suggest modest, perhaps negligible genetic effects on handedness. A related question concerning handedness is whether twins have higher rates of left-handedness than do singletons. We studied handedness, with information on forced right-handedness, in a sample of 30,161 subjects aged 18-69 from a questionnaire survey of the older Finnish Twin Cohort. Left-handedness was found to be more common in twins (8.1%) and triplets (7.1%) than in singletons (5.8%), whereas ambidextrousness was more common in triplets (6.4%) than in twins (3.4%) and singletons (3.5%). As in many other studies, males were more likely to be left-handed. Ambidextrous subjects were more likely to become right-handed writers even if not forced to use their right hand. We fit maximum likelihood models to our twin data to estimate the contribution of additive genetic, common environment and unique environmental effects to hand preference. Results, depending on the model, indicate that unique environmental effects account for most observed variance in handedness, both in childhood (92-100%) and adulthood (74-86%). When forced right-handedness was taken into account, estimates of familial effects increased. Concordance for left-handedness in twins is rare, and accordingly, very large samples are needed to detect the familial effects. Our results show that forced-handedness can have an effect on estimates of genetic effects.

Season of birth and handedness in Serbian high school students

BACKGROUND

Although behavioural dominance of the right hand in humans is likely to be under genetic control, departures from this population norm, i.e. left- or non-right-handedness, are believed to be influenced by environmental factors. Among many such environmental factors including, for example, low birth weight, testosterone level, and maternal age at birth, season of birth has occasionally been investigated. The overall empirical evidence for the season of birth effect is mixed.

METHODS

We have investigated the effect of season of birth in an epidemiologically robust sample of randomly selected young people (n = 977), all born in the same year. A Kolmogorov-Smirnov type statistical test was used to determine season of birth.

RESULTS

Neither the right-handed nor the non-right-handed groups demonstrated birth asymmetry relative to the normal population birth distribution. There was no between-group difference in the seasonal distribution of birth when comparing the right-handed to the non-right-handed groups.

CONCLUSION

The present study failed to provide support for a season of birth effect on atypical lateralisation of handedness in humans.

Season of birth and dopamine receptor gene associations with impulsivity, sensation seeking and reproductive behaviors

Background

Season of birth (SOB) has been associated with many physiological and psychological traits including novelty seeking and sensation seeking. Similar traits have been associated with genetic polymorphisms in the dopamine system. SOB and dopamine receptor genetic polymorphisms may independently and interactively influence similar behaviors through their common effects on the dopaminergic system.

Methodology/Principal Findings

Based on a sample of 195 subjects, we examined whether SOB was associated with impulsivity, sensation seeking and reproductive behaviors. Additionally we examined potential interactions of dopamine receptor genes with SOB for the same set of traits. Phenotypes were evaluated using the Sociosexual Orientation Inventory, the Barratt Impulsivity Scale, the Eysenck Impulsivity Questionnaire, the Sensation Seeking Scale, and the Delay Discounting Task. Subjects were also asked about their age at first sex as well as their desired age at the birth of their first child. The dopamine gene polymorphisms examined were Dopamine Receptor D2 (DRD2) TaqI A and D4 (DRD4) 48 bp VNTR. Primary analyses included factorial gender×SOB ANOVAs or binary logistic regression models for each dependent trait. Secondary analysis extended the factorial models by also including DRD2 and DRD4 genotypes as independent variables. Winter-born males were more sensation seeking than non-winter born males. In factorial models including both genotype and season of birth as variables, two previously unobserved effects were discovered: (1) a SOB×DRD4 interaction effect on venturesomeness and (2) a DRD2×DRD4 interaction effect on sensation seeking.

Conclusion

These results are consistent with past findings that SOB is related to sensation seeking. Additionally, these results provide tentative support for the hypothesis that SOB modifies the behavioral expression of dopaminergic genetic polymorphism. These findings suggest that SOB should be included in future studies of risky behaviors and behavioral genetic studies of the dopamine system.

Season of birth modulates mood seasonality in humans

We investigate whether mood seasonality is modulated by season of birth. A sample of 1682 university students were administered the Seasonal Pattern Assessment Questionnaire. We found that subjects born during spring or summer months had significantly higher Global Seasonality scores than those born during autumn or winter months.

Relation of handedness with season of birth of professional baseball players revisited

We reexamined the relationship between month and season of birth and handedness in male professional baseball players listed in Lahman's Baseball Archive Vol. 4.5 database. The sample of 8,016 individuals (left-handed = 1,658; right-handed = 6,358) played major league baseball between 1900-2001. A slightly higher, but statistically significant, percentage of left-handed players were born in June compared to other months, and in the winter-spring months, but we were unable to corroborate Rogerson's spring and early summer categorization of handedness as related to birth season. The data did not support the hypothesis that the prevalence of left-handedness is related to season of birth.