Season of birth and handedness in Serbian high school students

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.

Asymmetry of the Brain: Development and Implications

Although the left and right hemispheres of our brains develop with a high degree of symmetry at both the anatomical and functional levels, it has become clear that subtle structural differences exist between the two sides and that each is dominant in processing specific cognitive tasks. As the result of evolutionary conservation or convergence, lateralization of the brain is found in both vertebrates and invertebrates, suggesting that it provides significant fitness for animal life. This widespread feature of hemispheric specialization has allowed the emergence of model systems to study its development and, in some cases, to link anatomical asymmetries to brain function and behavior. Here, we present some of what is known about brain asymmetry in humans and model organisms as well as what is known about the impact of environmental and genetic factors on brain asymmetry development. We specifically highlight the progress made in understanding the development of epithalamic asymmetries in zebrafish and how this model provides an exciting opportunity to address brain asymmetry at different levels of complexity.

Handedness is modulated by sex and self-perception

The distribution of handedness is different for men and women. Less pronounced right- or left-handedness in men is well established and often attributed to direct effects of genetic factors. Many studies observing this sex difference assessed handedness via questionnaire. It may therefore be influenced by a sex-specific bias in self-perception. Permanent inadequate self-perceptions are key characteristics of hypochondriasis. The study therefore tested 1017 participants (614 females) on two standard questionnaires to assess handedness and hypochondriasis: the Edinburgh Inventory and the Whiteley Index. Effects of sex, hypochondriac traits and the direction of handedness (left-handers; right-handers) on the degree of handedness (DH; the strength of lateralisation) were tested with multivariate linear regression. In confirmation of previous results, the DH is lower (less pronounced) in right-handed males than right-handed females, but is similar across sexes in left-handers. Regression analyses showed that for right-handers, male sex and higher hypochondriac traits are independent predictors of lower DH. For left-handers, main effects of sex and hypochondriac traits are not significant whereas a significant interaction of these two factors indicates that in left-handed men higher hypochondriac traits are associated with a differentially stronger shift towards lower DH compared to left-handed women. In conclusion, the DH is modulated by sex-specific effects of self-perception for left-handers but not right-handers. The implications of this finding on current theories of the inheritance of handedness are discussed. The assessment of hypochondriac traits might be useful to control response bias in questionnaire-based studies of human handedness.

[Social aspects of left-handedness]

Throughout human history left-handedness has been considered as sinful. It has been associated with the devil, weakness, female gender, unhealthiness, evil, something that has to be turned to a "good"--right side by force. Left-handedness is being more and more acceptable at rational level, but in everyday life it is still considered to be unusual if someone writes with the left hand. Lessening of the number of lefthanders is associated with ageing. There are about 13% lefthanders among people in twenties and less than 1% lefthanders among those in eighties. This finding may be explaned with more pronounced socio-cultural pressure on left-handed people in the past, compared to nowadays. On the other hand, this may also support the hypothesis about a reduced life span of lefthanded people. With cross-exercising of left-handedness, certain typical characteristics and behavioural patterns appear in these people. This was a sort of provoked behaviour and an attack on the integrity of an emotional attitude toward oneself. Stuttering may also appear as a consequence of unsuccessful cross-exercising of left-handedness. The hypothesis about left-handedness as an advantage is supported with the reports about relatively more lefthanders in some specific groups such as: mathematicians, sculptors, architects, painters, musicians, actors, tennis players, as well as famous army commanders and rulers.