Functional dominance of the right hand is the norm across different populations, various geographical regions, and diverse cultures, with approximately 90% of humans exhibiting clear dominance of the right side of the body. This behavioural characteristic is considered as uniquely human, as there is no other species that displays such a large behavioural asymmetry at the population level. It is also widely accepted that this behavioural feature emerged at some point during the hominid evolution, and that this feature preceded the evolution of another uniquely human feature – language, and in particular, speech as its central component 1.

The transmission of handedness over many generations of humans is widely believed to be under genetic control 234, rather than resulting from learning. Converging lines of evidence provide support for the genetic hypothesis, including imaging studies on twins 5, meta-analysis of handedness in twins 6, and molecular genetic studies 78. To date, however, no gene for handedness has been identified. Genetic models of handedness 23 argue that the functional advantage of the right hand originates from a purely genetic effect, while left-handedness is a consequence of a random shift in hand dominance. Theory suggests that in individuals without the genetic disposition, both cerebral and hand dominance are randomly assigned. An implication of these models is that left-sided behavioural dominance is a benign genetic consequence, but not a pathology acquired during early brain development. Moreover, these models also argue that left-handedness may be beneficial. For example, there is empirical support for the notion that left-handers are somewhat better in visuospatial and visuomotor abilities than right-handers This may explain why left-handers are overrepresented in some groups with high demand on spatial skills such as architects 9, tennis players and cricketers 10, and musicians 11. In contrast to purely genetic models, the shift away from the "default" right-handedness has occasionally been labelled as "anomalous" (e.g. 12), "alinormal" (e.g. 13), or "atypical" (e.g. 14). The increased prevalence of left-handedness in populations with some medical conditions (e.g. Rett syndrome, schizophrenia, autism) is believed to originate from pathological processes that either overpower or disrupt the genetics of hand dominance.

The environmental factors believed to provide structural brain substrate for left-handedness include birth difficulties 15, prenatal ultrasound 16, maternal smoking during pregnancy 17, low birth weight 1819, diffuse brain damage 20, and testosterone level during early development 12. Another factor that has occasionally been considered as "trigger" for atypical lateralisation of hand preferences is season of birth. That season of birth can be a serious risk factor has already been established for various conditions, including brain tumours 21, proneness to road accidents 22, and schizophrenia 23. How season of birth may exert an influence on cerebral lateralisation is less clear. Season of birth may be conceptualised as a portmanteau term covering various environmental variables such as prenatal exposure to various hormones (e.g. testosterone), incidence of diseases, nutrition, and reproductive activity in humans. For obvious reasons, direct investigations between these factors and behavioural lateralisation are not always feasible. It has therefore been hypothesised that variation in the incidence of viral infections 2425 and prenatal exposure to testosterone 12 may be responsible for sinistral developmental trajectory. Similar to all other environmental factors responsible for left-handedness, the empirical evidence for the season of birth effect is indirect, speculative and, at best, mixed. In this paper we describe the effect of season of birth on atypical lateralisation of hand preferences in a large sample of students all born in the same year.

Neither the right-handed nor the non-right-handed groups showed a birth asymmetry relative to the normal population birth distribution. Furthermore, there was no between-groups difference in the seasonal distribution of birth comparing the right-handed to the non-right-handed groups. Figure 1a–c shows the cumulative proportional difference in the two sub-samples compared with the general population (a, b) and with each other (c).

The results of the present study do not support an association between the distribution of handedness and season of birth in young people.

It is worth noting that all of our analyses are based on a sample of subjects who were all born in a single year and compared with whole of population birth distribution for the same year. As far as we are aware, this is the first study to use this procedure within a variable population at risk model. Most season of birth studies create a composite reference population, summing the different normal population yearly birth distributions for the age range of the dependent group 33. In such a situation it can be difficult to definitively conclude that any seasonal difference between two composite birth distributions is related to the dependent sample, and is not merely an artefact associated with the "constructed" reference population. Furthermore, the sample comprised a randomly selected subset of all school-age children from Belgrade. Again, this approach has not been used previously to study season of birth effects in handedness. Others, for well-understood reasons, tend to use convenience samples. The effect such methodological differences may have has not been systematically studied in the seasonality literature, however, the advantages of random sampling, per se, have been well described (e.g. 34).

Although there is some evidence for a season of birth effect on human handedness, the empirical data are inconclusive as there are studies showing contradictory results. For example, several published studies 24253536 have suggested that distribution of birth is different in left-handers than in right-handers, whereas some studies reported a gender specific association (e.g. 2537). By contrast, quite a few studies 383940, including some that have re-examined previously published results (e.g. 41), failed to confirm the pathogenic effect of season of birth on atypical hand dominance. The lack of empirical consistency of findings makes this factor, which potentially may explain a certain proportion of variation in human handedness, remain obscure.

In conclusion, we found no evidence that season of birth possess an aetiological relevance for developing atypical lateralisation of hand preferences.

The author(s) declare that they have no competing interests.

The authors all contributed equally to the manuscript, were all involved in the drafting of the manuscript and gave final approval on the manuscript.