Latent variable analysis indicates that seasonal anisotropy accounts for the higher prevalence of left-handedness in men

Abstract

According to the Geschwind-Galaburda theory of cerebral lateralization, high intrauterine testosterone levels delay left brain hemisphere maturation and thus promote left-handedness. Human circulating testosterone levels are higher in the male fetus and also vary with length of photoperiod. Therefore, a higher prevalence of left-handedness, coupled with seasonal anisotropy (i.e., a non-uniform distribution of handedness across birth months or seasons), may be expected among men. Prior studies yielded inconsistent evidence for seasonal anisotropy and suffered from confounding and a number of shortcomings affecting statistical power. This study examined hand preference and associations of handedness with sex, age, and season of birth in independent discovery (n = 7658) and replication (n = 5062) samples from Central Europe with latent class analysis (LCA). We found clear evidence of a surplus of left-handed men born during the period November-January, which is consistent with predictions from the Geschwind-Galaburda theory. Moreover, seasonal anisotropy fully accounted for the higher prevalence of left-handedness among men, relative to women. Implications of these findings with regard to seasonal anisotropy research and handedness assessment and classification are discussed.

Asymmetry of the Brain: Development and Implications

“…Although not confirmed by other studies (85,114,159), independent reports in humans found significantly more LHs among the males born in winter (152,160). One hypothesis is that GE49CH26-Roussigne ARI 27 September 2015 14:54 seasonal variation in the mother's hormones, such as high levels of androgens during spring, could influence fetal development and contribute, together with genetic factors, to induce a left shift.…”

A large-scale population study of early life factors influencing left-handedness

“…In a number of studies, a stronger seasonal effect was found in males than in females (49,50). In other studies, more left-handers were found among children born in March-July (29,30,51), but in other studies in winter (49,50,52). In yet other studies, no effect of season was detected (31,(53)(54)(55).…”

“…In a number of studies, a stronger seasonal effect was found in males than in females (Badian, 1983;Stoyanov et al, 2011). In other studies, more lefthanders were found among children born in March-July Martin and Jones, 1999;Rogerson, 1994), but in other studies in winter (Badian, 1983;Stoyanov et al, 2011;Tran et al, 2014). In yet other studies, no effect of season was detected (Abel and Kruger, 2004;Cosenza and Mingoti, 1995;Karev, 2008;Tonetti et al, 2012).…”

Early- and Late-Light Embryonic Stimulation Modulates Similarly Chicks’ Ability to Filter out Distractors

“…There are also indications that light can affect the development of cerebral lateralization in human foetuses by modulating the available hormonal levels [54]. Despite assessing this hypothesis has proven to be very difficult, seasonal anisotropy has been recently shown with respect to the distribution of handedness as related to gender: longer photoperiods experienced during the first 14-18 weeks are associated with left-handed males [55]. Hence, at present, a hormonal modulation cannot be ruled out.…”