Season of birth, Geschwind and Galaburda hypothesis, and handedness

Abstract

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.

Keywords:

• Handedness
• Season of birth
• Geschwind and Galaburda hypothesis
• Testosterone

Acknowledgements

The authors would like to thank Dr George B. Karev for his valuable comments on an earlier version of this text.

Notes

¹Some of the original questions were modified: for example squash is not a popular game in Bulgaria while badminton is known as federball. Other questions were rephrased in a way that their answers can give information about activities of the non-dominant hand (for example, “in which hand do you: hold the nail when you hammer it; hold the box of matches when striking a match; hold the pack of cards when dealing; hold the needle when threading it). These questions were rearranged in the questionnaire in a way that should avoid automatisation of the answers (see Barnsley & Rabinovitch, 1970). Items that were ambiguous (such as "top hand on broom") were changed with more understandable ones (“which hand do you use when sweeping the floor”). Test–retest procedure and ? comparison with behavioural tests confirmed the reliability and the validity of the adapted questionnaire.

²Post-hoc analysis showed that sex had a significant main effect on handedness (Wald: χ²=18.39, df=1, p<.001): the incidence of non-right-handedness is higher in males.