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Seasonal trends in the birth of sinistrals.

This study examined the proposition that hand preference may change with season of birth in a group of 523 students born in southern Australia. Hand preference and performance measures revealed a higher incidence of sinistrality or weak dextrality for individuals born in winter and autumn compared to summer and spring. These seasonal effects tended to be more pronounced for females compared to males. Similar seasonal patterns have been observed in the northern hemisphere. Previous research has accounted for seasonal changes in hand preference in terms of changes in the frequency of viral infections. It is proposed that seasonal variations in hand preference can also be accounted for by seasonal variations in hormone levels. Higher levels of testosterone in spring compared to autumn have been reported in females. Exposure to higher levels of testosterone during the first trimester of foetal development is thought to promote the development of the right hemisphere. Individuals born in winter would have been exposed to the high testosterone levels associated with spring during the first critical trimester of foetal brain development. The lack of seasonal effect observed for males may be related to endocrinological differences between the male and female foetus.