The finger length ratio (2D:4D) is a biomarker for the prenatal levels of testosterone. The prenatal effects of testosterone on the development of brain hemispheres are considered as a key factor in the etiology of left-handedness. We tested whether 2D:4D correlates with the type of handedness in males (18 right-handers and 18 left-handers). Right hand 2D:4D, left hand 2D:4D, average 2D:4D, and the difference between right and left 2D:4D (D(r-l)) were compared. The value of D(r-l) in left-handers was significantly lower than D(r-l) in right-handers. This result can be interpreted as proof that left-handers were subjected to higher levels of testosterone in utero.

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.

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A total of 1985 women aged between 55 and 65 were distributed into two groups (145 left-handers and 1840 right-handers). They were asked to complete a questionnaire on the appearance of menopause, duration of menopausal transition and age of menopause. In left-handed women a significantly earlier appearance of premenopause was established together with a shorter menopause transition and an earlier occurrence of menopause. These results give grounds for a correlation between handedness, functional brain asymmetry, respectively and the genetically determined fading away of ovary steroidogenesis associated with the appearance and progression of the climacterium. In light of the available literature we assume that progressive reduction in ovarian function during climacterium is coupled with possible specific functioning of the hypothalamo-pituitary-gonadal axis, dependent on the type of hemispheric asymmetry.

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