Most of the mere populated left handed people, especially men, are born between the months October to February, finds study.
The study conducted by Ulrich Tran, Stefan Stieger, and Martin Voracek comprised of two large and independent samples of nearly 13000 adults from Austria and Germany and found that overall, 7.5 percent of women and 8.8 percent of men were left-handed.
Ulrich Tran, lead author of the study, said that this imbalance was caused since more left-handed men were born specifically during November, December, and January. On a monthly average, 8.2 percent of left-handed men were born during the period February to October, which rose to 10.5 percent during November to January.
The relative darkness during the period November to January may not be directly connected to this birth seasonality of handedness, and they assume that the relative brightness during the period May to July, half a year before, was its distal cause, added Ulrich Tran.
A theory, brought forth in the 1980's by US neurologists Norman Geschwind and Albert Galaburda, posits that testosterone delays the maturation of the left brain hemisphere during embryonic development. The left brain hemisphere is dominant among right-handers, the right brain hemisphere is dominant among left-handers. Intrauterine testosterone levels are higher in the male fetus, because of its own testosterone secretion, than in the female fetus. However, the testosterone level of the mother and external factors may also affect intrauterine testosterone levels. Specifically, more daylight may increase testosterone levels, making a seasonality effect plausible.
According to the findings, there was a small, but robust and replicable, effect of birth seasonality on handedness, which affected only men, however, the exact way of causation still needs to be investigated in future studies.