The reason for this may be lying in testosterone level changes in warmer months affecting a key part of the brain. A team of Austrian and German researchers have found that on a monthly average, 8.2 percent of left-handed men were born during the period of February to October. During November to January, this number rose to 10.5 percent.

"Presumably, the relative darkness during the period November to January is not directly connected to this birth seasonality of handedness. We assume that the relative brightness during the period from May to July, half a year before, is its distal cause," explained lead author Ulrich Tran from University of Vienna, Austria.

According to current findings, there is a small but robust and replicable effect of birth seasonality on handedness, affecting only men. These results are consistent with a hormonal basis of handedness, corroborating an old and controversial theory.

A theory, brought forth in 1980s by US neurologists Norman Geschwind and Albert Galaburda, said that testosterone delays the maturation of the left brain hemisphere during embryonic development. The left brain hemisphere is dominant among right-handers while the right brain hemisphere is dominant among left-handers.

Specifically, more daylight may increase testosterone levels, making a seasonality effect plausible, researchers noted. The study comprised two large and independent samples of nearly 13,000 adults from Austria and Germany.


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