Why More Left-Handed Men are Born in the Winter

Are you left-handed or right-handed? Whether or not you favor one hand or the other may just depend on when you were born. Scientists have found indirect evidence of a hormonal mechanism that promotes left-handedness among men.

 Around 90 percent of the general population is right-handed, and most tools that are sold are optimized for people who work primarily with their right hands. Yet there's that 10 percent of the population that's left-handed. In order to better understand what mechanisms drive left-handedness, the researchers examined nearly 13,000 adults from Austria and Germany.

In this particular population, about 7.5 percent of women and 8.8 percent of men were left-handed. Yet what particularly surprised the researchers wasn't the left-handedness, but rather when the people were born.

"We were surprised to see this imbalance was caused by more left-handed men being born specifically during November, December and January," said Ulrich Tran, the lead author of the new study, in a news release. "On a monthly average, 8.2 percent of left-handed men were born during the period February to October. During November to January, this number rose to 10.5 percent."

So what was causing this imbalance? It might just have to do with the amount of sunlight a person receives in addition to hormones. A theory, brought forth in the 1980s, states that testosterone delays the maturation of the left brain hemisphere during embryonic development. The left brain is actually dominant among right-handers, while the right brain is dominant among left-handers. It's possible that more daylight could actually increase testosterone levels and make a seasonality effect plausible.

"Presumably, the relative darkness during the period November to January is not directly connected to this birth seasonality of handedness," said Tran. "We assume that the relative brightness during the period May to July, half a year before, is its distal cause."

The findings reveal that it's certainly possible that seasons could affect hormones and thus play a role in left and right-handedness. That said, more studies need to be done before any firm conclusions are drawn.

The findings are published in the journal Cortex.