Mental Health

A Lot More Lefties Born in Winter

Christine Hsu
July 03, 2014 05:15 PM EDT

Men born in the winter are more likely to be left-handed, according to a new study.

Statistics show that only around 10 percent of the general population is left-handed, and researchers in the latest study wanted to understand why being right handed is more popular than being left handed.

Study authors Ulrich Tran, Stefan Stieger, and Martin Voracek analyzed 13,000 adults from Austria and Germany. Researchers said that using two samples could help researchers double-check replicability and robustness of the findings.

Study results revealed that 7.5 percent of women and 8.8 percent of men were left-handed.

"We were surprised to see that this imbalance was caused by more left-handed men being 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. During November to January, this number rose to 10.5 percent," lead study author Ulrich Tran said in a news release.

"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 May to July, half a year before, is its distal cause," Tran explained.

Researchers explain that a theory developed by American neurologists Norman Geschwind and Albert Galaburda suggests that testosterone stunts the growth of the brain's left hemisphere during embryonic development. Researchers explain that right-handed people have dominant left-brain hemisphere s, and left-handers have dominant right-brain hemispheres. Furthermore, intrauterine testosterone levels are higher in the male fetus than in the female fetus. However, the testosterone level of the mother and external factors could also influence intrauterine testosterone levels.  Researchers believe that more daylight may increase testosterone levels, which support the latest findings.

Researchers from the University of Vienna said that the latest findings suggest that there is a small, but robust and replicable, effect of birth seasonality on handedness in men, but not women. Researchers explain that these findings support previous findings linking hormones and to handedness.

The findings are published in the journal Cortex.