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Researchers at the School of Psychology, University of Vienna surveyed 13000 adults from Austria and Germany and found that most left-handed males are born during the winter. The difference is statistically significant, suggesting there’s a lot more at play than just chance. Namely, the findings seem to back the Geschwind-Galaburda theory of cerebral lateralization, which states high intrauterine testosterone levels delay left brain hemisphere maturation and thus promote left-handedness.

Lefties are cool

Only one in ten people in the world are left handed, and like most people living in a minority, lefties often get marginalized. Apparently, every mass produced good is made to work right handed, whether we’ll talking about cooking tools, musical instruments or gadgets. Some get even forced to write with their right hand in pre-school (which I believe is a really stupid thing to do!). Of course, if you’re a lefty, then by now you’ve learned to deal with it, and if you’ve been smart about it, you’ve learned to use it to your advantage. It’s no secret that lefties are much better are sports. For instance, just because all the other players are right handed, they find a lefty opponent unpredictable and harder to win against. Ask any right handed boxer about this and he’ll share some painful stories. Left handed people are also better artists, since they’re better at divergent thinking, a method of idea generation that explores many possible solutions. Also, people seem to vote for lefties: four of the last seven U.S. presidents were left-handed – quite a few, considering they’re a minority. Don’t feel too bad because you’re right handed.

Winter is coming… so are the lefties

Using a discovery-and-replication-sample design, the Austrian researchers found that, overall, 7.5 percent of women and 8.8 percent of men were left-handed. They were surprised to find that significantly more people were born left-handed in 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. Why?

Clearly, the discrepancy is too high to account for a statistical anomaly. Instead, the results seem to point to a theory brought forth in the 1980s by US neurologists Norman Geschwind and Albert Galaburda, that posits that testosterone delays the maturation of the left brain hemisphere during embryonic development. Right handed people are left-hemisphere dominant, while left handed people are right-hemisphere dominant.

Testosterone is the adrogen hormone responsible for the development of many male defining sexual characteristics. During fetal development, the testosterone level of the mother and external factors may affect the baby’s testosterone levels. Specifically, more daylight during the summer time may increase testosterone levels, making a seasonality effect plausible. According to the current findings, there is a small, but robust and replicable, effect of birth seasonality on handedness. Considering differences in seasonal left handedness were only measured in man gives further credence to the variable testosterone theory. Previous, similar studies however have delivered mixed and inconsistent evidence. The causation needs to be further investigated.

Findings appeared in the journal Cortex.