Winter Promotes Male Leftie Births

Science Fields

A survey conducted in Germany and Austria has shown that more left-handed men are born in winter months.

We can safely say that our civilization stands on the right hand. In our daily lives, great majority of manual tasks necessitate the the use of the use of right hands. And most industrial, technological and electronic products requiring manual handling are optimized for right hand use.

Statistics show that over ninety percent of people are right-handed, while the left-handed make up less than ten percent .

The findings of a comprehensive survey covering 13.000 randomly-picked adults in Germany and Austria tend to confirm this huge imbalance in favor of the right hand users. 


Researchers from University of Vienna Ulrich Tran, Stefan Stieger and Martin Voracek, have established the ratio of left handednes among men to be 8.8 percent, while it lagged at 7.5 percent among women. More surprising is the finding that this imbalance between male and female lefties is caused by a marked rise in the left handed male births in winter months. 

Looking at monthly averages of male births from February to end of October, the researchers found that the monthly average of the left-handed among boys born in this term was 8.2 percent. The average rate for the period from November to the end of January, however, was seen to jump to 10.5 percent.

Tran attributes this seasonality in leftie births less to the darkness of winter months than the relative brightness of the rest of the year.

The findings are seen as lending support to a controversial  theory, proposed by American neurologists Norman Geschwind and Albert Galaburda in 1980s, focusing on the effects of hormones on handedness. According to the two researchers, testosterone delays the development of the brain’s left hemisphere during embryonic growth. The left hemisphere is dominant in the right-handed people. In the left-handeds, it is the right hemisphere which calls the shots.

Because the embryo itself secretes testosterone, the intrauterine testosterone levels are higher in male fetuses compared to the females. However, the mothers’ testosterone levels as well as external factors, such as stronger daylight, influence the intrauterine testosterone levels. Thus they cause small seasonal fluctuations like those that broached at the survey of Tran and his fellow researchers, because of the dominance granted to the right hemisphere by the slower development of the left hemisphere caused by the excess of testosterone. 


  • 1. “More left-handed men are born during the winter”, University of Vienna, 3 Temmuz 2014