More left-handed men are born during the winter

Men born in November, December or January are more likely of being left-handed than during the rest of the year. While the genetic bases of handedness are still under debate, scientists at the Faculty of Psychology, University of Vienna, obtained indirect evidence of a hormonal mechanism promoting left-handedness among men. Psychologist Ulrich Tran and his colleagues published their findings in the scientific journal "Cortex". Various manual tasks in everyday life require the use of the right hand or are optimized for right-handers. Around 90 percent of the general population is right-handed, only about 10 percent is left-handed. The study of Ulrich Tran, Stefan Stieger, and Martin Voracek comprised two large and independent samples of nearly 13000 adults from Austria and Germany. As in modern genetic studies, where a discovery-and-replication-sample design is standard, the use of two samples allowed testing the replicability and robustness of findings within one-and-the-same study. Overall, 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", according to Ulrich Tran, lead author of the study. A hormonal cause during embryonic development