Males born during the winter months of November, December, and January are more likely to be left-handed than males born during other months.
There is evidence that the hormone testosterone and the availability of daylight may play a role in this phenomenon, according to psychologists at the University of Vienna.
The study showed a small, but consistent, effect of birth seasonality on left-handedness in men.
“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 psychologist Dr. Ulrich Tran, lead author of the study.
“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,” said Tran.
The study, published in the scientific journal Cortex, involved two large and independent samples of nearly 13,000 adults from Austria and Germany. As in modern genetic studies, the use of two samples allowed researchers to replicate and confirm the findings.
A previous study had suggested that the hormone testosterone delays the maturation of the left brain hemisphere in the fetus. The left brain hemisphere is dominant in right-handers, while the right brain hemisphere is dominant among left-handers.
While still in the womb, testosterone levels are higher in males, because of their own testosterone secretion, compared to female fetuses. However, the testosterone level of the mother and external factors may also affect testosterone levels in the womb.
In fact, more daylight may increase testosterone levels, making a seasonality effect more likely.
Prior studies on the subject have given mixed and inconsistent evidence. There was no clear indication of which season had an effect, and whether seasonality affects men, women, or both sexes equally.
According to the current findings, there is a small, but consistent, effect of birth seasonality on left-handed males. These results are consistent with the hormonal theory of handedness — an older controversial theory. The exact cause of this will need to be investigated in future studies.
Source: University of Vienna