The vast majority of humans are right-handed. Only about ten percent are left-hand dominant. But what causes the ten percent to prefer their opposite set of digits? Scientists have long traded theories on the matter and argued whether genetics are at play.

Recently, in a survey of handedness involving 13,000 Germans and Austrians, researchers found that male southpaws are slightly more likely to be born in November, December and January than other months -- proof, they say, of a hormonal mechanism at work.

The study was led by psychologist Ulrich Tran, a researcher at the University of Vienna, in Austria; the study was published this week in the scientific journal Cortex.

"Presumably, the relative darkness during the period November to January is not directly connected to this birth seasonality of handedness," said Tran. "We assume that the relative brightness during the period May to July, half a year before, is its distal cause."

Tran and his colleagues say their new research confirms an earlier theory, proffered in the 1980s by American neurologists Norman Geschwind and Albert Galaburda. They hypothesized that testosterone levels were essential in tipping the scales that determine handedness. Most right-handed people are left-brain dominant, and left-handed are right-brain dominant. And testosterone has been shown to delay the development of the left brain hemisphere during embryonic stages.

Thus, Geschwind and Galaburda concluded, heightened testosterone levels may enable a preference for the right side of the brain and the left hand.

The theory remains controversial, but Tran and his colleagues say the prevalence of left-handedness in winter babies corroborates the now-decades-old logic. Winter babies are conceived in spring, and in their earliest embryonic stages during summer, increased daylight hours can lead to heightened levels of testosterone in women.

Excited as Tran and his study co-authors are, the researchers acknowledge more research is needed determine "the exact way of causation."