Men born between the months of November and January may have an increased propensity to be left-handed, according to a recent study conducted at the University of Vienna.
Although the genetic basis for handedness is not yet clear, the study provides indirect evidence that hormones may play a role because their production levels fluctuate between the seasons due to the difference in darkness and light.
"Presumably, the relative darkness during the period November to January is not directly connected to this birth seasonality of handedness," says lead author and psychologist Ulrich Tran. "We assume that the relative brightness during the period May to July, half a year before, is its distal cause."
In compliance with modern genetic studies, researchers gathered two independent groups of 13,000 German and Austrian volunteers, one intended to test the credibility of findings, the other to test their replication.
In the study, 7.5 percent of women and 8.8 percent of men were left-handed. According to Tran, only 8.2 percent of left-handed men were born between February and October, while a solid 10.5 percent were born between November and January.
Published in the journal Cortex, Tran's study was inspired by a theory that emerged in the 1980s, saying that testosterone suppresses maturation of the left brain hemisphere in the developing fetus.
This theory rests upon the principles that the male fetus excretes more testosterone than the female, and that the left brain hemisphere is dominant in right-handed individuals. External factors could also determine intrauterine testosterone levels, according to the theory.
U.S. neurologists Norman Geschwind and Albert Galaburda, who developed the theory, say spring's surplus of daylight may increase testosterone levels, which corresponds with the findings of Tran's study.
Universally, 90 percent of people are right-handed and 10 percent are left-handed.
Past studies have also indicated that the left hemisphere dominates language in 90 percent of individuals.
A recent French study, published June 30 in the journal PLOS One, indicates that the hemisphere dominates language is not related to left or right handedness, although in a tiny percentage of lefties' language is dominated by the right hemisphere.
In this study, researchers from the Groupe d'Imagerie Neurofonctionnelle worked with 297 participants of which 153 were left-handed.
Using MRI technologies, they discovered three formulas of "language laterialization," which refers to the dominant hemisphere for language.
The most typical includes 88 percent of righties and 78 percent of lefties, all of whose left hemisphere dominates language.
Less common are "ambilateral" individuals, approximately 12 percent of righties and 15 percent of lefties without a dominant hemisphere.
The least typical group includes just seven percent of lefties with a dominant right hemisphere, meaning statistically that one percent of the entire population has an association between language and handedness while for the rest, it occurs at random.