Here is a bit of interesting news. Baby boys who are born in the winter are more likely to be left-handed than are those born in the summer. Psychologists at the University of Vienna conducted a study with 13,000 adults and found that, in general, 7.5% of women are left-handed and 8.8% of men are. Their findings were published in the scientific journal Cortex.
However, they found that 8.2% of the left-handers were born during the period from February to October, while from November to January, the number rose up to 10.5%. Researchers have thought that if an embryo is exposed to higher levels of testosterone in the womb, this might increase the chance that the baby will be left-handed. More daylight increases the levels of testosterone, and babies that are born in the winter were in embryo during the spring and summer.
As Ulrich Tran, the lead author of the study, said, “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.’
This theory, The Geschwind-Galaburda hypothesis, named after the neurologists who devised it, suggests that testosterone delays the left brain hemisphere’s maturation in the embryo.