Most Left Handed People Were Winter Babies

Researchers have found a remarkably clear correlation between winter-births and left handedness, where up to 9 percent of all children born in winter months are left handed, compared to significantly lower rates for the rest of the year. Analysis of this correlation supports a 35-year-old theory, according to a recent study.

Left handedness was once commonly associated with evil and even death, where the angel Gabriel - one of six angels of death - was called God's left hand, while black magic was commonly referred to as "the left hand path" during early colonial times.

In some small way, it may have been that these superstitions from an age-gone-by were taking inspiration in-part from the fact that left handed children were most commonly born during the winter season - a time when the days are shortest and life was hardest for growing hungry families.

According to a study published in the journal Cortex in a survey of nearly 13000 adults from Austria and Germany, a great majority of left handed people, especially among men, were born during November, December, and January, with only 82 percent being born between February and October, and 10.5 percent being born between November and January.

According to lead author Ulrich Tran, this phenomenon is best explained by the theory that light exposure can lead to varied testosterone exposure in the womb.

"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", he explained in a statement.

According to Tran, US neurologists Norman Geschwind and Albert Galaburda first argued that testosterone levels may influence handedness in the 1980s. They said that elevated levels may slightly delay the development of the left hemisphere of the brain, leading to a dominance of the right hemisphere - which is connected to the left hand.

Trans and his colleagues add to this theory, arguing that if a child's mother is exposed to more daylight while pregnant, her testosterone levels may rise high enough to affect the baby's own levels, potentially leading to a change in handedness.

The authors admit that other factors may play a part too, resulting in an estimated 10 percent of the world's populations being left-handed. A 2011 study published in Obstetrics and Gynecology revealed that there is also "a statistically significant - albeit weak - association between ultrasound screening during pregnancy and being non-right handed later in life."

Although, much like this latest study, the authors suggest that more research will be needed to find a clear causal relationship.