Baby boys born in winter months are more likely to be left-handed, study finds



Psychologists at the University of Vienna found baby boys born in the period October to February were more likely to be left-handed

Backed the theory left-handedness could have a hormonal cause

An embryo's exposure to higher levels of the male hormone testosterone in womb increases chances of being left-handed

More daylight during summer pregnancies could boost testosterone levels in baby boys born in winter months



Baby boys born in winter are more likely to be left-handed than those born in summer, new research suggests

Baby boys born in winter are more likely to be left-handed than those born in summer, new research suggests.

Psychologists at the University of Vienna studied 13,000 adults and found that overall, 7.5 per cent of women and 8.8 per cent of men were left-handed.

But they found that among the men, on a monthly average, 8.2 per cent of the left-handers were born during the period February to October.

Yet during November to January, this number rose to 10.5 per cent.

The researchers said their findings, which were published in the scientific journal Cortex, backed the theory that left-handedness in men could have a hormonal cause.

It’s thought that an embryo’s exposure to higher levels of the male hormone testosterone in the womb increases their chances of being left-handed.

More daylight may increase testosterone levels, and as baby boys born in the winter are in the embryonic stage during the spring and summer, they may be exposed to more testosterone.

Lead author of the study, Ulrich Tran, 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.’

The effect is explained for men, but not women, because male brains are exposed to substantially higher testosterone levels than female brains during prenatal development.

The Geschwind-Galaburda hypothesis, named after the neurologists who devised the theory, suggests that testosterone delays the maturation of the left brain hemisphere during embryonic development.

Psychologists at the University of Vienna said their findings backed the theory that left-handedness in men could have a hormonal cause. More daylight may increase testosterone levels, and as baby boys born in the winter are in the embryonic stage during the spring and summer, they may be exposed to more testosterone

The left brain hemisphere is dominant among right-handers, while the right brain hemisphere is dominant among left-handers.

In-womb levels of testosterone are higher in the male foetus, because of its own testosterone secretion, than in the female foetus.

However, the testosterone level of the mother and external factors may also affect levels of the hormone in the womb.