Winter babies, especially men, born between November and January are more likely to be left handed, says a study.
Winter babies, especially men, born between November and January are more likely to be left handed, a new study has claimed.
Various manual tasks in everyday life require the use of the right
hand or are optimised for right-handers. Around 90 per cent of the
general population is right-handed, only about 10 per cent is
left-handed.
The study by Ulrich Tran, Stefan Stieger, and Martin Voracek from the
University of Vienna comprised two large and independent samples of
nearly 13000 adults from Austria and Germany.
Overall, 7.5 per cent of women and 8.8 per cent of men were left-handed.
“We were surprised to see that this imbalance was caused by more
left-handed men being born specifically during November, December, and
January. On a monthly average, 8.2 per cent of left-handed men were born
during the period February to October. During November to January, this
number rose to 10.5 per cent,” according to Ulrich Tran, lead author of
the study.
“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,” said Tran.
A theory, brought forth in the 1980s by US neurologists Norman
Geschwind and Albert Galaburda, posits that testosterone delays the
maturation of the left brain hemisphere during embryonic development.
The left brain hemisphere is dominant among right-handers, the right
brain hemisphere is dominant among left-handers. Intra-uterine
testosterone levels 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 intra-uterine testosterone levels. Specifically, more
daylight may increase testosterone levels, making a seasonality effect
plausible.
Previous studies on the subject provided mixed and inconsistent
evidence. There was no clear indication which season has an effect, and
whether seasonality affects men, women or both sexes equally.
According to the current findings, there is a small, but robust and
replicable, effect of birth seasonality on handedness, affecting only
men.
These results are consistent with a hormonal basis of handedness, corroborating thus an old and controversial theory.
