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June 28, 2005
Vishy's Useless Factoid of the Day #14: Handedness and season of birth
Or, how I learned to stop worrying about when the stork dropped me off and hit the ball with my left hand.
Not all the endocrinal functions of the mysterious pineal gland are fully understood. In ancient cultures, such as that of India, it is known as the Third Eye and has been associated with giving special powers such as clairvoyance. It is activated by prolonged periods of darkness, and its function is to suppress the production of sex hormones. What good does this serve? When summer arrives and the days start to get longer, the pineal gland lets the sex organs do their work to increase libido. This evolutionary adaptation of the pineal gland results in babies being born at the start of spring, when fresh food is available. In sum, when the days are long, hormonal levels, especially those of sex hormones, are high.
It is well documented that men and women both secrete trace quantities of the other sex's sex hormones. In other words, women too secrete trace amounts of testosterone. In a pregnant woman, increased levels of testosterone are associated with increased asymmetry in how the analytic, visuo-spatial left brain hemisphere of her developing child dominates over the holistic, non-visual right brain hemisphere. Much like an embryo is female by default, neither hemisphere dominates the other one particularly strongly by default. Along comes testosterone to change everything. In addition to being produced by the pregnant mother, testosterone is also produced by genetically male embryos, starting at 8 weeks of gestation. It converts the so-far female embryo to a male embryo and skews the left hemisphere to dominate over the right hemisphere. High levels of testosterone are therefore more likely to lead to right handed people, whose left hemisphere dominates their right hemisphere. This effect is more pronounced when the fetus is exposed to high levels of testosterone in the first six
s of pregnancy.
So where does this leave us? If a baby is conceived in the winter months (between December and May in the Northern Hemisphere), it is exposed to lower levels of testosterone overall, which means a lower likelihood of left hemisphere dominance and a higher occurrence of non-right-handedness. These left-handed and ambidextrous babies are born from August to January. This effect is more pronounced in the temperate latitudes, where seasonal effects are stronger. I didn't find studies to corroborate a complementary seasonal pattern for handedness in the Southern Hemisphere. However, exposure to high hormonal levels has been linked with occurrence of schizophrenia and other psychological conditions. Complementary seasonal patterns have been observed in schizophrenic births in Europe and South Africa.
I am left-handed. I was born in September. I am the World's Largest Repository of Useless Knowledge.
Source for the above, with lots of citations
Posted by Vishy at June 28, 2005 12:09 AM