Being left-handed, I have done a bit of reading on the topic. So I have known for a while that one of the causes of left-handedness is the presence of higher than normal levels of testosterone in utero. If there is a high level of testosterone during certain stages of brain development, hemisphere dominance can change, and left-handedness can be a result.
A new study by Simon Baron-Cohen shows high levels of steroid hormones in utero for children who later show signs of autism. Testosterone is, of course, a steroid hormone. This made me wonder if there is a correlation between left-handedness and autism.
Not only is there, but we have known about it since 1983.While the general population shows 37% non-right handed dominance (meaning left-handedness or various forms of ambidexterity; left-handedness alone is about 18%), that number is almost twice as high in people with autism: 62%. This is pretty much a complete inversion of neurotypicals’ handedness. More recent papers all suggest people with autism may be three times more likely to be left-handed.
Of course, autism is not the only condition strongly associated with left-handedness. Dyslexia is as well. And so are many mental disorders. Equally, about half of lefties are clearly neurotypical (not autistic, dyslexic, etc.), so it’s important to understand that while the presence of left-handedness may indicate non-neurotypical neural architecture, it does not necessarily do so.
Still, the correlation between high testosterone levels and left-handedness and the correlation between high steroid levels (including testosterone) and autism points toward Baron-Cowan’s theory of autism as a more male brain. Now, given that I subscribe to the “intense world theory” of autism (at least for myself and my son), I have to wonder if there is a relationship between these high steroid levels and neurohyperactivity.
Now, here’s a fun fact
Both autism and left-handedness are found more often in males than females. Yet, left-handedness results in a 50% larger corpus callosum, and autism is in part caused by a more active brain. Women also have a 50% larger corpus callosum than men, and their brains are more active than are men’s brains. So, ironically, more testosterone to such a degree that it causes left-handedness and autism makes those male brains more like female brains in certain ways (but in other ways, very obviously not–such as the lower connectivity in autustic brains). Either way, these differences may explain why there’s a great deal more gender fluidity among autistics (and more homosexuality among left-handers as well). These differences may contribute to the greater creativity seen in both groups as well.