In 1982, one of my mentors, the late Norman Geschwind, and two colleagues – Al Galaburda and Peter Behan - proposed an extraordinary hypothesis. It was that the levels of testosterone to which a baby is exposed before birth influence the development of both the cerebral and immune systems. According to this theory, high levels of testosterone result in greater incidences of left-handedness, deviations from the standard distribution of cerebral functions and increased autoimmune dysfunction. If the theory is right, then male brains should mature later than female brains, and the left hemisphere should mature later than the right.
It is certainly true that if a boy gets a head injury or infection involving the brain, he is less likely to recover than would a girl, and boys are far more likely to have some types of neurodevelopmental problems like dyslexia.
For a while it seemed as if there was also a strong association between left-handedness and certain types of allergy, and also with inflammatory bowel disease. This association with immunity also seemed to be present in mice: those who had left paw preference had more reactive immune systems, and they were thought to be more likely to produce auto-antibodies, suggesting that the central nervous system was involved in the genesis of some autoimmune diseases. Over the years the data has become less clear-cut, but the idea of an association between anomalous cerebral asymmetry and autoimmune disease never completely went away.
The Geschwind-Galaburda hypothesis proposes that there should be a four-way association among neurodevelopmental disorders, special talents, non-right handedness, and immune disorders. In a huge study of 11,578 children, less than 1% had all four.
So where does this leave us?
The original theory was half right:
- There is indeed a link between testosterone and early brain development
- People who are left-handed or have a strong tendency toward left-handedness do seem to be at slightly increased risk of several autoimmune conditions
- People who are left-handed or have a strong tendency toward left-handedness may have a slightly increased risk of high blood pressure, asthma and migraine
- People who are left-handed or have mixed handedness are more likely to excel in certain disciplines: creative arts, music, computer programming and mathematics. What we don’t know is whether people with these special skills are more likely to have autoimmune diseases
- Amongst very successful tennis players, there are far more left-handers than would be predicted by chance. This supports the idea that support the notion that left-handed people have neurological advantages in performing certain tasks, such as visuospatial visuomotor cognitive tasks.
I was reminded of the way in which Nature seems to like to balance things out a bit: with some notable and famous exceptions, many successful athletes have not done so well academically and many academics would be unlikely to survive on the plains of Africa. Only some of these differences can be explained in terms of early direction and encouragement in school or while growing up: it seems that most of us cannot hope to become the kind of superman that Nietzsche used to dream about.
Perhaps it’s a way of stopping us from getting too full of ourselves.