Genetics research could soon solve the riddle of mathematical pre-eminence: why nearly all geniuses in the field over the last millennium were first-born boys. It is not surprising that boys should have predominated, given that girls in the past have been denied both encouragement and access to tutoring. But if you make a list of the 100 greatest mathematicians of all time, well over 90 per cent of them are eldest boys, even though there were often several younger brothers in the family. The correlation between birth order and mathematical pre-eminence is far too strong to be coincidental.
Similar, if less strong, correlations crop up in other fields that involve analytical or abstract thought. In 1864, Galton, who was one of the first scholars to study the effects of birth order on personality and intellect, observed that successful British scientists were most likely to be eldest sons and, failing that, second eldest sons. It seemed that the lower down the birth order you were, the less good your prospects.
Speculation over causes tended to focus on the relationship between birth order and exposure to intellectual stimuli at a formative age. Such theories have some credibility, because great mathematicians were almost always exposed to the subject at an early age. Isaac Newton helped his father with engineering calculations at the age of three.
Similar arguments have been used to explain a correlation between birth order and intelligence in both sexes. But this fails to explain the strength of the correlation for mathematical genius, or why in other fields the correlation between birth order and intelligence is weaker or non-existent. In literature, for example, boys and girls lower down the birth order have been equally likely to succeed.
Interest in the birth order riddle has recently been rejuvenated by the discovery of genes associated with neurological development in the womb. At first sight, this seems irrelevant to the cause, for there is no inherent genetic advantage in being high up the birth order. A first or last born is equally likely to inherit a particular set of genes conducive to mathematical genius or anything else. What has become clear, though, is that the expression of genes that play vital roles in brain development is controlled by the mother's response to the developing foetus, and this changes with successive pregnancies. It turns out that this has more serious consequences for boys than for girls.
This discovery was made initially in the light of a different phenomenon, the fact that the likelihood of a boy being homosexual is strongly related to birth order. A boy with an elder brother is about twice as likely to be homosexual as one who is an only child or who has just sisters, and the probability continues to double with each additional elder brother. Yet having elder sisters has no bearing on sexual orientation for boys.
Then, during the 1990s, it was discovered that the mother's immune system reacts against the proteins coded for by three genes on the Y chromosome which are closely related with brain development in the womb. As only boys have a Y chromosome, only in boys are these particular proteins involved in the development of brain structure.
It has now been shown that during successive pregnancies, mothers tend to develop progressively stronger immunity against these proteins involved in male brain development. This "sexual antagonism" appears to explain why later born boys are more likely to be homosexual, having brains that in some respects are more female-or, at any rate, less male. It also explains why there is no corresponding correlation between birth order and lesbianism, for there is no equivalent antagonism between mothers and girls in the womb.
More controversially, this antagonism could also explain the correlation between birth order and mathematical potential. If so, the question is, what are the masculine brain features conducive to mathematical ability? There is, after all, no basic anatomical difference between female and male human brains. But it has been found from testing brains obtained after autopsy that women tend to have a larger corpus callosum (the bundle of fibres connecting the hemispheres of the brain).
It is now well established that while both brain hemispheres are involved in nearly every human activity, the right hemisphere is used more for intuitive processes, while the left is the seat of language, logic and analytical thought.
It may be that extreme analytical ability is associated with a highly specialised left hemisphere operating on its own with less involvement of the right hemisphere. This would accord with the tendency among great mathematicians of being late to talk. A delay in verbal development is associated with a separation between the brain's hemispheres, allowing the left to become exceptionally specialised in analytical activity to the detriment of communication skills.
Boys are much more likely than girls to be late in learning to talk and to experience greater than average separation between the hemispheres. It is reasonable, then, to suggest that sexual antagonism in the womb could, by retarding male brain development, decrease the likelihood of this separation occurring. This in turn reduces, alas, the chance of becoming a great mathematician.
