It seems preposterous that the length of children’s fingers could point towards success in exams. Yet that was precisely what was reported this week in the British Journal of Psychology. Scientists from the University of Bath found that seven-year-olds whose ring fingers were longer than their index fingers tended to get higher marks at maths. Though the results were due to the children’s brains, the shape of their hands appeared to matter.

The probable reason why owes nothing to palmistry. It is well established that the length of the ring finger, relative to the index finger, is affected by exposure to the male hormone testosterone in the womb. Men tend to have longer ring fingers, while in women the index finger is usually longer or both are about the same size.

The fingers, of course, are irrelevant to a child’s educational prognosis. Their relative lengths, however, might reflect the influence of prenatal hormones on the brain. There is some evidence that high testosterone exposure can improve spatial reasoning, and this skill is as useful in maths as in map-reading. The contours of the hand may thus be a signpost that is suggestive of mathematical ability.

Ring finger length has been linked to other traits that could be influenced by testosterone. John Manning, of the University of Central Lancashire, has found that professional footballers tend to have long ring fingers, and that internationals often have a longer digit than club journeymen. Tim Spector, of King’s College London, has made a similar discovery for female athletes.

A Liverpool University team has found that men with shorter ring fingers may have a slightly higher risk of an early heart attack, but a lower risk in middle or old age. There is some evidence that a longer ring finger can be predictive of a higher sperm count, and there is copious academic literature on sexual orientation. Lesbians tend to have a more masculine finger profile than straight women. The pattern is more complicated among gay men, but there are indications that longer ring fingers, signifying high prenatal testosterone, are sometimes associated with homosexuality.

This is valuable research that casts new light on how human physiology is influenced by hormones in the womb. But the information it reveals must be handled with great care, because the associations between fingers and physical or mental traits are only associations. They reflect averages that may apply at the level of the general population, but which mean little or nothing for individuals.

Nobody would argue with the statement that men are generally taller than women, but it is equally uncontroversial that some women are taller than some men. So it is with ring fingers. The Bath research, for example, suggests that testosterone might be one factor that affects the mathematical aptitude of young children, but it is only one factor. Social and environmental pressures are also going to be involved, as well as other biological ones such as genes.

Thus, while it is tempting and well-intentioned to use traits like this predictively, perhaps to tailor educational strategies to children so they benefit most, that would be utter folly. Many children with the “wrong” finger profile are going to be perfectly good at maths, or even excel.

Ray Blanchard, of the University of Toronto, makes a similar point about research into homosexuality: “There’s no way anybody could use this to screen a date.”

This is worth remembering when it comes to research into the origins of all sorts of human traits, whether genetic, hormonal or environmental. Many such factors are now understood but with a few exceptions, such as the gene that triggers Huntington’s disease, they contribute or predispose to an effect but do not invariably cause it. By recognising that, we can avoid misusing science to create misleading pigeonholes.

Mark Henderson is Science Editor of The Times

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