Differences in our digits, visible in the womb, can predict much about the life that lies ahead of us, argues John Manning.

Think of a foetus and its environment. What comes to mind? An individual suspended in a watery membrane of peace, security and protection? A gentle slumber before an awakening in a cruel Darwinian world? Wrong. This is a desperate time, a time for precision. The foetus must develop and perfect the reproductive system, brain, heart and blood vessels, the immune system and so on. Get it wrong and infertility, mental illness, early heart attacks and cancers may be the legacy.

Since these things are common and a part of the human condition, the implication is that we often make mistakes in utero . But that does not mean we must sit back and wait for prenatal errors to catch us up. Suppose there is a trait that is fixed early, a sort of living fossil in our body that bears testament to what happened during the formation of our major organ systems. I believe this trait exists and that it is to be found in the fingers - or to be more precise, it is in the relative lengths of our index and ring fingers.

The answer, as with so many things, lies in sex. In the early foetus, the testes begin to function at about week eight. At this stage males and females are bathed in high concentrations of oestrogen. In males, the testosterone from their testes pervades the foetus, cancelling the effects of oestrogen and masculinising the brain, the heart and immune system. Understand this very early process and we may understand the many behaviours and diseases that have different patterns of expression in males and females.

ÖйúAƬ

But which traits should we look at? Obviously, those that show sex differences. There are many of these. In comparison with women, men are on average taller, they have larger jaws in proportion to the size of their faces, and they have a lower percentage of body fat, which they deposit around the waist rather than the hips and buttocks. In common with other biologists, I have studied these and other sex-dependent traits. But they all share a fatal flaw - these sex differences arise or are accentuated at puberty. They are of interest because we use them in our mate choices. But as markers of early foetal conditions, they are useless.

The search for a trait that is fixed early, that differs between the sexes, but does not change at puberty was inevitably going to be a difficult one. I had almost given up hope of finding it, but it turned out to be in a part of the body that is often a focus of our attention. By an extraordinary quirk of evolution, the genes that control the development of the fingers are also those responsible for the differentiation of the testes and the ovaries. They therefore have an affect on our fertility.

ÖйúAƬ

The fingers bear the stamp of our early foetal environment, they are our window into our early sexual development, and it turns out that the ring and index fingers are most important. In men, the ring finger tends to be longer than the index. This is the digit that reflects "male" influences before birth. In women, the index finger is dominant, reflecting female influences. It must be emphasised that what is measured here is the relative lengths of index and ring finger. If the index is longer than the ring finger, that is a feminine-type hand; if the ring finger is longer, that is a masculine hand.

What does this finger ratio tell us? In men, we find sperm counts, testosterone and even family size linked to long ring fingers. In women, high oestrogen and large families are associated with long index fingers. These are weak relationships, but real across many human groups with different mean family sizes and different access to contraception. Geography also plays an important role in the overall picture of finger ratios.

But what of variation between ethnic groups? One of the startling facts of the finger ratio is that it differs markedly between human populations. Caucasians have in general a female-type ratio while still showing sex differences within their populations. Japanese and black populations have in general very male-type ratios. Caucasian, oriental and black populations represent large and variable groups within which we find further variation in the finger ratio. Among Caucasians, the English, Polish and Spanish have rather feminised fingers; Germans, Hungarians, Finns and Indians are rather more masculinised. These geographical differences in fingers tend to be larger than the sex differences. Nevertheless, the sex differences are found within all populations.

I believe these differences reflect pre-disposition to disease. Consider heart attacks and breast cancer. The rates of these diseases between populations vary to such an extent that factors such as diet are unlikely to explain all of the observed differences. Heart attacks are more common in men, particularly youngish men, than in women. Surprisingly both women and men have essentially the same kind of breast tissue but breast cancer is much more common in women (about 0.07 per cent of breast cancers occur in men). There is evidence that testosterone protects against heart attack in males. Accordingly, long ring fingers are rare in men who have suffered an early heart attack. In women, oestrogen is associated with breast cancer. As expected, women with long index fingers are commonly found among breast cancer patients who develop the tumour when young. It is not surprising then that Japanese and many black populations show low rates of heart disease and breast cancer compared with Caucasians.

ÖйúAƬ

But what of the delights of robust health, of athleticism and sports, of romance and music? Long ring fingers are found in men who can run fast, who have good judgement of distance and shapes and who excel at sports. But what of male traits that are valued by females? Darwin suggested that music is essentially about courtship. Male musicians, particularly good musicians, have long ring fingers. Are their female admirers reacting to music as a signal of male fertility?

So in men, long ring fingers may indicate fertility, athleticism and an efficient heart, but there may also be a dark side to this trait in males. A number of developmental disorders are more common in boys than in girls, including autism, attention deficit disorder and dyslexia. Poor language ability, short attention spans and poor reading ability are all male traits and are commonly seen in an intense form in developmental disorders.

It could be that these children are showing some form of "extreme male brain" in which these disorders are magnified. For example, children with autism usually have little or no language ability, but may also have surprising "islets" of ability in music. There is evidence that children suffering from developmental disorders have in fact, very long ring fingers. So perhaps it pays to be "male" but not too "male". Prenatal development is a finely balanced process.

Our fingers may bear witness to other abilities and diseases: excellence in mathematics, spatial orientation and languages, a pre-disposition to schizophrenia, psychoticism, asthma, strokes, deep vein thromboses and cancers, a number of deficiencies of the immune system and infections by parasites such as those that lead to sleeping sickness. These are present more commonly or exclusively in one sex or the other. Our fingers may predict our depth of talent or our degree of susceptibility in all of these.

ÖйúAƬ

This analysis indicates that the "nature versus nurture" debate can be better characterised as "prenatal versus postnatal" influences - it is often the case that both genes and the environment influence traits. The finger ratio is no exception because genes and environment have equal effects on the variation between individuals. A more important question is, at what stage in development do these genetic and environmental factors have their most important effects?

So here is a tale of two fingers. It is one of sex and race, of fertility and disease, and of profound mental defects and human abilities. The foetal period can be the best of times but it may also be the worst. What happened to you and what you inflicted on yourself in that time of deceptively tranquil watery peace had a profound effect. It continues to do so.

ÖйúAƬ

John Manning is reader in biological sciences at the University of Liverpool and author of the forthcoming Digit Ratio: A Pointer to Fertility, Behaviour, and Health , published by Rutgers University Press.