The idea that the shape of your hands indicates something profound about the films you like, your athletic ability and your prowess in the stock market seems bizarre.
And yet for many decades now, scientists have noticed an extraordinary link between the ratio of two digits on the hand — the ring and the index fingers, known in scientists' jargon as 2D and 4D — and a host of seemingly unrelated traits.
Evidence is growing that this "digit ratio", especially when applied to the right hand, is a fundamental indicator of the appeal, aggression and diseases suffered by men. Last week, for example, strong evidence has emerged of a link between the "2D:4D finger ratio" and a man's likelihood of developing prostate cancer.
Specifically, men whose index fingers are longer than their ring fingers are less likely to develop the disease, according to scientists at the Institute Of Cancer Research.
Working out your digit ratio is not just a matter of looking at your hand and comparing the position of the tips of the fingers. You must measure the distance from the midpoint of the lowest crease at the base of the finger, on the palm side, to the very end of the fleshy tip (the fingernail does not count!).
A long index finger also correlates with a lower risk of early heart disease and, in women, a higher risk of breast cancer and greater fertility. People with relatively long index fingers are also more likely to suffer from schizophrenia, allergies, eczema and hay fever. Young boys are more likely to be clingy and anxious than their low-ratio peers but also less attention-seeking and better-behaved in school.
A short index finger is a more masculine hand. It correlates with higher male fertility, higher levels of aggression and increased aptitude for both sports and music.
Women who have this masculine finger pattern are more likely to display higher levels of aggression — and enjoy greater professional success.
The extraordinary thing is that these assertions are based on scientific evidence. It was as long ago as the late 1700s that people noticed that a greater proportion of men have shorter index fingers than do women. But it was not until the 1980s that scientists began to wonder if the digit ratio could be linked to more.
The first such study was conducted on women and found a link between a short index finger and female assertiveness. Since then, the floodgates have opened, showing links between the digit ratio and more than 100 psychological traits and propensities to various illnesses.
So can finger length really determine your behaviour and vulnerability to certain diseases?
According to developmental biologist Dr John Manning, who has been analysing digit ratios for more than 20 years, this subtle difference in finger lengths is linked to a foetus's exposure in the womb to sex hormones, notably the "masculine" hormone testosterone.
Put simply, more testosterone equals a greater chance of a more "masculine" hand — one with a relatively short index finger. And it is this exposure to testosterone in the womb that has very profound effects on our behaviour and susceptibility to diseases.
Studies have found that foetuses which have had a high exposure to testosterone tend to be associated with an extroverted personality, a willingness to take risks, higher levels of aggression, stronger muscles and a much greater likelihood of playing an instrument well.
Of course, the rules are not hard and fast, but people of both sexes with relatively short index fingers tend to be more adventurous.
People with short index fingers make better soldiers, engineers, speculators and chess players, and are better at solving problems such as crosswords. They are also more likely to be left-handed.
But short index fingers have also been linked to a higher chance of ending up in prison, being murdered, going mad — and in children, higher rates of hyperactivity and attention deficit disorder. Those with short index fingers may be poorer and find it harder to hold down a job. They also suffer more infectious diseases.
But why did this relationship between testosterone levels in the womb and finger length evolve?
What Professor Manning has noticed is that there may be an element of "sex selection" going on with hands as well. Women often remark on "masculine" hands and Manning speculates this might be a subconscious assessment of the digit ratio. It may be, as Manning says, that a long index finger in men evolved for functional reasons. Anthropologists have speculated that, along with our ability to use fire, one of the reasons why humans became successful was our ability to project our strength from a distance by using weapons such as the spear, slingshot and the bow-and-arrow.
Scientists have found that a longer wedding-ring finger can help increase accuracy when throwing objects. And men who could throw well killed more animals, ate better and thus made better mates. So they would have been preferred as partners, thus ensuring that the masculinity-long ring finger link was passed on.
Nailbiting test of health
Doctors can tell a great deal about how healthy we are by looking at our fingernails. Sudden changes in the shape, thickness or colour of our nails are usually a sign that something is amiss. A whitening nail bed is often a sign of anaemia. White nails could signify that something is wrong with your liver, spotting on the nails could be due to calcium deficiency and horizontal grooves can be a sign of diabetes, circulatory disease or malnutrition.
