Men: what your finger size says about you

In palmistry, each of our fingers reveals something about our personality and character. A prominent middle finger, for example, is said to represent a serious and intense nature, while a little finger that stands out naturally from the hand shows a very independent and outspoken person.

While palm reading sceptics would dismiss that as quackery, new scientific studies actually show the fingers - in particular, the lengths of the index and ring - are associated with many traits from sexual orientation to sperm counts, musical ability to sporting prowess, and even risk of health problems such as autism, depression, heart attack and breast cancer.

The relative lengths of the index and ring fingers, also known as the 2D:4D ratio or digit ratio, depends on one's exposure to the male sex hormone testosterone and the female sex hormone oestrogen while still a foetus in their mother's belly. Previous studies have shown that a higher exposure to testosterone relative to oestrogen leads to longer ring finger in comparison to the index, or a lower digit ratio.

In one of the latest published studies on digit ratio, researchers at the University of Cambridge's Division of Biological Anthropology found distance runners with lower digit ratios clocked faster times - a correlation notably strong in men, but also present in women.

The scientists correlated the finding with a man's "reproductive potential", which, like endurance running ability, has a negative relationship with digit ratio.

"The observation that endurance running ability is connected to reproductive potential in men suggests that women in our hunter-gatherer past were able to observe running as a signal for a good breeding partner," says the study's lead author Dr Danny Longman, a postdoctoral fellow.

Published a fortnight ago in the journal PLOS ONE, the study involved 542 runners who participated in the 2013 Robin Hood Half Marathon in Nottingham. Among the male participants, the average digit ratio was 0.94 for the left hand and 0.97 for the right; among the women, it was 0.98 and 1.01 respectively.

(The best way to find out your digit ratio is to photocopy each hand with the palm flat and face down, fingers splayed naturally, and measure each finger where it meets the palm of the hand to the tip. Divide the length of the index by the ring finger.)

This bizarre difference between the sexes was first noted by Dr Frank Baker, a professor of anatomy at the University of Georgetown in Washington DC, in his report Anthropological Notes on the Human Hand published in January 1888 in the journal American Anthropologist.

In 2011, University of Florida biologists conducted tests on mice - which have a digit length ratio similar to humans - and discovered why: the developing digits of male and female embryos are packed with receptors for sex hormones.

Hormonal signals govern the rate at which skeletal precursor cells divide, and different finger bones have different levels of sensitivity to testosterone (a type of androgen) and oestrogen.

In the study, which appeared in the Proceedings of the National Academy of Sciences, the biologists were able to control the gene-signalling effects of these sex hormones in the mice. When they blocked testosterone receptors, they got a female digit ratio. When they added testosterone, they got super male ratios, and when they added oestrogen, super female ratios.

"There is growing evidence that a number of adult diseases have fetal origins," said the study's co-researcher Dr Martin Cohn. "With the data, we've shown that the digit ratio reflects one's prenatal androgen and oestrogen activity, and that could have some explanatory power."

Digit ratio may be used as a "moderate indicator" of schizophrenia, concluded scientists in a recent study published in March in Clinical Anatomy. Among males, they found that digit ratio was "statistically significantly lower" among 103 diagnosed schizophrenic patients than 100 matched healthy individuals.

Another study, featured in 2008 in Arthritis & Rheumatism, suggests that having uncommonly long ring fingers raises the risk of developing osteoarthritis of the knee, independent of other risk factors and particularly among women. Here are six more things your fingers say about you.

Dealing with others

Men with a lower digit ratio are on average nicer towards women, according to a McGill University study published in February in the journal Personality and Individual Differences.

The researchers say this finding might help explain why these men tend to have more children.

"When with women, men with smaller ratios were more likely to listen attentively, smile and laugh, compromise or compliment the other person," says Debbie Moskowitz, lead author and a psychology professor at McGill.

They acted that way in sexual relationships, but also with female friends or colleagues.

These men were also less quarrelsome with women than with men, whereas the men with larger ratios were equally quarrelsome with both.

For women, however, digit ratio variation did not seem to predict how they behaved.

Aggressive behaviour

Both men and women with smaller digit ratios self-reported themselves to be more verbally aggressive, in a 2012 study that appeared in the Journal of Communication.

The researchers noted that while a small degree of verbal aggression may be beneficial for a person (such as being able to stand up for yourself if attacked), higher degrees of verbal aggression have been shown to be detrimental to one's personal life (for example, problems maintaining close personal relationships, loss of job).

Another study found that men with shorter index fingers are more likely to pick fights.

Sexual orientation

Various studies show that gay men have a larger digit ratio compared to heterosexual men, suggesting that sexual orientation may be influenced by prenatal testosterone levels.

But there have been conflicting studies on digit ratio and its correlation with lesbians and straight women, so the jury is still out. In one study published in 2007 in the Archives of Sexual Behaviour, ethnicity showed to affect the results: differences in digit ratio between gay and straight men were strongest in whites, and there was no evidence of such a difference among black or Chinese participants.

Spatial and verbal ability

A smaller digit ratio is associated with a larger difference between ability in maths and literacy, favouring the former, according to a study of children published in 2007 in the British Journal of Psychology.

Comparing finger lengths with the SAT scores of seven-year-olds, the researchers found a clear link between a smaller digit ratio and higher numeracy scores in boys, and a larger digit ratio and higher literacy scores in girls.

"Testosterone has been argued to promote development of the areas of the brain which are often associated with spatial and mathematical skills," said Dr Mark Brosnan, head of the department of psychology at the University of Bath, who led the study. "Oestrogen is thought to do the same in the areas of the brain which are often associated with verbal ability."

Risk-taking

Men with a lower digit ratio were found to take more risks across three domains: recreational, social and financial, in a 2010 study from Concordia University. However, the link was not observed in women.

In another study, published in 2009 in the Proceedings of the National Academy of Sciences, Rockefeller University researchers in New York found that male traders engaged in high-frequency trading were more likely to be more successful and stay in the business for longer if they had a smaller digit ratio.

Penis size

The lower the digit ratio, the more well hung a man is, found a University of Incheon study that appeared in 2011 in the Asian Journal of Andrology. The researchers had measured the penile length - flaccid and stretched - of 144 Korean male patients under anaesthesia for urological surgery.