Hands can give away a lot about the owner. Callouses can indicate manual work, chewed nails a nervous disposition, and now finger length can reveal to the careful observer the length of a man's penis.
Researchers in Korea have established that the ratio between the length of the second and the fourth finger indicates how well nature has endowed a man.
The bigger the difference between the two fingers, the smaller the size of a man's penis. Or, to look at the other way, the longer his ring finger is compared to his index finger, the better endowed he is.
"Based on this evidence, we suggest that digit ratio can predict adult penile size," they concluded.
Researchers believe the link between finger ratios and penis length is caused by exposure to the androgen sex hormones, specifically testosterone, at key points of development in the womb.
They wrote: "The fourth digit length and digit ratio were associated with stretched penile length."
Androgens in the womb are necessary for the normal development of the penis. Some of the genes that control the development of male sexual organs have also been found to play a role in the growth of ovaries and fingers.
The study is one of a growing number that link differences in finger length with a variety of attributes that are believed to be set as the baby is developing in the mother's womb.
It has previously been shown that fingers can give away sexual orientation. In women, the greater the difference between ring and and index finger, the more likely they are to be gay.
More recently, scientists discovered a link between cancer and finger length. Men whose ring fingers are shorter than their index fingers have been found to be significantly less prone to suffer from prostate cancer.
The link between penile length and finger-length ratios was discovered by researchers in Korea who measured the penises, both flaccid and stretched out, of 140 men.
Fingers revealed nothing about the size of a flaccid penis – though there has been a previously established link with a man's height – but there was a clear correlation between the digit ratio and stretched penises. The fingers on the right hands were measured for the study.
Volunteers for the study had to be anaesthetised for the stretched measurements to be taken because the penises were pulled out as far as possible, a procedure the researchers feared would be too painful for the patients if awake.
The research team noted in their report, published in the Asian Journal of Andrology, that stretching out a penis is indicative of its length when erect. They said previous studies had shown the stretched measurement provides "a reliable estimate" of erect length.