Ring finger length has previously been linked to sperm count, aggressive behaviour, sexual orientation and sports prowess, while high levels of testosterone are associated with a higher sex drive.
Now, researchers at Florida University have found that the longer a man`s ring finger is in relation to his index finger, the more attractive he is to be to the opposite sex.
They discovered its size of the fourth finger is actually determined by levels of male and female sex hormones in the womb. The more testosterone -- the male sex hormone -- a baby is exposed to, the longer his ring finger is likely to be.
The study explains for the first time why men`s fourth fingers are usually longer than their index fingers, while for women it is the other way round.
Dr Martin Cohn and Zhengui Zheng, who led the study, say the fourth finger is packed with receptors for sex hormones.
In their experiment, the researchers took groups of pregnant mice -- a species with a similar finger-length ratio to humans -- and controlled the genes which affect their oestrogen and testosterone levels.
Increased testosterone gave the baby mice a bigger right paw, but made little difference to the left. This, the team says, is because the right paw -- like the human left hand -- is more receptive to sex hormones.
By comparison, mice exposed to higher levels of oestrogen had a shorter right paw and a more "feminised" appearance. All the male mice in this group had genital defects.
Dr Cohn said: "Sex hormones affect finger length, and the ratio is fixed before the bone has even developed. This happens during a narrow window of development but it stays fixed throughout life."
The findings have been published in the `Proceedings of the National Academy of Sciences` journal.
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