The longer a man's fourth or ring finger is compared to his index finger, the more likely he is to be judged attractive by women, according to a study released Wednesday.
The results, published in the British Royal Society's journal Biological Sciences, unveil intricate links between foetal exposure of males to hormones, the development of certain physical traits, and what turns on the opposite sex.
It also adds to a growing body of research -- conducted under the banner of evolutionary psychology -- suggesting that the drivers of human behaviour are found, more than previously suspected, in "nature" rather than "nurture."
Earlier studies had already shown that the size ratio between the fourth and second fingers, especially of the right hand, is a reliable indicator of the extent a man was exposed to testosterone while still in the womb.
The bigger the gap between a longer ring finger and a shorter index, the greater the likely impact of the hormone.
For the new study, scientists led by Camille Ferdenzi of the University of Geneva designed an experiment to find out if women are drawn to the telltale signs of high testosterone levels in men -- a symmetrical face, a deeper voice, a particular body odour -- who have this more "masculine" finger configuration.
More than 80 women university students between 18 and 34 looked at pictures of 49 similarly aged men, and were asked to evaluate them for masculinity and attractiveness.
Smaller groups of women listened to recordings of the male voices, and smelled samples of their body odour, taken from cotton pads worn under the arm for 24 hours.
"The aim was to understand what makes a man attractive," and whether at least some of those qualities "were in part conditioned by the foetal environment," Ferdenzi said in an interview.
For the visual test, the results were unambiguous. "The longer the ring finger compared to the index -- that is, the greater the exposure to testosterone -- the more attractive the face was rated," she said by phone.
"We also found that attractiveness and symmetry in the face are highly correlated."
Such a preference might have evolved to boost a female's chances of reproductive success through mating with a more virile partner, she said.
Surprisingly, however, women did not consistently tag the same men as "masculine".
Nor did their preferences for voice or odours correspond to the longer ring-finger males.
"There wasn't any relation between the 2D-4D" -- 2nd digit, 4th digit -- "ratio and the reactions of the women to odour," Ferdenzi said.
One reason, she speculated, may be that voice and body odour are more dependent on fluctuating levels of adult testosterone than on pre-natal testosterone.
The ring-index finger ratio has also proven to be a useful indicator for gauging the risk of prostate cancer, likewise tied to high levels of testosterone.
Research published in December showed that the chances of developing the disease drop by a third in men whose index finger is longer than their ring finger.
Other studies have also found a link between exposure to hormones before birth and the development of other diseases, including breast cancer and osteoarthritis.
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