Research published in the journal of Personality and Individual Differences, suggest that impulsive men who take risks in both their personal and processional lives have ratio of less than one when the length of their second digit (index finger) is divided by that of their fourth digit (ring finger). This has been dubbed the 2D/4D ratio.
For example, if the second digit was 7 .6 cm long and the 4th 8.5 cm in length, the ratio would be 7.6/8.5 = 0.89 indicating a higher level of risk taking and impulsivity that in a man whose 2nd digit was 7.5 cm and whose 4th was 7 giving a ratio of 1.1.
In other words, the lower the ratio the more impulsive a risk taker the man is likely to be.
The most likely explanation for the 2D/4D effect is that prenatal exposure to the male hormone testosterone not only slows the growth of the index finger relative to the other four fingers, excluding the thumb, but also influences the development of the foetal brain.
As a result finger length offers a, literally, handy measure of prenatal testosterone exposure.
In the study, the index fingers of some 400 male and female students were compared with their ring fingers. Researchers reported that that men, but not women, with low ratios were more likely to act impulsively and engage in risky activities.
Senior researcher on the study, Gad Saad, of Concordia University and the John Molson School of Business, commented: “Our findings show an association between high testosterone and risk-taking among males in three domains: recreational, social and financial.”
Why are the effects found only in men?
The researchers suggest a possible explanation lies in the fact that, unlike males, women do not use risky behaviour as a mating signal.