Gad Saad, who likes to link evolution to consumption and marketing phenomena, says the length between the second and fourth finger is an indicator of high levels of prenatal testosterone and therefore risk-taking and potential financial success in adult men.

His hypothesis suggests that alpha males may take greater risks in relationships, in sports and in the financial markets, which we all sort of knew because that is the origin of the term 'alpha male'.   

Saad, Concordia University Research Chair in Evolutionary Behavioral Sciences and Darwinian Consumption as well as a marketing professor at the John Molson School of Business, and his team surveyed risk-taking among 413 male and female students - because surveys are how all evolutionary research is apparently done in psychology, it seems.

"Prenatal testosterone exposure not only influences fetal brain development," adds study co-author and graduate student, Zack Mendenhall, "but it also slows the growth of the index finger relative to the sum of the four fingers excluding the thumb."

They say the change in finger length produced by testosterone provides a handy measure of prenatal testosterone exposure. The study compared the length of the index finger with all four digits (known as the rel2 ratio) and found that those with lower ratios were more likely to engage in risk-taking. These findings were further confirmed by the additional measurement of the ratio between the index and ring finger. These correlations were only observed in men. 

"A possible explanation for the null effects in women is that they do not engage in risky behaviour as a mating signal, whereas men do," says Saad.  "Previous studies have linked high testosterone levels with risky behaviour and financial success.  We investigated the relationship between prenatal testosterone and various risk proclivities. Our findings show an association between high testosterone and risk-taking among males in three domains: recreational, social and financial."

"Since women tend to be attracted to men who are fit, assertive and rich, men are apt to take risks with sports, people and money to be attractive to potential mates. What's interesting is that this tendency is influenced by testosterone exposure – more testosterone in the womb can lead to more risks in the rink, the bar and the trading floor in later in life," says first author and Concordia doctoral student, Eric Stenstrom.

Citation: Eric Stenstrom, Gad Saad, Marcelo V. Nepomuceno and Zack Mendenhall, 'Testosterone and domain-specific risk: Digit ratios (2D:4D and rel2) as predictors of recreational, financial, and social risk-taking behaviors', Personality and Individual Differences 2010