Risk-taking behavior in men is closely associated with levels of testosterone. Erin Stenstrom and his colleagues from Concordia University found that not only did testosterone levels in the womb affect men’s proclivity for risk-taking, but that those prenatal testosterone levels could be predicted by looking at men’s finger length.
The researchers surveyed 413 male and female students for finger length and risky behavior, such as aggressive sports play. They found correlations for the men, but not for the women. In particular, the length of the index finger in comparison to the other finger lengths tells the tale. Baby boys who were exposed to higher amounts of testosterone in the womb had smaller ratios of index finger length to ring finger length, and of index finger length to the length of the other four fingers combined
As to why the effect is only seen in men, Gad Saad explains:
A possible explanation for the null effects in women is that they do not engage in risky behavior as a mating signal, whereas men do.