Recent research suggests that prenatal testosterone levels contribute to the relative sizes of fingers on the hand among men. Specifically, when testosterone levels are high, the second (index) digit of the hand tends to be small relative to the fourth (ring) finger. Thus, the finger-length ratio is correlated inversely with characteristics associated with elevated testosterone, such as aggressiveness. There is also some evidence that the ratio is predictive of aggressive behavior among females.
An important implication of this research is that much of our psychological makeup is determined by our biology. Finger lengths predict about five percent of the variance in aggressive traits, which is not trivial.
Of equal interest is the finding that the ratio of lengths between the fourth (ring) and fifth (little) fingers is related to neuroticism among both females and males. The ratio between second and fourth fingers also appears to be predictive of depression among men.
Given that aggressiveness among traders is plausibly related to risk seeking and neuroticism is related to emotional self control, it may well be that one's trading psychology is related to simple finger measurements. The ability to aggressively take controlled and prudent risk is surely an important component of trading success.
A Personality Test for Traders
Interpreting the Personality Questionnaire for Traders