The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens e.g. testosterone while in the uterus and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher androgen exposure.
Source: Wikipedia/Digit_ratio

Pablo Brañas speaks about several research papers that discuss fetal exposure to testosterone (digit ratio) and its implications on various characteristics of human behavior, such as performance in math, sports and arts. He mentions works that link this topic to risk aversion thus determining certain conduct when making economic decisions, also noticing how ratios vary among different ethnic groups. He shares two papers carried out by him, along with fellow scholars. One shows a different approach to sampling for this type of investigation; and the other presents a hypothesis about the impact of testosterone exposure on altruism. After commenting on the results, he discusses the ongoing investigative work on second to fourth digit ratio (2D:4D), explaining the relation between certain questions and answers, depending on people's 2D:4D ratio.