“The digit ratio is the ratio of the lengths of different digits or fingers. The 2D:4D ratio is the most studied digit ratio and is calculated by dividing the length of the index finger of a given hand by the length of the ring finger of the same hand. A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.” Source: Wikipedia
Although it has been extensively studied since the late 1800s, and it is known that the ratio is affected by prenatal hormones, there is as yet no explanation as to why the hormone levels affect the ratio.
Analysis of the ratio has been linked to a range of extremely diverse characteristics - such as mathematical competence, video-game addiction and the success of Sumo wrestlers - many of the findings are yet to be generally accepted.