Digit ratio

For the observation about the frequencies of leading digits of numbers, see Benford's law.

Hand with index finger shorter than the ring finger, resulting in a small 2D:4D ratio.

Image shows the measurement procedure of dorsal digit length using vernier calipers. The hand is placed on the edge of a table so that fingers are rested on table while making an angle of 90 degrees to the palm.

The digit ratio is the ratio taken of the lengths of different digits or fingers on a hand.

The most commonly studied digit ratio is that of the 2nd (index finger) and 4th (ring finger), also referred to as the 2D:4D ratio, measured on the palm side. It is proposed that the 2D:4D ratio indicates the degree to which an individual has been exposed to androgens during key stages of fetal development. A lower ratio (relatively shorter index finger) has been associated with higher androgen exposure, which would be the physiological norm for males but may also occur in some exceptional circumstances in females. The latter include developmental disorders such as congenital adrenal hyperplasia.

The 2D:4D ratio has been postulated to correlate with a range of physical and cognitive traits in childhood and adulthood, including personality traits such as assertiveness in women, aggressiveness in men, and cognitive abilities such as numerical skills. It has also been shown to vary considerably between racial groups with males having, on average, lower 2D:4D ratio than females.

Studies in this field have drawn criticism over questionable statistical significance and difficulties in reproducing their findings as well as lack of high quality research protocols.