The ratio of the length of the index finger to the length of the fourth finger (ring finger), measuring from the basal crease near the palm to the tip of the finger. Counting the thumb as digit 1, the ratio compares digits 2 and 4 and is sometimes called the 2D : 4D ratio. In men, the index finger is usually shorter than the fourth finger, the ratio averaging about 0.96, whereas in women, the ratio is close to 1.00. The English evolutionary biologist John T. Manning (born 1942) discovered this sexual dimorphism and first described it in an article co-authored with others in the journal Human Reproduction in 1998. A low digit ratio tends to be associated with high concentrations of testosterone in the blood. The sex difference is believed to arise from the effects of testosterone on the developing embryo during the 8th to the 13th week after conception, promoting growth of the fourth finger. In his book Digit Ratio: A Pointer to Fertility, Behavior and Health (2000), Manning summarized further evidence suggesting that, in men, a low digit ratio tends to be associated with fertility, assertiveness or aggressiveness, proclivity for homosexuality or bisexuality, and musical and sporting aptitude, and a high digit ratio with increased risk of heart disease; in women, a low digit ratio tends to be associated with assertiveness or aggressiveness and a proclivity for homosexuality or bisexuality, and a high digit ratio with fertility and an increased risk of breast cancer. See also induction (5). 2D:4D abbrev.