Digit ratio

The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the midpoint of bottom crease (where the finger joins the hand) to the tip of the finger. 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 prenatal androgen exposure. There are also studies that suggest that the 2D:4D ratio is also influenced by prenatal estrogen exposure, and that it thus correlates negatively not with prenatal testosterone alone, but rather with the prenatal testosterone to estrogen ratio (T:O). The 2D:4D ratio 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 longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio lower than 1. The 2D:4D digit ratio is sexually dimorphic: although the second digit is typically shorter in both females and males, the difference between the lengths of the two digits is greater in males than in females. A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits. That a greater proportion of men have shorter index fingers than ring fingers than do women was noted in the scientific literature several times through the late 1800s, with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930, after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr Glenn Wilson of King's College, London, published a study examining the correlation between assertiveness in women and their digit ratio. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero. In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children and further developed the idea that the index was a marker of prenatal sex hormones. Since then research on the topic has burgeoned around the world. A 2009 study in Biology Letters argues: 'Sexual differences in 2D:4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women,' which may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios. A 2011 paper by Zhengui Zheng and Martin J. Cohn reports 'the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development'.The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood. During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio. Importantly, there has been no correlation between the sex hormone levels of an adult and the individual's 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon.