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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. 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 prenatal androgen exposure. The 2D:4D ratio is calculated by dividing the length of the index finger of the right hand by the length of the ring finger of the right hand. A longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio of less than 1. The 2D:4D digit ratio is sexually dimorphic: while 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. ==History of digit ratio research== 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. A major problem with the research on this topic comes from the contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio,〔 but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation with the 2D:4D. So there should be a correlation with one or the other but not both. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「digit ratio」の詳細全文を読む スポンサード リンク
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