指长比为手指底部折痕到手指前端中点(如图)不同手指长度的比例。[1] 一些科学家认为食指和无名指的指长比(2D:4D)受胎儿暴露在子宫的雄激素（如睾酮）和雌激素不同程度结果所致，因而2D:4D被视为产前雄激素曝露量的天然测量指标,低2D:4D比率表示胎儿在子宫内暴露在较高浓度的雄激素环境。[2][3] 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. 较长的食指将导致指长比大于1，而较长的无名指将导致小于1的比率。
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,[6][7] with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930,[8] after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr Glenn Wilson（英语：Glenn Wilson (psychologist)） of 伦敦国王学院 published a study examining the correlation between assertiveness in women and their digit ratio.[9] This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.[10] Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.[9] In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children[11] 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 异速生长 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,"[12] 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."[13]
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.[14] 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.[15]
Importantly, there has been no correlation between the sex hormone levels of an adult and the individual’s 2D:4D,[16] 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,[16] 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.
指长比的正态分布
A visualization of the distributions: Men (blue), women (green), and the whole population (red). Based on a population at University of Alberta and assuming normal distribution.
Males with CAH have more masculine (smaller) digit ratios than control males,[18][19] which also suggests that prenatal androgens affect digit ratios, since amniocentesis samples show that prenatal levels of testosterone are in the high normal range in males with CAH, while levels of the weaker androgen androstenedione（英语：androstenedione） are several fold higher than in control males.[22][23][24] These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
Digit ratio in men with 克氏综合征, who have reduced testosterone secretion throughout life compared to control males, are greater (i.e., more feminine) than in their fathers or control males.[25]
Digit ratio in men correlates with genetic variation in the 雄激素受体 gene.[26] Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater, more feminine, digit ratios. There are reports of a failure to replicate this finding.[27] However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by secreting more testosterone,[28] probably as a result of reduced negative feedback on gonadotropins. Thus, it is not clear that 2D:4D would be expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen.
XY individuals with androgen insensitivity syndrome（英语：androgen insensitivity syndrome） (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratios is unrelated to the Y chromosome per se.[29]
The sex difference in 2D:4D is present before birth in humans,[30][31] which rules out any social influences that might affect digit growth differentially in the two sexes. Because all somatic sex differences in mammals to date have been found to be due to either androgenic masculinization or effects of the sex chromosomes, and as the AIS finding rules out a role for sex chromosomes in the sex difference in digit ratios, the prenatal sexual dimorphism also indicates that androgens act before birth to affect digit ratios. [来源请求]
The ratio of testosterone to 雌二醇 measured in 33 羊膜穿刺术 samples correlates with the child's subsequent 2D:4D ratio.[32]
In 雉s, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the 卵.[33]
Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.[34]
There is evidence that this reflects 胎儿 exposure to the 激素s 睾酮[35] and 雌激素.[来源请求]
Several studies present evidence that digit ratios are heritable.[36][37]
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, and when examined researchers found no difference in estrogen levels between males and females.[38]
指长比影响的解释
目前尚不清楚为何产前激素会影响指长比. 还有其它类似特征的证据,例如耳声传射和手臂到躯干长度之比也显示了类似的效果. 同源异形基因 responsible for both digit and penis development[39] have been implicated in affecting these multiple traits (基因多效性). Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes. Likewise, it is unclear why digit ratio on the right hand should be more responsive than that on the left hand, as is indicated by the greater sex difference on the right than the left.[40]
It should be noted, however, that the errors associated with each given 2D:4D mean are such that there could in fact be no ethnic variation. For example, the ratio for Han children (0.954±−0.032) allows for a ratio as low as 0.922. The ratio for Jamaican children (0.935±0.035) allows for a ratio as high as 0.970. Research of higher accuracy is therefore necessary to determine whether or not there is ethnic variation in 2D:4D ratios.
Sexual preference for more masculine men among women[96] and gay men[109] with high digit ratio; a preference for a masculine facial type means a more "feminized" mindset.
Lesbians are more likely to be femme（英语：butch and femme） and less likely to be butch（英语：butch and femme） with a high digit ratio.[98][110] Identical female twins discordant for sexual orientation still show the difference (lesbian less than straight, on average) in digit ratio.[100][111]
Homosexuality for men, according to some studies.[99][107][112] Other studies have disputed this; some have shown that the digit ratio in homosexual men is similar to,[97][104][105][106][113] or lower than,[101][103][108] that of heterosexual men. One study concluded that differences are dependent on geographical variation, with gay men having lower or similar ratios to straight men in Europe, but higher or similar in the United States.[114] But this finding has been questioned in a meta-analysis including 18 studies, which suggested that ethnicity, rather than geography, explained the differences previously found in men of different sexual orientations. The meta-analysis concluded that there are no significant sexual orientation differences in digit ratio exist in men.[115]
Close
参见：Biology and sexual orientation和Neuroscience and sexual orientation § 2D:4D digit ratio
跨性别女性的变性者
一份来自德国的研究发现，跨性别女性的变性者有较顺性别男性高的指长比, but one that was comparable to cisgender women.[116]
指长比与发展
There is some evidence that 2D:4D ratio may also be indicative for human development and growth. Ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.[117] Moreover, studies about 2D:4D correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development. Prenatal sex steroid ratios (in terms of 2D:4D) and actual chromosomal sex dimorphism were found to operate differently on human faces, but affect male and female face shape by similar patterns.[118] Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express greater levels of facial symmetry（英语：facial symmetry）.[119] However, exposure to very high levels of testosterone and/or estrogen in the womb may have negative effects as well.
Dennis McFadden与其同僚宣称某些人科动物(如大猩猩和黑猩猩)后肢指长比具有两性异形的现象。[99]
Emma Nelson和Susanne Shultz目前正在调查2D:4D与灵长类动物的交配战略和人类社会演化的关联性。[123]
Sexual dimorphism in hind limb 2D:4D has been demonstrated in mice by two studies by both John Manning and Marc Breedlove（英语：Marc Breedlove）'s research groups. There is some evidence to suggest that this effect is not seen in all mouse strains.[来源请求]
Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.[来源请求]
Front limb D2:D3 has shown to be influenced by prenatal alcohol exposure in female rats.[来源请求]
Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.[124]
Peter L. Hurd（英语：Peter L. Hurd）, Theodore Garland, Jr.（英语：Theodore Garland, Jr.）, and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolution（英语：experimental evolution）). These high-runner mice exhibit increased 2D:4D. This apparent "feminization" is opposite to the relation seen between 2D:4D and physical fitness in human beings, and is difficult to reconcile with the idea that 2D:4D is a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes.[125]
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Manning JT, Bundred PE. The ratio of second to fourth digit length and age at first myocardial infarction in men: a link with testosterone?. British Journal of Cardiology. 2001, 8 (12): 720–3. ISSN 0969-6113.
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Fink B, Manning JT, Neave N, Tan U; Manning; Neave; Tan. Second to fourth digit ratio and hand skill in Austrian children. Biological Psychology. November 2004, 67 (3): 375–84. PMID 15294393. doi:10.1016/j.biopsycho.2004.03.012.
Benderlioglu Z, Nelson RJ; Nelson. Digit length ratios predict reactive aggression in women, but not in men. Hormones and Behavior. December 2004, 46 (5): 558–64. PMID 15555497. doi:10.1016/j.yhbeh.2004.06.004.
Beech, John R.; MacKintosh, Isla C. Do differences in sex hormones affect handwriting style? Evidence from digit ratio and sex role identity as determinants of the sex of handwriting. Personality and Individual Differences. July 2005, 39 (2): 459–68. doi:10.1016/j.paid.2005.01.024.
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Austin, Elizabeth J.; Manning, John T.; McInroy, Katherine; Mathews, Elizabeth. A preliminary investigation of the associations between personality, cognitive ability and digit ratio. Personality and Individual Differences. November 2002, 33 (7): 1115–24. doi:10.1016/S0191-8869(02)00002-8.
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S.J. Robinson, J.T. Manning; Manning. The ratio of 2nd to 4th digit length and male homosexuality. Evolution and Human Behavior. 2000, 21 (5): 333–345. PMID 11053694. doi:10.1016/S1090-5138(00)00052-0.
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Churchchill AJG, Manning JT, Peters M; Manning; Reimers. The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D). Archives of Sexual Behavior. 2007, 36 (2): 251–260. PMID 17394056. doi:10.1007/s10508-006-9166-8.
M.V. Voracek, J.T. Manning & I. Ponocny; Manning; Ponocny. Digit ratio (2D:4D) in homosexual and heterosexual men from Austria. Archives of Sexual Behaviour. 2005, 34 (3): 335–340. PMID 15971016. doi:10.1007/s10508-005-3122-x.
Ronalds, G; Phillips, DI; Godfrey, KM; Manning, JT. The ratio of second to fourth digit lengths: A marker of impaired fetal growth?. Early human development. 2002, 68 (1): 21–6. PMID 12191526. doi:10.1016/s0378-3782(02)00009-9.
Fink, Bernhard; Manning, John T.; Neave, Nick; Grammer, Karl. Second to fourth digit ratio and facial asymmetry. Evolution and Human Behavior. March 2004, 25 (2): 125–32. doi:10.1016/S1090-5138(03)00084-9.
Talarovičová A, Kršková L, Blažeková J; Krsková; Blazeková. Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood. Hormones and Behavior. January 2009, 55 (1): 235–9. PMID 19022257. doi:10.1016/j.yhbeh.2008.10.010.
