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無名指長於食指, 意味胎兒期時在子宮中暴露於較高的睪固酮環境

指長比為手指底部摺痕到手指前端中點(如圖)不同手指長度的比例。[1] 一些科學家認為食指和無名指的指長比(2D:4D)受胎兒暴露在子宫的雄激素（如睾酮）和雌激素不同程度結果所致，因而2D:4D被視為產前雄激素曝露量的天然測量指標,低2D:4D比率表示胎兒在子宮內暴露在較高濃度的雄激素環境。[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. 較長的食指將導致指長比大於1，而較長的無名指將導致小於1的比率。

指長比2D:4D為两性异形，無論女性或男性，食指通常較短，但一般而言男性的指長差大於女性(意謂男性的2D:4D比率較女性小).[4]

部分研究已顯示2D:4D與各種生理和行為特徵之間具有關聯性.[5]

指長比研究史

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.

指長比的常態分布

 
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.

根據一份來自阿爾伯塔大學136名男性與137名女性的研究[17]

設其為正态分布, 2D:4D比率的95%信賴區間，男性落於0.889-1.005，而女性落於0.913-1.017 .

雄激素影響指長比的證據

患有先天性腎上腺增生症 (CAH)的女性具有較高濃度的雄激素, 她們有著較平均值低的2D:4D比.[18][19][20] 其它可能的生理影響包括陰蒂肥厚與陰道淺短.[21]

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. [來源請求]

The ratio of testosterone to 雌二醇 measured in 33 羊膜穿刺術 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 胎兒 exposure to the 激素s 睾酮[35] and 雌激素.[來源請求]

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]

指長比影響的解釋

目前尚不清楚為何產前激素會影響指長比. 還有其它類似特徵的證據,例如耳聲傳射和手臂到軀幹長度之比也顯示了類似的效果. 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]

2D:4D的種族差異

Manning與其同事已表明，2D:4D比率在不同民族之間的差異很大。Manning等人發現漢族兒童有最高的2D:4D平均值(0.954±−0.032), 柏柏人次之(0.950±0.033), 然後是維吾爾族(0.946±0.037), 牙買加兒童的2D:4D平均比值最低(0.935±0.035).[41][42] 不同種族間的差異遠大於性別差異; Manning說：「一個 波蘭人跟一個芬兰人的差異，大於一個男人跟一個女人」[43]

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.

2D:4D與特質間的關聯

有些作者認為指長比與人生中的健康、行為，甚或 人类的性 有關聯. 以下是一些非全面的清單，整理一些指長比與特質有關的發表.

低指長比

高指長比

生理與疾病
  • Increased risk of 前列腺癌 and prostate diseases in males.[44][45]
  • Slower utero fetal development in both sexes.[44]
  • Increased reproductive success in males.[46]
精神障礙
生理和競爭行為 運動中有較多的攻擊性行為.[68]
  • Reduced performance in sports[69]
  • Reduced financial trading ability[70]
  • Right handedness skills[71] (inconclusive)[72]
認知與人格
管理
感官知覺
  • Smell perception[93]
  • Color perception[94]
  • Tactile perception[95]
性取向
  • 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]
参见：Biology and sexual orientation和Neuroscience and sexual orientation § 2D:4D digit ratio

跨性別女性的變性者

一份來自德國的研究發現，跨性別女性的變性者有較順性別男性高的指長比, but one that was comparable to cisgender women.[116]

指長比與發展

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.

舊石器時代手模中的指長比

部分研究者正試圖藉由鑽研史前歐洲和印尼的石洞壁画中舊石器時代的手模來研究當時的2D:4D.[120][121][122]

其它動物

  • Dennis McFadden與其同僚宣稱某些人科動物(如大猩猩和黑猩猩)後肢指長比具有兩性異形的現象。[99]
  • Emma Nelson和Susanne Shultz目前正在調查2D:4D與靈長類動物的交配戰略和人類社會演化的關聯性。[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.[來源請求]
  • 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.[來源請求]
  • Front limb D2:D3 has shown to be influenced by prenatal alcohol exposure in female rats.[來源請求]
  • 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]

參見

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  45. ^ Sudhakar, Hanumanthaiah Honnamachanahalli; Manjunatha, Revanna; Madhusudhana, Heragu Rangegowda. Relationship Between Second to Fourth Digit Ratios and Benign Prostatic Hyperplasia in Aging Men. Journal of Clinical and Diagnostic Research : JCDR. 2015-05-01, 9 (5): PC01–PC03. ISSN 2249-782X. PMC 4484107  . PMID 26155515. doi:10.7860/JCDR/2015/11992.5937. 
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  60. ^ 60.0 60.1 Carré, Justin M.; Ortiz, Triana L.; Labine, Brandy; Moreau, Benjamin J. P.; Viding, Essi; Neumann, Craig S.; Goldfarb, Bernard. Digit ratio (2D:4D) and psychopathic traits moderate the effect of exogenous testosterone on socio-cognitive processes in men. Psychoneuroendocrinology. 2015-12-01, 62: 319–326. ISSN 1873-3360. PMID 26356040. doi:10.1016/j.psyneuen.2015.08.023. 
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  71. ^ 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. 
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  75. ^ Butovskaya, Marina; Burkova, Valentina; Karelin, Dmitry; Fink, Bernhard. Digit ratio (2D:4D), aggression, and dominance in the Hadza and the Datoga of Tanzania. American Journal of Human Biology. 2015-10-01, 27 (5): 620–627. ISSN 1520-6300. PMID 25824265. doi:10.1002/ajhb.22718. 
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  78. ^ Neave N, Laing S, Fink B, Manning JT; Laing; Fink; Manning. Second to fourth digit ratio, testosterone and perceived male dominance. 皇家學會報告. October 2003, 270 (1529): 2167–72. PMC 1691489  . PMID 14561281. doi:10.1098/rspb.2003.2502. 
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  80. ^ Sluming, Vanessa A.; Manning, John T. Second to fourth digit ratio in elite musicians Evidence for musical ability as an honest signal of male fitness. Evolution and Human Behavior. January 2000, 21 (1): 1–9. doi:10.1016/S1090-5138(99)00026-4. 
  81. ^ Nye, John V. C.; Androuschak, Gregory; Desierto, Desirée; Jones, Garett; Yudkevich, Maria. 2D:4D Asymmetry and Gender Differences in Academic Performance. PLoS ONE. 2012, 7 (10): e46319 [2014-09-20]. PMID 23056282. doi:10.1371/journal.pone.0046319. 
  82. ^ Finger Ratio Predicts Maths Ability?. http://ibmathsresources.com/. [2014-09-20]. 
  83. ^ 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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  87. ^ Brosnan MJ. Digit ratio as an indicator of numeracy relative to literacy in 7-year-old British schoolchildren. British Journal of Psychology. February 2008, 99 (Pt 1): 75–85. PMID 17535470. doi:10.1348/000712607X197406. 简明摘要 – LiveScience（英语：LiveScience） (22 May 2007). 
  88. ^ Fink, Bernhard; Manning, John T; Neave, Nick. Second to fourth digit ratio and the ‘big five’ personality factors. Personality and Individual Differences: 495–503. doi:10.1016/j.paid.2003.09.018. 
  89. ^ Borniger, Jeremy C.; Chaudhry, Adeel; Muehlenbein, Michael P. Relationships among Musical Aptitude, Digit Ratio and Testosterone in Men and Women. PLoS ONE. 2013-03-08, 8 (3). ISSN 1932-6203. PMC 3592910  . PMID 23520475. doi:10.1371/journal.pone.0057637. 
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  96. ^ 96.0 96.1 Csathó A, Osváth A, Bicsák E, Karádi K, Manning J, Kállai J; Osváth; Bicsák; Karádi; Manning; Kállai. Sex role identity related to the ratio of second to fourth digit length in women. Biological Psychology. February 2003, 62 (2): 147–56. PMID 12581689. doi:10.1016/S0301-0511(02)00127-8. 
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  98. ^ 98.0 98.1 Tortorice JL. Written on the body: butch vs. femme（英语：butch and femme） lesbian gender identity and biological correlates of low digit ratio. Rutgers University. 2002. OCLC 80234273. 
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  102. ^ Putz, David A.; Gaulin, Steven J. C.; Sporter, Robert J.; McBurney, Donald H. Sex hormones and finger length: What does 2D:4D indicate? (PDF). Evolution and Human Behavior. May 2004, 25 (3): 182–99. doi:10.1016/j.evolhumbehav.2004.03.005. 
  103. ^ 103.0 103.1 Rahman Q. Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation. Psychoneuroendocrinology. May 2005, 30 (4): 382–91. PMID 15694118. doi:10.1016/j.psyneuen.2004.10.006. 
  104. ^ 104.0 104.1 Kraemer B, Noll T, Delsignore A, Milos G, Schnyder U, Hepp U; Noll; Delsignore; Milos; Schnyder; Hepp. Finger length ratio (2D:4D) and dimensions of sexual orientation. Neuropsychobiology. 2006, 53 (4): 210–4. PMID 16874008. doi:10.1159/000094730. 
  105. ^ 105.0 105.1 Wallien MS, Zucker KJ, Steensma TD, Cohen-Kettenis PT; Zucker; Steensma; Cohen-Kettenis. 2D:4D finger-length ratios in children and adults with gender identity disorder. Hormones and Behavior. August 2008, 54 (3): 450–4. PMID 18585715. doi:10.1016/j.yhbeh.2008.05.002. 
  106. ^ 106.0 106.1 Grimbos T, Dawood K, Burriss RP, Zucker KJ, Puts DA; Dawood; Burriss; Zucker; Puts. Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women. Behav Neurosci. 2010, 124 (2): 278–287. PMID 20364887. doi:10.1037/a0018764. 
  107. ^ 107.0 107.1 Hirashi K, Sasaki S, Shikishima C, Ando J; Sasaki; Shikishima; Ando. The second to fourth digit ratio (2D:4D) in a Japanese twin sample: heritability, prenatal hormone transfer, and association with sexual orientation. Arch Sex Behav. Jun 2012, 41 (3): 711–24. PMID 22270254. doi:10.1007/s10508-011-9889-z. 
  108. ^ 108.0 108.1 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. 
  109. ^ McIntyre MH. Digit ratios, childhood gender role behavior, and erotic role preferences of gay men. Archives of Sexual Behavior. December 2003, 32 (6): 495–6. PMID 14627046. doi:10.1023/A:1026054625638. 
  110. ^ Brown WM, Finn CJ, Cooke BM, Breedlove SM; Finn; Cooke; Breedlove. Differences in finger length ratios between self-identified 'butch' and 'femme' lesbians (PDF). Archives of Sexual Behavior. February 2002, 31 (1): 123–7. PMID 11910785. doi:10.1023/A:1014091420590. 
  111. ^ Hiraishi K, Sasaki S, Shikishima C, Ando J.; Sasaki; Shikishima; Ando. The second to fourth digit ratio (2D:4D) in a Japanese twin sample: heritability, prenatal hormone transfer, and association with sexual orientation. Archives of Sexual Behavior. 2012, 41 (3): 711–24. PMID 22270254. doi:10.1007/s10508-011-9889-z. 
  112. ^ 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. 
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  115. ^ Teresa Grimbos; Kenneth Zucker（英语：Kenneth Zucker）; Khytam Dawood; Robert P. Burriss. Sexual Orientation and the Second to Fourth Finger Length Ratio: A Meta-Analysis in Men and Women (PDF). Behavioral Neuroscience. 2010, 124 (2): 278 –287. PMID 20364887. doi:10.1037/a0018764. 
  116. ^ Schneider HJ, Pickel J, Stalla GK; Pickel; Stalla. Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure. Psychoneuroendocrinology. February 2006, 31 (2): 265–9. PMID 16140461. doi:10.1016/j.psyneuen.2005.07.005. 
  117. ^ 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. 
  118. ^ Fink B; Grammer K; Mitteroecker P; et al. Second to fourth digit ratio and face shape. 皇家學會報告. October 2005, 272 (1576): 1995–2001. PMC 1559906  . PMID 16191608. doi:10.1098/rspb.2005.3179. 
  119. ^ 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. 
  120. ^ Snow, Dean R. Sexual dimorphism in Upper Palaeolithic hand stencils. Antiquity. 2006, 80 (308): 390–404. 
  121. ^ Chazine, Jean-Michel; Noury, Arnaud. Sexual Determination of Hand Stencils at the Masri II Cave. Inora Newsletter. 2006, 44: 21–6. 
  122. ^ Nelson, Emma C.; Manning, John T.; Sinclair, Anthony G. M. Using the length of the 2nd to 4th digit ratio (2D:4D) to sex cave art hand stencils: factors to consider (PDF). Before Farming. 2006, 1 (6): 1–7. 
  123. ^ Nelson, Emma. Investigating relationships between the second-to-fourth digit ratio (2D:4D), social and bonding behaviours in non-human anthropoids. [29 October 2009]. （原始内容存档于8 November 2011）. [自述来源]
  124. ^ 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. 
  125. ^ Yan RH, Malisch JL, Hannon RM, Hurd PL, Garland T; Malisch; Hannon; Hurd; Garland Jr. Svensson, Erik I. , 编. Selective Breeding for a Behavioral Trait Changes Digit Ratio. PLoS ONE. 2008, 3 (9): e3216. PMC 2528935  . PMID 18797502. doi:10.1371/journal.pone.0003216. 

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