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The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the 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 androgen exposure.
2D:4D is sexually dimorphic: in males, the second digit tends to be shorter than the fourth, and in females the second tends to be the same size or slightly longer than the fourth. However, homosexual men tends to have a higher 2D:4D digit ratios than heterosexual men. This trait may be better considered 'sexually differentiated' rather than 'sexually dimorphic' in recognition of the fact that the effect size is fairly small (2D:4D distributions of the two sexes overlap to a great degree), especially compared to other sexually dimorphic traits e.g. height.
History of digit ratio researchEdit
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 large sample (201 men, 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 . Digit ratio research has since exploded, as perusal of citations at the end of this article will attest. In 2002 Manning of the University of Liverpool published a book summarizing all such research on the topic to that point, asserting that prenatal testosterone affects digit ratios and their psychological correlates.
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.
Evidence of androgen effect on digit ratioEdit
- Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average.
- Males with CAH have more masculine (smaller) digit ratios than control males , 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 are several fold higher than in control males . These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
- Digit ratio in men correlates with genetic variation in the androgen receptor gene. Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios.
- XY individuals with 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 is unrelated to the Y chromosome per se.
- The sex difference in 2D:4D is present before birth in humans, 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.
- The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlates with the child's subsequent 2D:4D ratio. The effect of a child's sex is confusingly controlled for in this study.[original research?]
- In pheasants, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the egg.
Explanation of the digit ratio effectEdit
It is not clear why digit ratio ought to be influenced by prenatal hormones. There is evidence of other similar traits, e.g. otoacoustic emissions and arm-to-trunk length ratio, which show similar effects. Hox genes responsible for both digit and penis development  have been implicated in this pleiotropy. 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.
Geographic/Ethnic variation in 2D:4DEdit
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups. This variation is far larger than the differences between sexes, in Manning's words "There’s more difference between a Pole and a Finn than a man and a woman."Template:Cite quote The variation appears to be correlated with latitude, such that more northerly populations have higher digit ratios.
Correlation between digit ratio and traitsEdit
Some authors suggest that digit ratio correlates with health, behavior, and even sexuality in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with digit ratio.
Physiology and diseaseEdit
Physical and competitive abilityEdit
Cognition and personalityEdit
- Assertiveness in women
- Masculinity of Handwriting
- Perceived 'dominance' and masculinity of man's face
- Exam scores: a higher ratio is correlated with higher exam scores among male students
- Musical ability
- Bem sex role score in women; erotic role preference in men.
- Lesbians vs. straight women; butch vs. femme lesbians.
- Gay vs straight men and the very odd Europe vs. North American straight man effect.
- Difference in digit ratio between identical female twins discordant for sexual orientation
- Fraternal birth order effect on digit ratio.
- A recent study in Germany has found a correlation between digit ratio and male to female transsexualism. Male to female transsexuals (Transwomen) were found to have a higher digit ratio than control males, but one that was comparable to control females.
Digit ratio and handedness, autism, other immune diseasesEdit
There is some evidence that testosterone facilitates the differentiation of the brain both prenatally and postnatally. There have been many extensions of this, such as the Geschwind–Galaburda hypothesis, that immune diseases and autism are related to prenatal testosterone, this also explaining why more men are left-handed, autistic, etc. than women. Prenatal exposure to testosterone is thought to promote the development of the right-hemisphere and increase the incidence of sinistrality. As such low 2D:4D was found to be associated with improved left-hand performance.
Digit ratio and developmentEdit
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. 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. However, exposure to very high levels of testosterone and/or estrogen in the womb may have also negative effects. Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express lower levels of facial symmetry.
Digit ratio and palaeolithic hand stencilsEdit
It is generally assumed that creating cave art was a male behavior. 2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves.
Digit ratio research in animalsEdit
- Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including chimpanzees, gorillas and orangutans.
- Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.
- Sexual dimorphism in hind limb 2D:4D has been demonstrated in mice by two studies by both John Manning and Marc Breedlove's research groups. There is some evidence to suggest that this effect is not seen in all mouse strains.
- 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.
- Front limb D2:D3 has shown to be influenced by prenatal alcohol exposure in female rats
- 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.
- Peter L. Hurd, 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). 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.
- ↑ Churchchill AJG, Manning JT, Peters M. 2007. The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D). Archives of Sexual Behavior. 36(2)251-260
- ↑ Ecker A (1875). Einige Bemerkungen über einen Schwankenden Charakter in den Hand des Menschen[Some remarks about a varying character in the hand of humans]. Archiv fur Anthropologie 8: 68-74.
- ↑ Baker F (1888). Anthropological notes on the human hand. The American Anthropologist 1: 51-75.
- ↑ George R (1930). Human finger types. Anatomical Record 46: 199-204.
- ↑ 5.0 5.1 5.2 (1983). Finger-length as an index of assertiveness in women. Personality and Individual Differences 4 (1): 111–2.
- ↑ Manning JT, Scutt D, Wilson J, Lewis-Jones DI (1998). The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen.. Hum Reprod 13: 3000-3004.
- ↑ Manning, John T. (2002). Digit ratio: a pointer to fertility, behavior, and health, New Brunswick, N.J: Rutgers University Press.
- ↑ Kratochvíl L, Flegr J (October 2009). Differences in the 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line. Biology Letters 5 (5): 643–6.
- ↑ 9.0 9.1 Brown WM, Hines M, Fane BA, Breedlove SM (December 2002). Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia. Hormones and Behavior 42 (4): 380–6.
- ↑ 10.0 10.1 Okten A, Kalyoncu M, Yariş N (December 2002). The ratio of second- and fourth-digit lengths and congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Early Human Development 70 (1-2): 47–54.
- ↑ Pang S, Levine LS, Cederqvist LL, Fuentes M, Riccardi VM,Holcombe JH, Nitowsky HM, Sachs G, Anderson CE, Duchon MA,Owens R, Merkatz I, New MI (1980). Amniotic fluid concentrations of delta5 and delta4 steroids in fetuses with congenital adrenal hyperplasia. J Clin Endocrinol Metab 51: 223–229.
- ↑ Dorr, H. G., and Sippell, W. G. (1993). Prenatal dexamethasone treatment in pregnancies at risk for congenital adrenal hyperplasia due to 21-hydroxylase deficiency: Effect on midgestational amniotic fluid steroid levels. J. Clin. Endocrinol. Metab. 76: 117-120.
- ↑ LWudy, S. A., Dorr, H. G., Solleder, C., Djalali, M., and Homoki, J. (1999). Profiling steroid hormones in amniotic fluid of midpregnancy by routine stable isotope dilution/gas chromatography mass spectrometry: Reference values and concentrations in fetuses at risk for 21-hydroxylase deficiency.. J. Clin. Endocrinol. Metab. 84: 2724–2728.
- ↑ (2003). The second to fourth digit ratio and variation in the androgen receptor gene. Evolution and Human Behavior 24: 399–405.
- ↑ Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S (November 2009). Fingers as a marker of prenatal androgen exposure. Endocrinology 150 (11): 5119–24.
- ↑ Malas MA, Dogan S, Evcil EH, Desdicioglu K. (2006). Fetal development of the hand, digits and digit ratio (2D:4D).. Early Hum Dev 82: 469–475.
- ↑ Galis F, Ten Broek CM, Van Dongen S, Wijnaendts LC (2009). Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D). Arch Sex Behav 38: Mar 20. [Epub ahead of print].
- ↑ Lutchmaya S, Baron-Cohen S, Raggatt P, Knickmeyer R, Manning JT (April 2004). 2nd to 4th digit ratios, fetal testosterone and estradiol. Early Human Development 77 (1-2): 23–8.
- ↑ 19.0 19.1 Romano M, Leoni B, Saino N (February 2006). Examination marks of male university students positively correlate with finger length ratios (2D:4D). Biological Psychology 71 (2): 175–82.
- ↑ McIntyre MH (2006). The use of digit ratios as markers for perinatal androgen action. Reproductive Biology and Endocrinology 4: 10.
- ↑ 21.0 21.1 Paul SN, Kato BS, Hunkin JL, Vivekanandan S, Spector TD (December 2006). The big finger: the second to fourth digit ratio is a predictor of sporting ability in women. British Journal of Sports Medicine 40 (12): 981–3.
- ↑ Gobrogge, K.L., S.M.Breedlove & K.L.Klump (2008). Genetic and environmental influences on 2d:4d finger length ratios: a study of monozygotic and dizygotic male and female twins.. Archives Sexual Behavior 37: 112-118.
- ↑ Dickman S. (Mar 1997). HOX gene links limb, genital defects.. Science 275 (5306): 1568-9.
- ↑ 24.0 24.1 Manning JT, Barley L, Walton J, et al. (May 2000). The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success. evidence for sexually antagonistic genes?. Evolution and Human Behavior 21 (3): 163–183.
- ↑ Manning JT, Stewart A, Bundred PE, Trivers RL (November 2004). Sex and ethnic differences in 2nd to 4th digit ratio of children. Early Human Development 80 (2): 161–8.
- ↑ Manning JT, Scutt D, Wilson J, Lewis-Jones DI (November 1998). The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Human Reproduction 13 (11): 3000–4.
- ↑ Manning JT, Bundred PE (2001). The ratio of second to fourth digit length and age at first myocardial infarction in men: a link with testosterone?. British Journal of Cardiology 8 (12): 720–3.
- ↑ Fink B, Manning JT, Neave N (April 2006). The 2nd-4th digit ratio (2D:4D) and neck circumference: implications for risk factors in coronary heart disease. International Journal of Obesity 30 (4): 711–4.
- ↑ Manning JT, Baron-Cohen S, Wheelwright S, Sanders G (March 2001). The 2nd to 4th digit ratio and autism. Developmental Medicine and Child Neurology 43 (3): 160–4.
- ↑ 30.0 30.1 Bailey AA, Hurd PL (March 2005). Finger length ratio (2D:4D) correlates with physical aggression in men but not in women. Biological Psychology 68 (3): 215–22.
- ↑ Arató M, Frecska E, Beck C, An M, Kiss H (January 2004). Digit length pattern in schizophrenia suggests disturbed prenatal hemispheric lateralization. Progress in Neuro-psychopharmacology & Biological Psychiatry 28 (1): 191–4.
- ↑ McFadden, D., Westhafer, J.G., Pasanen, E.G., Carlson, C.L., and Tucker, D.M. (2005). Physiological evidence of hypermasculinization in boys with the inattentive subtype of attention-deficit/hyperactivity disorder (ADHD). Clinical Neuroscience Research 5: 233-245.
- ↑ Stevenson JC, Everson PM, Williams DC, Hipskind G, Grimes M, Mahoney ER. (2007). Attention deficit/hyperactivity disorder (ADHD) symptoms and digit ratios in a college sample. Am J Hum Biol 19: 41-50.
- ↑ Martel, M.M, K.L.Gobrogge, S.M.Breedlove & J.T.Nigg (2008). Masculinized finger length ratios of boys, but not girls, are associated with attention-deficit/hyperactivity disorder. Behavioral Neuroscience 122: 273-281.
- ↑ Martel, M.M. (2009). Conscientiousness as a mediator of the association between masculinized finger-length ratios and attention-deficit/hyperactivity disorder (ADHD).. J Child Psychol Psychiatry 50: 790-798..
- ↑ Manning, John (2002). Digit ratio: a pointer to fertility, behavior, and health, New Brunswick, N.J: Rutgers University Press.Template:Pn
- ↑ Manning JT, Taylor RP (January 2001). Second to fourth digit ratio and male ability in sport: implications for sexual selection in humans. Evolution and Human Behavior 22 (1): 61–69.
- ↑ Coates JM, Gurnell M, Rustichini A (January 2009). Second-to-fourth digit ratio predicts success among high-frequency financial traders. Proceedings of the National Academy of Sciences of the United States of America 106 (2): 623–8.
- ↑ Benderlioglu Z, Nelson RJ (December 2004). Digit length ratios predict reactive aggression in women, but not in men. Hormones and Behavior 46 (5): 558–64.
- ↑ (July 2005). 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 39 (2): 459–68.
- ↑ Neave N, Laing S, Fink B, Manning JT (October 2003). Second to fourth digit ratio, testosterone and perceived male dominance. Proceedings. Biological Sciences 270 (1529): 2167–72.
- ↑ Burriss RP, Little AC, Nelson EC (June 2007). 2D:4D and sexually dimorphic facial characteristics. Archives of Sexual Behavior 36 (3): 377–84.
- ↑ (November 2002). A preliminary investigation of the associations between personality, cognitive ability and digit ratio. Personality and Individual Differences 33 (7): 1115–24.
- ↑ Fink et al. 2004[verification needed]
- ↑ (October 2005). Second-to-fourth digit ratio related to Verbal and Numerical Intelligence and the Big Five. Personality and Individual Differences 39 (5): 959–66.
- ↑ Brosnan MJ (February 2008). Digit ratio as an indicator of numeracy relative to literacy in 7-year-old British schoolchildren. British Journal of Psychology 99 (Pt 1): 75–85.
- ↑ (January 2000). Second to fourth digit ratio in elite musicians Evidence for musical ability as an honest signal of male fitness. Evolution and Human Behavior 21 (1): 1–9.
- ↑ 48.0 48.1 Csathó A, Osváth A, Bicsák E, Karádi K, Manning J, Kállai J (February 2003). Sex role identity related to the ratio of second to fourth digit length in women. Biological Psychology 62 (2): 147–56.
- ↑ McIntyre MH (December 2003). Digit ratios, childhood gender role behavior, and erotic role preferences of gay men. Archives of Sexual Behavior 32 (6): 495–6.
- ↑ 50.0 50.1 Williams TJ, Pepitone ME, Christensen SE, et al. (March 2000). Finger-length ratios and sexual orientation. Nature 404 (6777): 455–6.
- ↑ Tortorice JL (2002). Written on the body: butch/femme lesbian gender identity and biological correlates.
- ↑ 52.0 52.1 52.2 McFadden D, Shubel E (December 2002). Relative lengths of fingers and toes in human males and females. Hormones and Behavior 42 (4): 492–500.
- ↑ 53.0 53.1 Hall LS, Love CT (February 2003). Finger-length ratios in female monozygotic twins discordant for sexual orientation. Archives of Sexual Behavior 32 (1): 23–8.
- ↑ Rahman Q, Wilson GD (April 2003). Sexual orientation and the 2nd to 4th finger length ratio: evidence for organising effects of sex hormones or developmental instability?. Psychoneuroendocrinology 28 (3): 288–303.
- ↑ (May 2004). Sex hormones and finger length: What does 2D:4D indicate?. Evolution and Human Behavior 25 (3): 182–99.
- ↑ Rahman Q (May 2005). Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation. Psychoneuroendocrinology 30 (4): 382–91.
- ↑ Kraemer B, Noll T, Delsignore A, Milos G, Schnyder U, Hepp U (2006). Finger length ratio (2D:4D) and dimensions of sexual orientation. Neuropsychobiology 53 (4): 210–4.
- ↑ Wallien MS, Zucker KJ, Steensma TD, Cohen-Kettenis PT (August 2008). 2D:4D finger-length ratios in children and adults with gender identity disorder. Hormones and Behavior 54 (3): 450–4.
- ↑ Brown WM, Finn CJ, Cooke BM, Breedlove SM (February 2002). Differences in finger length ratios between self-identified 'butch' and 'femme' lesbians. Archives of Sexual Behavior 31 (1): 123–7.
- ↑ Schneider HJ, Pickel J, Stalla GK (February 2006). Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure. Psychoneuroendocrinology 31 (2): 265–9.
- ↑ Geschwind and Galaburda, 1985[verification needed]
- ↑ Baron-Cohen S, Wheelwright S (April 2004). The empathy quotient: an investigation of adults with Asperger syndrome or high functioning autism, and normal sex differences. Journal of Autism and Developmental Disorders 34 (2): 163–75.
- ↑ Fink B, Manning JT, Neave N, Tan U (November 2004). Second to fourth digit ratio and hand skill in Austrian children. Biological Psychology 67 (3): 375–84.
- ↑ Fink B, Grammer K, Mitteroecker P, et al. (October 2005). Second to fourth digit ratio and face shape. Proceedings. Biological Sciences 272 (1576): 1995–2001.
- ↑ (March 2004). Second to fourth digit ratio and facial asymmetry. Evolution and Human Behavior 25 (2): 125–32.
- ↑ (2006). Sexual dimorphism in Upper Palaeolithic hand stencils. Antiquity 80 (308): 390–404.
- ↑ (2006). Sexual Determination of Hand Stencils at the Masri II Cave. Inora Newsletter 44: 21–6.
- ↑ (2006). Using the length of the 2nd to 4th digit ratio (2D:4D) to sex cave art hand stencils: factors to consider. Before Farming 1 (6): 1–7.
- ↑ Nelson, Emma Investigating relationships between the second-to-fourth digit ratio (2D:4D), social and bonding behaviours in non-human anthropoids. URL accessed on [[Template:Date]].Template:Self-published inline
- ↑ Talarovičová A, Kršková L, Blažeková J (January 2009). Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood. Hormones and Behavior 55 (1): 235–9.
- ↑ Yan RH, Malisch JL, Hannon RM, Hurd PL, Garland T (2008). Selective breeding for a behavioral trait changes digit ratio. PLoS ONE 3 (9): e3216.
- Mills, Michael E. (October 2002). Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning. Human Nature Review 2: 418–23.
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