The digit ratio is the ratio of the lengths of different digits or fingers. The 2D:4D ratio is the most studied digit ratio and 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 number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
The digit length is typically measured on palmar (ventral) hand, from the midpoint of bottom crease (where the finger joins the hand) to the tip of the finger. However, recently measurement of digit on dorsal hand, from tip of finger to proximal phalange-bone protrusion (which occurs when digits are bent at 90 degree angle to the palm), has also gained acceptance. A study has shown that, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio. Moreover, differential placing of flexion creases is a factor in palmar digit ratio.
Other digit ratios are also similarly calculated in the same hand.
The ratio of two digits in particular – the palmar 2nd (index finger) and 4th (ring finger) – is affected by fetal exposure to hormones, in particular to testosterone, and androgen; 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 palmar 2D:4D ratio is 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).
In keeping with these hormonal differences, the digit ratios are sexually dimorphic, being lower in men than in women. In palmar digit ratios, strong sexual dimorphism occurs in those of digit 2. In dorsal digit ratios, in contrast, strong sexual dimorphism occurs in those of digit 5, with women having shorter fifth digits on average. Overall, the report of sexual dimorphism is much stronger in dorsal digit ratios than in palmar digit ratios, especially as compared to the classic, palmar 2D:4D ratio. Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio. Thus, while most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.
Experimental studies have shown prenatal testosterone injection produces male-typical changes in dermatoglyphics and in palmar digit length, but not in bone digit length. Moreover, this effect occurs in digit 2 but not in digit 4. Additionally, human epidermal tissues have only androgen receptors and no estrogen receptors-α. Thus, it is likely that dermatoglyphic tissues in fingers may be more sensitive to prenatal testosterone effect, whereas, as reported above, bone digit ratios may be sensitive to testosterone-to-estrogen ratio. Hence, the palmar 2D:4D ratio reflects a combination of two different hormonal sensitivities. In support of this, a 2019 study has shown that differential placing of flexion creases contributes to sex differences in the palmar 2D:4D ratio.
History of 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, Glenn Wilson of King's College London published a study examining the correlation between assertiveness in women and their digit ratio, which found that women with a lower 2D:4D ratio reported greater assertiveness. 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.
- Males: mean 0.947, standard deviation 0.029
- Females: mean 0.965, standard deviation 0.026
Assuming a normal distribution, the above lead to 95% prediction intervals for 2D:4D ratio of 0.889–1.005 for males and 0.913–1.017 for females.
- Males: mean 0.951, standard deviation 0.035
- Females: mean 0.968, standard deviation 0.028
Evidence of androgen effect
The sex difference in 2D:4D is present before birth in humans. The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlated with the child's subsequent 2D:4D ratio. The conclusion of this research supported to an association between low 2D:4D and high levels of testosterone compared with estrogen, and high 2D:4D with low testosterone relative to estrogen.
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.
In a non-clinical sample of women, digit ratio correlated with anogenital distance in the expected direction. In other words, women with a greater anogenital distance, indicating greater prenatal androgen exposure, had a smaller digit ratio.
Disorders of sex development
Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average. Other possible physiological effects include an enlarged clitoris and shallow vagina. Males with CAH have more smaller (more masculine) digit ratios than control males, suggesting that prenatal androgens affect digit ratios. 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 may correlate with genetic variation in the androgen receptor gene. Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater, i.e. more feminine, digit ratios. There are reports of a failure to replicate this finding. However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by secreting more testosterone, 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 (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.
Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.
Explanation of the digit ratio effect
It is not clear why digit ratio is 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 affecting these multiple traits (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. 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. However, because no right–left difference has been found in sexual dimorphism of bone digit ratios (2D:4D) and because differential placing of flexion creases contributes to sex differences in palmar digit ratio, the study of right–left difference in placing of flexion creases may clarify this right–left difference in palmar 2D:4D ratio.
One study on mice from 2011 suggests that the 2D:4D ratio correlates with prenatal sex hormone levels because the androgen receptor and estrogen receptor activity is higher in digit 4 than in digit 2. Inactivation of AR decreases growth of digit 4, which causes a higher 2D:4D ratio, whereas inactivation of estrogen receptor alpha (ER-α) increases growth of digit 4, which leads to a lower 2D:4D ratio.
Geographic and ethnic variation in 2D:4D
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups. In a study with Han, Berber, Uygur and Jamaican children as subjects, Manning et al. found that Han children had the highest mean values of 2D:4D (0.954±−0.032), they were followed by the Berbers (0.950±0.033), then the Uygurs (0.946±0.037), and the Jamaican children had the lowest mean 2D:4D (0.935±0.035). 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."
The standard deviations associated with each given 2D:4D mean are considerable. For example, the ratio for Han children (0.954±−0.032) allows for a ratio as low as 0.922, while the ratio for Jamaican children (0.935±0.035) allows for a ratio as high as 0.970. Thus, some ethnic groups' confidence intervals overlap.
In 2007 Manning et al. also found that mean 2D:4D varied across ethnic groups with higher ratios for Whites, Non-Chinese Asians, and Mid-Easterners and lower ratios in Chinese and Black samples.
Two studies explored the question of whether geographical differences in 2D:4D ratios were caused by gene pool differences or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). The conclusions were that geographical differences in 2D:4D ratio were caused by genetic pool differences, not by geographical latitude.
Consanguinous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring, which may account for some of the geographical and ethnic variation in 2D:4D ratios, as consanguinity rates depend on, among others, religion, culture, and geography.
Correlation with traits
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 either high or low digit ratio.
Low digit ratio
High digit ratio
|Physiology and disease||
|Physical and competitive behavior||Increased aggressive behavior in sports.|
|Cognition and personality||
Male-to-female transgender women
A study in Germany has found a correlation between digit ratio and male-to-female transgender women. Trans women were found to have a higher digit ratio than males. This was not true for trans men, however, who were within the average range for females.
Digit ratio and development
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. 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.
Palaeolithic hand stencils
- Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.
- Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.
- 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.
- 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.
- Anogenital distance
- Body mass index
- Chiromancy (palm reading)
- Handedness and sexual orientation
- Waist–hip ratio
- Wlodarski R, Manning J, Dunbar RI (February 2015). "Stay or stray? Evidence for alternative mating strategy phenotypes in both men and women". Biology Letters. 11 (2): 20140977. doi:10.1098/rsbl.2014.0977. PMC 4360109. PMID 25652222.
- Mayhew TM, Gillam L, McDonald R, Ebling FJ (November 2007). "Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle". Journal of Anatomy. 211 (5): 630–638. doi:10.1111/j.1469-7580.2007.00801.x. PMC 2375787. PMID 17764524.
- Kumar, Sanjay; Voracek, Martin; Singh, Maharaj (6 March 2017). "Sexual Dimorphism in Digit Ratios Derived from Dorsal Digit Length among Adults and Children". Frontiers in Endocrinology. 8: 41. doi:10.3389/fendo.2017.00041. ISSN 1664-2392. PMC 5337486. PMID 28321205.
- Auger, J.; Eustache, F. (August 2011). "Second to fourth digit ratios, male genital development and reproductive health: A clinical study among fertile men and testis cancer patients: Finger ratios and male reproductive health". International Journal of Andrology. 34 (4pt2): e49–e58. doi:10.1111/j.1365-2605.2010.01124.x. PMID 21091719.
- Kumar, Sanjay; Voracek, Martin; Singh, Maharaj (2 August 2019). "Differential placing of flexion creases contributes to sex differences in the second-to-fourth digit ratio (2D:4D)". Frontiers in Endocrinology. 10: 537. doi:10.3389/fendo.2019.00537. ISSN 1664-2392. PMC 6688535. PMID 31428058.
- Zheng Z, Cohn MJ (September 2011). "Developmental basis of sexually dimorphic digit ratios". Proceedings of the National Academy of Sciences of the United States of America. 108 (39): 16289–16294. doi:10.1073/pnas.1108312108. PMC 3182741. PMID 21896736.
- Hönekopp J, Bartholdt L, Beier L, Liebert A (May 2007). "Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: new data and a meta-analytic review". Psychoneuroendocrinology. 32 (4): 313–21. doi:10.1016/j.psyneuen.2007.01.007. PMID 17400395.
- 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. doi:10.1006/hbeh.2002.1830. PMID 12488105.
- 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. doi:10.1016/s0378-3782(02)00073-7. PMID 12441204.
- Rivas MP, Moreira LM, Santo LD, Marques AC, El-Hani CN, Toralles MB (2014). "New studies of second and fourth digit ratio as a morphogenetic trait in subjects with congenital adrenal hyperplasia". American Journal of Human Biology. 26 (4): 559–561. doi:10.1002/ajhb.22545. PMID 24668932.
- Manning JT, Kilduff LP, Trivers R (January 2013). "Digit ratio (2D:4D) in Klinefelter's syndrome". Andrology. 1 (1): 94–99. doi:10.1111/j.2047-2927.2012.00013.x. PMID 23258636.
- Ventura T, Gomes MC, Pita A, Neto MT, Taylor A (February 2013). "Digit ratio (2D:4D) in newborns: Influences of prenatal testosterone and maternal environment". Early Human Development. 89 (2): 107–112. doi:10.1016/j.earlhumdev.2012.08.009. hdl:10400.17/772. PMID 23017880.
- McIntyre MH (February 2006). "The use of digit ratios as markers for perinatal androgen action". Reproductive Biology and Endocrinology. 4: 10. doi:10.1186/1477-7827-4-10. PMC 1409789. PMID 16504142.
- 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): 23–28. doi:10.1016/j.earlhumdev.2003.12.002. PMID 15113628.
- Malas MA, Dogan S, Evcil EH, Desdicioglu K (July 2006). "Fetal development of the hand, digits and digit ratio (2D:4D)". Early Human Development. 82 (7): 469–475. doi:10.1016/j.earlhumdev.2005.12.002. PMID 16473482.
- Dean A, Sharpe RM (June 2013). "Clinical review: Anogenital distance or digit length ratio as measures of fetal androgen exposure: relationship to male reproductive development and its disorders". The Journal of Clinical Endocrinology and Metabolism. 98 (6): 2230–2238. doi:10.1210/jc.2012-4057. PMID 23569219.
- Loehlin, John C.; Medland, Sarah E.; Martin, Nicholas G. (April 2009). "Relative Finger Lengths, Sex Differences, and Psychological Traits". Archives of Sexual Behavior. 38 (2): 298–305. doi:10.1007/s10508-007-9303-z. ISSN 0004-0002. PMID 18340520.
- McFadden, Dennis; Shubel, Erin (December 2002). "Relative lengths of fingers and toes in human males and females". Hormones and Behavior. 42 (4): 492–500. doi:10.1006/hbeh.2002.1833. PMID 12488115.
- Voracek, Martin (February 2009). "Comparative study of digit ratios (2D:4D and Other) and novel measures of relative finger length: Testing magnitude and consistency of sex differences across samples". Perceptual and Motor Skills. 108 (1): 83–93. doi:10.2466/pms.108.1.83-93. ISSN 0031-5125. PMID 19425449.
- Hönekopp, Johannes; Watson, Steven (September 2010). "Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand". American Journal of Human Biology. 22 (5): 619–630. doi:10.1002/ajhb.21054. PMID 20737609.
- Jamison, Cheryl Sorenson; Jamison, Paul L.; Meier, Robert J. (July 1994). "Effect of prenatal testosterone administration on palmar dermatoglyphic intercore ridge counts of rhesus monkeys (Macaca mulatta)". American Journal of Physical Anthropology. 94 (3): 409–419. doi:10.1002/ajpa.1330940309. ISSN 0002-9483. PMID 7943194.
- Abbott, Andrew D.; Colman, Ricki J.; Tiefenthaler, Ross; Dumesic, Daniel A.; Abbott, David H. (22 August 2012). Helle, Samuli (ed.). "Early-to-Mid Gestation Fetal Testosterone Increases Right Hand 2D∶4D Finger Length Ratio in Polycystic Ovary Syndrome-Like Monkeys". PLOS One. 7 (8): e42372. Bibcode:2012PLoSO...742372A. doi:10.1371/journal.pone.0042372. ISSN 1932-6203. PMC 3425513. PMID 22927929.
- Lofeu, Leandro; Brandt, Renata; Kohlsdorf, Tiana (December 2017). "Phenotypic integration mediated by hormones: Associations among digit ratios, body size and testosterone during tadpole development". BMC Evolutionary Biology. 17 (1): 175. doi:10.1186/s12862-017-1021-0. ISSN 1471-2148. PMC 5541650. PMID 28768472.
- Pelletier, G.; Ren, L. (April 2004). "Localization of sex steroid receptors in human skin". Histology and Histopathology. 19 (2): 629–636. doi:10.14670/HH-19.629. ISSN 0213-3911. PMID 15024720.
- Baker F (1888). "Anthropological Notes on the Human Hand". American Anthropologist: 51–76. doi:10.1525/aa.1888.1.1.02a00040.
- George R (1930). "Human finger types". The Anatomical Record. 46 (2): 199–204. doi:10.1002/ar.1090460210.
- Wilson GD (1983). "Finger-length as an index of assertiveness in women". Personality and Individual Differences. 4: 111–112. doi:10.1016/0191-8869(83)90061-2.
- Wilson G (2010). "Fingers to feminism: The rise of 2D:4D". Quarterly Review. 4: 25–32.
- 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. doi:10.1093/humrep/13.11.3000. PMID 9853845.
- 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. doi:10.1098/rsbl.2009.0346. PMC 2781964. PMID 19553247.
- Garn SM, Burdi AR, Babler WJ, Stinson S (November 1975). "Early prenatal attainment of adult metacarpal-phalangeal rankings and proportions" (PDF). American Journal of Physical Anthropology. 43 (3): 327–32. doi:10.1002/ajpa.1330430305. hdl:2027.42/37557. PMID 1211429.
- van Anders SM, Vernon PA, Wilbur CJ (March 2006). "Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins". Hormones and Behavior. 49 (3): 315–9. doi:10.1016/j.yhbeh.2005.08.003. hdl:2027.42/83921. PMID 16143332.
- 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. doi:10.1016/j.biopsycho.2004.05.001. PMID 15620791. Lay summary – LiveScience (2 March 2005).
- Dalton PS, Ghosal S (2018). "Self-confidence, Overconfidence and Prenatal Testosterone Exposure: Evidence from the Lab". Frontiers in Behavioral Neuroscience. 12: 5. doi:10.3389/fnbeh.2018.00005. PMC 5797613. PMID 29441000.
- Galis F, Ten Broek CM, Van Dongen S, Wijnaendts LC (February 2010). "Sexual dimorphism in the prenatal digit ratio (2D:4D)". Archives of Sexual Behavior. 39 (1): 57–62. doi:10.1007/s10508-009-9485-7. PMC 2811245. PMID 19301112.
- 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. doi:10.1136/bjsm.2006.027193. PMC 2577466. PMID 17008344.
- Gobrogge KL, Breedlove SM, Klump KL (February 2008). "Genetic and environmental influences on 2D:4D finger length ratios: a study of monozygotic and dizygotic male and female twins". Archives of Sexual Behavior. 37 (1): 112–8. doi:10.1007/s10508-007-9272-2. PMID 18074216.
- Barrett ES, Parlett LE, Swan SH (April 2015). "Stability of proposed biomarkers of prenatal androgen exposure over the menstrual cycle". Journal of Developmental Origins of Health and Disease. 6 (2): 149–57. doi:10.1017/S2040174414000646. PMC 5119464. PMID 25584807.
- Ciumas C, Lindén Hirschberg A, Savic I (May 2009). "High fetal testosterone and sexually dimorphic cerebral networks in females". Cerebral Cortex. 19 (5): 1167–74. doi:10.1093/cercor/bhn160. PMID 18854582.
- McAnulty RD, Burnette MM (2006). Sex and sexuality. 1. Greenwood Publishing Group. p. 165. ISBN 9780313049194.
- 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 (August 1980). "Amniotic fluid concentrations of delta 5 and delta 4 steroids in fetuses with congenital adrenal hyperplasia due to 21 hydroxylase deficiency and in anencephalic fetuses". The Journal of Clinical Endocrinology and Metabolism. 51 (2): 223–9. doi:10.1210/jcem-51-2-223. PMID 6447160.
- Dörr HG, Sippell WG (January 1993). "Prenatal dexamethasone treatment in pregnancies at risk for congenital adrenal hyperplasia due to 21-hydroxylase deficiency: effect on midgestational amniotic fluid steroid levels". The Journal of Clinical Endocrinology and Metabolism. 76 (1): 117–20. doi:10.1210/jc.76.1.117. PMID 8421074.
- Wudy SA, Dörr HG, Solleder C, Djalali M, Homoki J (August 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". The Journal of Clinical Endocrinology and Metabolism. 84 (8): 2724–8. doi:10.1210/jc.84.8.2724. PMID 10443667.
- Manning JT, Bundred PE, Newton DJ, Flanagan BF (2003). "The second to fourth digit ratio and variation in the androgen receptor gene". Evolution and Human Behavior. 24 (6): 399–405. doi:10.1016/S1090-5138(03)00052-7.
- Hampson E, Sankar JS (2012). "Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D:4D ratio". Evol Hum Behav. 33 (5): 557–561. doi:10.1016/j.evolhumbehav.2012.02.003.
- Crabbe P, Bogaert V, De Bacquer D, Goemaere S, Zmierczak H, Kaufman JM (September 2007). "Part of the interindividual variation in serum testosterone levels in healthy men reflects differences in androgen sensitivity and feedback set point: contribution of the androgen receptor polyglutamine tract polymorphism". The Journal of Clinical Endocrinology and Metabolism. 92 (9): 3604–10. doi:10.1210/jc.2007-0117. PMID 17579205.
- 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. doi:10.1210/en.2009-0774. PMC 2775980. PMID 19819951.
- 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. doi:10.1016/j.biopsycho.2005.03.006. PMID 15978716.
- Dickman S (March 1997). "HOX gene links limb, genital defects". Science. 275 (5306): 1568. doi:10.1126/science.275.5306.1568. PMID 9072822.
- Hönekopp J, Watson S (2010). "Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand". American Journal of Human Biology. 22 (5): 619–30. doi:10.1002/ajhb.21054. PMID 20737609.
- Xi, Huanjiu; Li, Ming; Fan, Yingnan; Zhao, Liguang (February 2014). "A Comparison of Measurement Methods and Sexual Dimorphism for Digit Ratio (2D:4D) in Han Ethnicity". Archives of Sexual Behavior. 43 (2): 329–333. doi:10.1007/s10508-013-0179-9. ISSN 0004-0002. PMC 3890058. PMID 24013635.
- Galis, Frietson; Ten Broek, Clara M. A.; Van Dongen, Stefan; Wijnaendts, Liliane C. D. (February 2010). "Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D)". Archives of Sexual Behavior. 39 (1): 57–62. doi:10.1007/s10508-009-9485-7. ISSN 0004-0002. PMC 2811245. PMID 19301112.
- Robertson, J.; Zhang, W.; Liu, J. J.; Muir, K. R.; Maciewicz, R. A.; Doherty, M. (13 November 2007). "Radiographic assessment of the index to ring finger ratio (2D:4D) in adults". Journal of Anatomy. 0 (1): 071115125032002––. doi:10.1111/j.1469-7580.2007.00830.x. ISSN 0021-8782. PMC 2423386. PMID 18005121.
- Manning JT, Barley L, Walton J, Lewis-Jones DI, Trivers RL, Singh D, Thornhill R, Rohde P, Bereczkei T, Henzi P, Soler M, Szwed A (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. doi:10.1016/S1090-5138(00)00029-5. PMID 10828555.
- 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. doi:10.1016/j.earlhumdev.2004.06.004. PMID 15500996.
- Williams TJ, Pepitone ME, Christensen SE, Cooke BM, Huberman AD, Breedlove NJ, Breedlove TJ, Jordan CL, Breedlove SM (March 2000). "Finger-length ratios and sexual orientation". Nature. 404 (6777): 455–6. Bibcode:2000Natur.404..455W. doi:10.1038/35006555. PMID 10761903.
- "STUDY ON THE DIGIT RATIO OF HUI AND HAN ETHNIC GROUPS IN NINGXIA--《Acta Anatomica Sinica》2008年02期". en.cnki.com.cn. Retrieved 7 November 2016.
- Manning JT, Churchill AJ, Peters M (April 2007). "The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D)". Archives of Sexual Behavior. 36 (2): 223–33. doi:10.1007/s10508-007-9171-6. PMID 17373585.
- Loehlin JC, McFadden D, Medland SE, Martin NG (December 2006). "Population differences in finger-length ratios: ethnicity or latitude?". Archives of Sexual Behavior. 35 (6): 739–42. CiteSeerX 10.1.1.579.6269. doi:10.1007/s10508-006-9039-1. PMID 16688484.
- Xu Y, Zheng Y (6 May 2015). "The digit ratio (2D:4D) in China: A meta-analysis". American Journal of Human Biology. 27 (3): 304–9. doi:10.1002/ajhb.22639. PMID 25284473.
- Ozener B, Hurd PL, Duyar I (2014). "Inbreeding is associated with lower 2D: 4D digit ratio". American Journal of Human Biology. 26 (2): 183–8. doi:10.1002/ajhb.22491. PMID 24288299.
- Tadmouri GO, Nair P, Obeid T, Al Ali MT, Al Khaja N, Hamamy HA (October 2009). "Consanguinity and reproductive health among Arabs". Reproductive Health. 6: 17. doi:10.1186/1742-4755-6-17. PMC 2765422. PMID 19811666.
- Manning JT (2002). Digit Ratio: A Pointer To Fertility, Behavior And Health. New Brunswick: Rutger University Press. pp. 65–68. ISBN 978-0813530291.
- Sudhakar HH, Manjunatha R, Madhusudhana HR (May 2015). "Relationship Between Second to Fourth Digit Ratios and Benign Prostatic Hyperplasia in Aging Men". Journal of Clinical and Diagnostic Research. 9 (5): PC01–3. doi:10.7860/JCDR/2015/11992.5937. PMC 4484107. PMID 26155515.
- Choi IH, Kim KH, Jung H, Yoon SJ, Kim SW, Kim TB (September 2011). "Second to fourth digit ratio: a predictor of adult penile length". Asian Journal of Andrology. 13 (5): 710–4. doi:10.1038/aja.2011.75. PMC 3739592. PMID 21725330.
- Muller DC, Baglietto L, Manning JT, McLean C, Hopper JL, English DR, Giles GG, Severi G (October 2012). "Second to fourth digit ratio (2D:4D), breast cancer risk factors, and breast cancer risk: a prospective cohort study". British Journal of Cancer. 107 (9): 1631–6. doi:10.1038/bjc.2012.418. PMC 3493764. PMID 22990654.
- Kyriakidis I, Papaioannidou P, Pantelidou V, Kalles V, Gemitizis K (December 2010). "Digit ratios and relation to myocardial infarction in Greek men and women". Gender Medicine. 7 (6): 628–36. doi:10.1016/j.genm.2010.11.00. PMID 21195362.
- 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. ISSN 0969-6113.
- 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. doi:10.1038/sj.ijo.0803154. PMID 16261185.
- Walsh F (1 December 2010). "Index finger length prostate cancer clue". BBC News. Retrieved 1 December 2010.
- Rahman AA, Lophatananon A, Stewart-Brown S, Harriss D, Anderson J, Parker T, Easton D, Kote-Jarai Z, Pocock R, Dearnaley D, Guy M, O'Brien L, Wilkinson RA, Hall AL, Sawyer E, Page E, Liu JF, UK Genetic Prostate Cancer Study Collaborators, British Association of Urological Surgeons' Section of Oncology, Eeles RA, Muir K (4 January 2011). "Hand pattern indicates prostate cancer risk". British Journal of Cancer. 104 (1): 175–177. doi:10.1038/sj.bjc.6605986. PMC 3039824. PMID 21119657.
- Ronalds G, Phillips DI, Godfrey KM, Manning JT (June 2002). "The ratio of second to fourth digit lengths: a marker of impaired fetal growth?". Early Human Development. 68 (1): 21–26. doi:10.1016/S0378-3782(02)00009-9. PMID 12191526.
- Klimek M, Galbarczyk A, Nenko I, Alvarado LC, Jasienska G (2014). "Digit ratio (2D:4D) as an indicator of body size, testosterone concentration and number of children in human males". Annals of Human Biology. 41 (6): 518–23. doi:10.3109/03014460.2014.902993. PMID 24766144.
- McFadden D, Westhafer JG, Pasanen EG, Carlson CL, Tucker DM (2005). "Physiological evidence of hypermasculinization in boys with the inattentive subtype of attention-deficit/hyperactivity disorder (ADHD)". Clinical Neuroscience Research. 5 (5–6): 233–245. doi:10.1016/j.cnr.2005.09.004.
- 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". American Journal of Human Biology. 19 (1): 41–50. doi:10.1002/ajhb.20571. PMID 17160985.
- Martel MM, Gobrogge KL, Breedlove SM, Nigg JT (April 2008). "Masculinized finger-length ratios of boys, but not girls, are associated with attention-deficit/hyperactivity disorder". Behavioral Neuroscience. 122 (2): 273–81. doi:10.1037/0735-7044.122.2.273. PMC 2902868. PMID 18410167.
- Martel MM (July 2009). "Conscientiousness as a mediator of the association between masculinized finger-length ratios and attention-deficit/hyperactivity disorder (ADHD)". Journal of Child Psychology and Psychiatry, and Allied Disciplines. 50 (7): 790–8. doi:10.1111/j.1469-7610.2009.02065.x. PMC 4311552. PMID 19298468.
- 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. doi:10.1111/j.1469-8749.2001.tb00181.x. PMID 11263685.
- Quartier A, Chatrousse L, Redin C, Keime C, Haumesser N, Maglott-Roth A, Brino L, Le Gras S, Benchoua A, Mandel JL, Piton A (August 2018). "Genes and Pathways Regulated by Androgens in Human Neural Cells, Potential Candidates for the Male Excess in Autism Spectrum Disorder". Biological Psychiatry. 84 (4): 239–252. doi:10.1016/j.biopsych.2018.01.002. PMID 29428674.
- Quinton SJ, Smith AR, Joiner T (September 2011). "The 2 to 4 digit ratio (2D:4D) and eating disorder diagnosis in women". Personality and Individual Differences. 51 (4): 402–405. doi:10.1016/j.paid.2010.07.024. PMC 3134962. PMID 21765573.
- Carré JM, Ortiz TL, Labine B, Moreau BJ, Viding E, Neumann CS, Goldfarb B (December 2015). "Digit ratio (2D:4D) and psychopathic traits moderate the effect of exogenous testosterone on socio-cognitive processes in men". Psychoneuroendocrinology. 62: 319–26. doi:10.1016/j.psyneuen.2015.08.023. PMID 26356040.
- Kornhuber J, Erhard G, Lenz B, Kraus T, Sperling W, Bayerlein K, Biermann T, Stoessel C (April 2011). "Low digit ratio 2D:4D in alcohol dependent patients". PLOS One. 6 (4): e19332. Bibcode:2011PLoSO...619332K. doi:10.1371/journal.pone.0019332. PMC 3081847. PMID 21547078.
- Lenz B, Bouna-Pyrrou P, Mühle C, Kornhuber J (August 2018). "Low digit ratio (2D:4D) and late pubertal onset indicate prenatal hyperandrogenziation in alcohol binge drinking". Progress in Neuro-Psychopharmacology & Biological Psychiatry. 86: 370–378. doi:10.1016/j.pnpbp.2018.02.012. PMID 29499227.
- Bailey AA, Hurd PL (2005). "Depression in men is associated with more feminine finger length ratios". Personality and Individual Differences. 39 (4): 829–836. doi:10.1016/j.paid.2004.12.017.
- 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. doi:10.1016/j.pnpbp.2003.09.020. PMID 14687873.
- Blanchard A, Lyons M (2010). "An investigation into the relationship between digit length ratio (2D: 4D) and psychopathy". The British Journal of Forensic Practice. 12 (2): 23–31. doi:10.5042/bjfp.2010.0183.
- Kornhuber J, Zenses EM, Lenz B, Stoessel C, Bouna-Pyrrou P, Rehbein F, Kliem S, Mößle T (2013). "Low 2D:4D values are associated with video game addiction". PLOS One. 8 (11): e79539. Bibcode:2013PLoSO...879539K. doi:10.1371/journal.pone.0079539. PMC 3827365. PMID 24236143.
- Evardone M, Alexander GM (June 2009). "Anxiety, sex-linked behaviors, and digit ratios (2D:4D)". Archives of Sexual Behavior. 38 (3): 442–55. doi:10.1007/s10508-007-9260-6. PMC 2768336. PMID 17943431.
- Mailhos A, Buunk AP, Del Arca D, Tutte V (September 2016). "Soccer players awarded one or more red cards exhibit lower 2D:4D ratios". Aggressive Behavior. 42 (5): 417–26. doi:10.1002/ab.21638. PMID 26699684.
- 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. doi:10.1016/S1090-5138(00)00063-5. PMID 11182575.
- 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. Bibcode:2009PNAS..106..623C. doi:10.1073/pnas.0810907106. PMC 2626753. PMID 19139402. Lay summary – Time (12 January 2009).
- 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. CiteSeerX 10.1.1.490.5559. doi:10.1016/j.biopsycho.2004.03.012. PMID 15294393.
- Titus-Ernstoff (2003). "Psychosexual Characteristics of Men and Women Exposed Prenatally to Diethylstilbestrol". CDC. 14 (2): 155–60. doi:10.1097/01.EDE.0000039059.38824.B2. PMID 12606880.
- Schuurmans-Stekhoven JB, Buckingham, RM (2010). "Ratio or Length? A Proposed Methodology for Understanding Digit Ratio and Personality Using a Female Sample". Journal of Individual Differences. 31 (3): 150–157. doi:10.1027/1614-0001/a000023.CS1 maint: multiple names: authors list (link)
- 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. doi:10.1016/j.yhbeh.2004.06.004. PMID 15555497.
- Liu J, Portnoy J, Raine A (August 2012). "Association between a marker for prenatal testosterone exposure and externalizing behavior problems in children". Development and Psychopathology. 24 (3): 771–82. doi:10.1017/S0954579412000363. PMC 4247331. PMID 22781854.
- Butovskaya M, Burkova V, Karelin D, Fink B (1 October 2015). "Digit ratio (2D:4D), aggression, and dominance in the Hadza and the Datoga of Tanzania". American Journal of Human Biology. 27 (5): 620–7. doi:10.1002/ajhb.22718. PMID 25824265.
- Joyce CW, Kelly JC, Chan JC, Colgan G, O'Briain D, Mc Cabe JP, Curtin W (November 2013). "Second to fourth digit ratio confirms aggressive tendencies in patients with boxers fractures". Injury. 44 (11): 1636–9. doi:10.1016/j.injury.2013.07.018. PMID 23972912.
- Cho KJ, Kim S (1 December 2017). "Utilization of Second Digit to Fourth Digit Ratio (2D : 4D) as One of Physical Markers to Evaluate Aggression in Elementary School Students". Korean Journal of Physical Anthropology (in Korean). 30 (4): 153. doi:10.11637/kjpa.2017.30.4.153. ISSN 1225-150X.
- Butovskaya M, Burkova V, Mabulla A (29 January 2010). "Sex differences in 2D: 4D ratio, aggression and conflict resolution in African children and adolescents: a cross‐cultural study". Journal of Aggression, Conflict and Peace Research. 2 (1): 17–31. doi:10.5042/jacpr.2010.0002. ISSN 1759-6599.
- Williams JH, Greenhalgh KD, Manning JT (May 2003). "Second to fourth finger ratio and possible precursors of developmental psychopathology in preschool children". Early Human Development. 72 (1): 57–65. doi:10.1016/s0378-3782(03)00012-4. PMID 12706312.
- Beech JR, Mackintosh IC (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. doi:10.1016/j.paid.2005.01.024.
- 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. doi:10.1098/rspb.2003.2502. PMC 1691489. PMID 14561281.
- Burriss RP, Little AC, Nelson EC (June 2007). "2D:4D and sexually dimorphic facial characteristics". Archives of Sexual Behavior. 36 (3): 377–84. doi:10.1007/s10508-006-9136-1. PMID 17203400.
- Sluming VA, Manning JT (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. doi:10.1016/S1090-5138(99)00026-4.
- Nye JV, Androuschak G, Desierto D, Jones G, Yudkevich M (2012). "2D:4D asymmetry and gender differences in academic performance". PLOS One. 7 (10): e46319. Bibcode:2012PLoSO...746319N. doi:10.1371/journal.pone.0046319. PMC 3466225. PMID 23056282.
- Sánchez Á, Sánchez-Campillo J, Moreno-Herrero D, Rosales V (2014). "2D:4D values are associated with mathematics performance in business and economics students". Learning and Individual Differences. 36: 110–116. doi:10.1016/j.lindif.2014.10.001.
- Skalná, Z., Senko, T., Olexová, L., Sedliak, M., & Kršková, L. VPLYV MIERY PRENATÁLNEJ EXPOZÍCIE TESTOSTERÓNU A JEHO POSTNATÁLNEJ KONCENTRÁCIE NA POČET SEXUÁLNYCH PARTNERIEK A SCHOPNOSŤ EMPATIZÁCIE U MUŽOV ZO SLOVENSKA. SLOVENSKÁ ANTROPOLÓGIA, 42.
- McIntyre MH, Barrett ES, McDermott R, Johnson DD, Cowden J, Rosen SP (2007). "Finger length ratio (2D:4D) and sex differences in aggression during a simulated war game". Personality and Individual Differences. 42 (4): 755–764. doi:10.1016/j.paid.2006.08.009.
- Stenstrom E, Saad G, Nepomuceno MV, Mendenhall Z (2011). "Testosterone and domain-specific risk: Digit ratios (2D:4D and rel2) as predictors of recreational, financial, and social risk-taking behaviors". Personality and Individual Differences. 51 (4): 412–416. doi:10.1016/j.paid.2010.07.003.
- Siurana D, Gadea M, Espert R (2017). "Efectos organizacionales y activacionales de la testosterona sobre la asunción de riesgos en conductas económicas: una revisión sistemática". Suma Psicológica (in Spanish). 24 (2): 142–152. doi:10.1016/j.sumpsi.2017.06.003.
- Millet K, Dewitte S (January 2006). "Second to fourth digit ratio and cooperative behavior". Biological Psychology. 71 (1): 111–5. CiteSeerX 10.1.1.518.4422. doi:10.1016/j.biopsycho.2005.06.001. PMID 16024156.
- Crocchiola D (2014). "Art as an Indicator of Male Fitness: Does Prenatal Testosterone Influence Artistic Ability?". Evolutionary Psychology. 12 (3): 147470491401200. doi:10.1177/147470491401200303.
- 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. doi:10.1348/000712607X197406. PMID 17535470. Lay summary – LiveScience (22 May 2007).
- Hoskin AW, Ellis L (1 February 2015). "Fetal Testosterone and Criminality: Test of Evolutionary Neuroandrogenic Theory". Criminology. 53 (1): 54–73. doi:10.1111/1745-9125.12056. ISSN 1745-9125.
- Ellis L, Hoskin AW (March 2015). "Criminality and the 2D:4D ratio: testing the prenatal androgen hypothesis". International Journal of Offender Therapy and Comparative Criminology. 59 (3): 295–312. doi:10.1177/0306624X13503813. PMID 24013770. S2CID 29218986.
- Weisman O, Pelphrey KA, Leckman JF, Feldman R, Lu Y, Chong A, Chen Y, Monakhov M, Chew SH, Ebstein RP (August 2015). "The association between 2D:4D ratio and cognitive empathy is contingent on a common polymorphism in the oxytocin receptor gene (OXTR rs53576)". Psychoneuroendocrinology. 58: 23–32. doi:10.1016/j.psyneuen.2015.04.007. PMID 25935637.
- Eichler A, Heinrich H, Moll GH, Beckmann MW, Goecke TW, Fasching PA, Muschler MR, Bouna-Pyrrou P, Lenz B, Kornhuber J (April 2018). "Digit ratio (2D:4D) and behavioral symptoms in primary-school aged boys". Early Human Development. 119: 1–7. doi:10.1016/j.earlhumdev.2018.02.012. PMID 29486372.
- Austin EJ, Manning JT, McInroy K, Mathews E (November 2002). "A preliminary investigation of the associations between personality, cognitive ability and digit ratio". Personality and Individual Differences. 33 (7): 1115–24. doi:10.1016/S0191-8869(02)00002-8.
- Fink et al. 2004[verification needed]
- Luxen MF, Buunk BP (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. doi:10.1016/j.paid.2005.03.016.
- Burton LA, Guterman E, Baum G (23 April 2013). "Effect of Prenatal Androgen on Adult Personality: Greater Openness with More Female-Typical 2D:4D Digit Ratios". Current Psychology. 32 (2): 197–202. doi:10.1007/s12144-013-9170-9. ISSN 1046-1310.
- Voracek, M. (July 2009). "Who wants to believe? Associations between digit ratio (2D:4D) and paranormal and superstitious beliefs". Personality and Individual Differences. 47 (2): 105–109. doi:10.1016/j.paid.2009.01.051.
- Fink B, Manning JT, Neave N (2004). "Second to fourth digit ratio and the 'big five' personality factors". Personality and Individual Differences. 37 (3): 495–503. doi:10.1016/j.paid.2003.09.018.
- Borniger JC, Chaudhry A, Muehlenbein MP (2013). "Relationships among musical aptitude, digit ratio and testosterone in men and women". PLOS One. 8 (3): e57637. Bibcode:2013PLoSO...857637B. doi:10.1371/journal.pone.0057637. PMC 3592910. PMID 23520475.
- Poulin M, O'Connell RL, Freeman LM (2004). "Picture recall skills correlate with 2D:4D ratio in women but not men". Evolution and Human Behavior. 25 (3): 174–181. doi:10.1016/j.evolhumbehav.2004.03.004.
- Watts TM, Holmes L, Raines J, Orbell S, Rieger G (November 2018). "Finger Length Ratios of Identical Twins with Discordant Sexual Orientations" (PDF). Archives of Sexual Behavior. 47 (8): 2435–2444. doi:10.1007/s10508-018-1262-z. PMID 29978344.
- 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. doi:10.1016/S0301-0511(02)00127-8. PMID 12581689.
- Tortorice JL (2002). "Written on the body: butch vs. femme lesbian gender identity and biological correlates of low digit ratio". Rutgers University. OCLC 80234273. Cite journal requires
- McFadden D, Shubel E (December 2002). "Relative lengths of fingers and toes in human males and females". Hormones and Behavior. 42 (4): 492–500. doi:10.1006/hbeh.2002.1833. PMID 12488115.
- 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. doi:10.1023/A:1021837211630. PMID 12597269.
- 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. doi:10.1016/S0306-4530(02)00022-7. PMID 12573297.
- Putz DA, Gaulin SJ, Sporter RJ, McBurney DH (2004). "Sex hormones and finger length". Evolution and Human Behavior. 25 (3): 182–199. doi:10.1016/j.evolhumbehav.2004.03.005.
- Rahman Q (May 2005). "Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation". Psychoneuroendocrinology. 30 (4): 382–91. doi:10.1016/j.psyneuen.2004.10.006. PMID 15694118.
- 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. doi:10.1159/000094730. PMID 16874008.
- 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. doi:10.1016/j.yhbeh.2008.05.002. PMID 18585715.
- Grimbos T, Dawood K, Burriss RP, Zucker KJ, Puts DA (April 2010). "Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women". Behavioral Neuroscience. 124 (2): 278–87. doi:10.1037/a0018764. PMID 20364887. S2CID 2777884.
- Hiraishi K, Sasaki S, Shikishima C, Ando J (June 2012). "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. 41 (3): 711–24. doi:10.1007/s10508-011-9889-z. PMID 22270254.
- Robinson SJ, Manning JT (September 2000). "The ratio of 2nd to 4th digit length and male homosexuality". Evolution and Human Behavior. 21 (5): 333–345. doi:10.1016/S1090-5138(00)00052-0. PMID 11053694.
- 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. doi:10.1023/A:1026054625638. PMID 14627046.
- 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. doi:10.1023/A:1014091420590. PMID 11910785.
- Peters M, Manning JT, Reimers S (April 2007). "The effects of sex, sexual orientation, and digit ratio (2D:4D) on mental rotation performance". Archives of Sexual Behavior. 36 (2): 251–60. doi:10.1007/s10508-006-9166-8. PMID 17394056.
- Yule MA, Brotto LA, Gorzalka BB (February 2014). "Biological markers of asexuality: Handedness, birth order, and finger length ratios in self-identified asexual men and women". Archives of Sexual Behavior. 43 (2): 299–310. doi:10.1007/s10508-013-0175-0. PMID 24045903.
- Voracek M, Manning JT, Ponocny I (June 2005). "Digit ratio (2D:4D) in homosexual and heterosexual men from Austria". Archives of Sexual Behavior. 34 (3): 335–40. doi:10.1007/s10508-005-3122-x. PMID 15971016.
- 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. doi:10.1016/j.psyneuen.2005.07.005. PMID 16140461.
- Ronalds G, Phillips DI, Godfrey KM, Manning JT (June 2002). "The ratio of second to fourth digit lengths: a marker of impaired fetal growth?". Early Human Development. 68 (1): 21–6. doi:10.1016/s0378-3782(02)00009-9. PMID 12191526.
- Fink B, Grammer K, Mitteroecker P, Gunz P, Schaefer K, Bookstein FL, Manning JT (October 2005). "Second to fourth digit ratio and face shape". Proceedings. Biological Sciences. 272 (1576): 1995–2001. doi:10.1098/rspb.2005.3179. PMC 1559906. PMID 16191608.
- Fink B, Manning JT, Neave N, Grammer K (March 2004). "Second to fourth digit ratio and facial asymmetry". Evolution and Human Behavior. 25 (2): 125–32. doi:10.1016/S1090-5138(03)00084-9.
- Snow DR (2006). "Sexual dimorphism in Upper Palaeolithic hand stencils". Antiquity. 80 (308): 390–404. doi:10.1017/S0003598X00093704.
- Chazine JM, Noury A (2008). "Sexual Determination of Hand Stencils on the Main Panel of the Gua Masri II Cave (East-Kalimantan/Borneo – Indonesia)". International Newsletter on Rock Art. 28: 16–22.
- Nelson E, Manning J, Sinclair A (2006). "News Using the length of the 2nd to 4th digit ratio (2D:4D) to sex cave art hand stencils: Factors to consider". Before Farming. 2006: 1–7. doi:10.3828/bfarm.2006.1.6.
- Shultz S, Opie C, Nelson E, Atkinson QD, Dunbar RI (2014). "Evolution of Primate Social Systems: Implications for Hominin Social Evolution". In Dunbar RI, Gamble C, Gowlett JA (eds.). Lucy to Language: The Benchmark Papers. pp. 317–405. ISBN 978-0-19-965259-4.
- Tyler Burley N (May 2006). "An Eye for Detail: Selective Sexual Imprinting in Zebra Finches". Evolution. 60 (5): 1076–1085. doi:10.1554/05-399.1. JSTOR 4095408.
- Talarovicová A, Krsková L, Blazeková 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. doi:10.1016/j.yhbeh.2008.10.010. PMID 19022257.
- Yan RH, Malisch JL, Hannon RM, Hurd PL, Garland T (September 2008). "Selective breeding for a behavioral trait changes digit ratio". PLOS One. 3 (9): e3216. Bibcode:2008PLoSO...3.3216Y. doi:10.1371/journal.pone.0003216. PMC 2528935. PMID 18797502.
|Wikimedia Commons has media related to Digit ratio.|
- PubMed listing of papers on digit ratios
- Mills ME (October 2002). "Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning". Human Nature Review. 2: 418–23.