The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the midpoint of bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens e.g. testosterone while in the uterus and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher prenatal androgen exposure. The 2D:4D ratio is calculated by dividing the length of the index finger of the right hand by the length of the ring finger of the right hand. A longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio of less than 1.
The 2D:4D digit ratio is sexually dimorphic: while the second digit is typically shorter in both females and males, the difference between the lengths of the two digits is greater in males than in females.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
- 1 History of digit ratio research
- 2 Digit ratio distribution
- 3 Evidence of androgen effect on digit ratio
- 4 Explanation of the digit ratio effect
- 5 Geographic and ethnic variation in 2D:4D
- 6 Correlation between digit ratio and traits
- 7 Digit ratio and palaeolithic hand stencils
- 8 Digit ratio research in animals
- 9 See also
- 10 References
- 11 External links
History of digit ratio research
That a greater proportion of men have shorter index fingers than ring fingers than do women was noted in the scientific literature several times through the late 1800s, with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930, after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr Glenn Wilson of King's College, London published a study examining the correlation between assertiveness in women and their digit ratio. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero. In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children and further developed the idea that the index was a marker of prenatal sex hormones. Since then research on the topic has burgeoned around the world.
A 2009 study in Biology Letters argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women," which may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios.
A 2011 paper by Zhengui Zheng and Martin J. Cohn reports "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development." The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual’s adulthood. During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio.
Importantly, there has been no correlation between the sex hormone levels of an adult and the individual’s 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon.
A major problem with the research on this topic comes from the contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio, but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation with the 2D:4D. So there should be a correlation with one or the other but not both.
Digit ratio distribution
From a study of 136 males and 137 females:
- 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% confidence intervals for 2D:4D ratio of 0.889-1.005 for males and 0.913-1.017 for females.
Evidence of androgen effect on digit ratio
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 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 with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to control males, are greater (i.e., more feminine) than in their fathers or control males.
Digit ratio in men correlates 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, 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.
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.
Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.
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.
Explanation of the digit ratio effect
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 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.
Geographic and ethnic variation in 2D:4D
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."
Correlation between digit ratio and 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|
|Cognition and personality|
Male-to-female transsexual women
A study in Germany has found a correlation between digit ratio and male-to-female transsexualism. Trans women were found to have a higher digit ratio than cisgender males, but one that was comparable to control 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. However, exposure to very high levels of testosterone and/or estrogen in the womb may have negative effects as well.
Digit ratio and palaeolithic hand stencils
Digit ratio research in animals
- 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.
- 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.
- Anogenital distance
- Waist–hip ratio
- Body mass index
- Handedness and sexual orientation
- Chiromancy — hand analysis
- T M Mayhew, L Gillam, R McDonald, and F J P Ebling (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.
- Wlodarski, R.; Manning, J.; Dunbar, R. I. M. (4 February 2015). "Stay or stray? Evidence for alternative mating strategy phenotypes in both men and women". Biology Letters 11 (2): 20140977–20140977. doi:10.1098/rsbl.2014.0977.
- 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. doi:10.1525/aa.1888.1.1.02a00040.
- George R (1930). "Human finger types". Anatomical Record 46 (2): 199–204. doi:10.1002/ar.1090460210.
- Wilson, Glenn D. (1983). "Finger-length as an index of assertiveness in women". Personality and Individual Differences 4 (1): 111–2. 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; Scutt; Wilson; Lewis-Jones (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 (11): 3000–3004. doi:10.1093/humrep/13.11.3000. PMID 9853845.
- Kratochvíl L, Flegr J; Flegr (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.
- Zhengui Z., Cohn M. J.; Cohn (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.
- Garn S. M., Burdi A. R., Babler W. J., Stinson S.; Burdi; Babler; Stinson (1975). "Early prenatal attainment of adult metacarpal-phalangeal rankings and proportions". American Journal of Physical Anthropology 43 (3): 327–332. doi:10.1002/ajpa.1330430305. PMID 1211429.
- van Anders SM, Vernon PA, Wilbur CJ; Vernon; Wilbur (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. PMID 16143332.
- Hönekopp Johannes, Bartholdt Luise, Beier Lothar, Liebert Andreas; Bartholdt; Beier; Liebert (2007). "Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: New data and a meta-analytic review". Psychoneuroendocrinology 32 (4): 313–321. doi:10.1016/j.psyneuen.2007.01.007. PMID 17400395.
- Bailey AA, Hurd PL; Hurd (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).
- Brown WM, Hines M, Fane BA, Breedlove SM; Hines; Fane; Breedlove (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; Kalyoncu; Yariş (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.
- Ciumas C, Lindén Hirschberg A, Savic I.; Lindén Hirschberg; Savic (2009). "High fetal testosterone and sexually dimorphic cerebral networks in females". Cereb Cortex 19 (5): 1164–72. doi:10.1093/cercor/bhn160. PMID 18854582.
- Richard D. McAnulty, M. Michele Burnette (2006) Sex and sexuality, Volume 1, Greenwood Publishing Group, p.165
- 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; Levine; Cederqvist; Fuentes; Riccardi; Holcombe; Nitowsky; Sachs; Anderson; Duchon; Owens; Merkatz; New (1980). "Amniotic fluid concentrations of delta5 and delta4 steroids in fetuses with congenital adrenal hyperplasia". J Clin Endocrinol Metab 51 (2): 223–229. doi:10.1210/jcem-51-2-223. PMID 6447160.
- 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 (1): 117–120. doi:10.1210/jc.76.1.117. PMID 8421074.
- 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 (8): 2724–2728. doi:10.1210/jc.84.8.2724. PMID 10443667.
- Manning JT, Kilduff LP, Trivers R; Kilduff; Trivers (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.
- Manning, John T.; Bundred, Peter E.; Newton, Darren J.; Flanagan, Brian F. (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; Hampson (2012). "Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D:4D ratio". Evol Hum Behav 33 (4): 557–561. doi:10.1016/j.genm.2012.05.001. PMID 22728214.
- Crabbe P, Bogaert V, De Bacquer D, Goemaere S, Zmierczak H, Kaufman JM.; Bogaert; De Bacquer; Goemaere; Zmierczak; Kaufman (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". J Clin Endocrinol Metab 92 (9): 3604–10. doi:10.1210/jc.2007-0117. PMID 17579205.
- Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S; Bryk; Nowak; Quigley; Moffat (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.
- Malas MA, Dogan S, Evcil EH, Desdicioglu K.; Dogan; Evcil; Desdicioglu (2006). "Fetal development of the hand, digits and digit ratio (2D:4D)". Early Hum Dev 82 (7): 469–475. doi:10.1016/j.earlhumdev.2005.12.002. PMID 16473482.
- Galis F, Ten Broek CM, Van Dongen S, Wijnaendts LC; Ten Broek; Van Dongen; Wijnaendts (2009). "Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D)". Arch Sex Behav 38 (1): 57–62. doi:10.1007/s10508-009-9485-7. PMC 2811245. PMID 19301112.
- Lutchmaya S, Baron-Cohen S, Raggatt P, Knickmeyer R, Manning JT; Baron-Cohen; Raggatt; Knickmeyer; Manning (April 2004). "2nd to 4th digit ratios, fetal testosterone and estradiol". Early Human Development 77 (1–2): 23–8. doi:10.1016/j.earlhumdev.2003.12.002. PMID 15113628.
- Romano M, Leoni B, Saino N; Leoni; Saino (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.
- Zheng Z, Cohn MJ; Cohn (2011). "Developmental basis of sexually dimorphic digit ratio". PNAS 108 (39): 16289–94. doi:10.1073/pnas.1108312108. PMC 3182741. PMID 21896736.
- McIntyre MH (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.
- Paul SN, Kato BS, Hunkin JL, Vivekanandan S, Spector TD; Kato; Hunkin; Vivekanandan; Spector (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, K.L., S.M.Breedlove & K.L.Klump; Breedlove; 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 (1): 112–118. doi:10.1007/s10508-007-9272-2. PMID 18074216.
- Lutchmaya S., Baron-Cohen S., Raggatt P., Knickmeyer R., Manning J. T.; Baron-Cohen; Raggatt; Knickmeyer; Manning (2004). "2nd To 4th Digit Ratios, Fetal Testosterone and Estradiol". Early human development 77 (1–2): 23–8. doi:10.1016/j.earlhumdev.2003.12.002. PMID 15113628.
- Dickman S. (Mar 1997). "HOX gene links limb, genital defects". Science 275 (5306): 1568–9. doi:10.1126/science.275.5306.1568. PMID 9072822.
- Honekopp J, Watson S; Watson (2010). "Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand". American Journal of Human Biology. online (5): 619–30. doi:10.1002/ajhb.21054. PMID 20737609.
- Manning, J. T.; 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. doi:10.1016/S1090-5138(00)00029-5. PMID 10828555.
- Manning JT, Stewart A, Bundred PE, Trivers RL; Stewart; Bundred; Trivers (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.
- Terrance, J.; Williams, Michelle E. Pepitone, Scott E. Christensen, Bradley M. Cooke, Andrew D. Huberman, Nicholas J. Breedlove, Tessa J. Breedlove, Cynthia L. Jordan and S. Marc Breedlove (30 March 2000). "Finger-length ratios and sexual orientation". Nature 404 (6777): 455–456. doi:10.1038/35006555. PMID 10761903., available on-line at "Finger-length ratios and sexual orientation". University of Nebraska-Lincoln. (quoted from New Scientist)
- Manning JT, Scutt D, Wilson J, Lewis-Jones DI; Scutt; Wilson; Lewis-Jones (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.
- 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; Manning; Neave (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, Fergus (1 December 2010). "Index finger length prostate cancer clue". BBC News. Retrieved 1 December 2010.
- Ronalds G, Phillips DIW, Godfrey KM, Manning JT. The ratio of second to fourth digit lengths: a marker of impaired fetal growth? Early Hum. Dev. 2002;68:21–6.
- Klimek M, Galbarczyk A, Nenko I, Alvarado LC, Jasienska G.; Galbarczyk; Nenko; Alvarado; Jasienska (2014). "Digit ratio (2D:4D)as an indicator of body size, testosterone concentration and number of children in human males". Ann Hum Biol.: 1. doi:10.3109/03014460.2014.902993. PMID 24766144.
- 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 (5–6): 233–245. doi:10.1016/j.cnr.2005.09.004.
- Stevenson JC, Everson PM, Williams DC, Hipskind G, Grimes M, Mahoney ER.; Everson; Williams; Hipskind; Grimes; Mahoney (2007). "Attention deficit/hyperactivity disorder (ADHD) symptoms and digit ratios in a college sample". Am J Hum Biol 19 (1): 41–50. doi:10.1002/ajhb.20571. PMID 17160985.
- Martel, M.M, K.L.Gobrogge, S.M.Breedlove & J.T.Nigg; Gobrogge; Breedlove; Nigg (2008). "Masculinized Finger-Length Ratios of Boys, but Not Girls, Are Associated With Attention-Deficit/Hyperactivity Disorder". Behavioral Neuroscience 122 (2): 273–281. doi:10.1037/0735-7044.122.2.273. PMC 2902868. PMID 18410167.
- 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 (7): 790–798. doi:10.1111/j.1469-7610.2009.02065.x. PMID 19298468.
- Manning JT, Baron-Cohen S, Wheelwright S, Sanders G; Baron-Cohen; Wheelwright; Sanders (March 2001). "The 2nd to 4th digit ratio and autism". Developmental Medicine and Child Neurology 43 (3): 160–4. doi:10.1017/S0012162201000317. PMID 11263685.
- Klump, K. L., Gobrogge, K. L., Perkins, P. S., Thorne, D., Sisk, C. L., Breedlove, S.M.; Gobrogge; Perkins; Thorne; Sisk; Breedlove (2006). "Preliminary evidence that gonadal hormones organize and activate disordered eating". Psychol Med 36 (4): 539–546. doi:10.1017/S0033291705006653. PMID 16336745.
- Smith, A. R., Hawkeswood, S. E., Joiner, T. E.; Hawkeswood; Joiner (2009). "The measure of a man: Associations between digit ratio and disordered eating in males". Int J Eat Disord 28 (1): 191–4. doi:10.1002/eat.20736. PMID 19718667.
- Bailey, A. & Hurd, P., Allison A.; Hurd, Peter L. (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; Frecska; Beck; An; Kiss (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. (May 2010). "An Investigation into the Relationship between Digit Length Ratio and Psychopathy". British Journal of Forensic Practice 12 (2): 23. doi:10.5042/bjfp.2010.0183.
- Kornhuber J, Erhard G, Lenz B, Kraus T, Sperling W, Bayerlein K, Biermann T, Stoessel C; Erhard; Lenz; Kraus; Sperling; Bayerlein; Biermann; Stoessel (April 2011). Zhang, Xiang Yang, ed. "Low Digit Ratio 2D∶4D in Alcohol Dependent Patients". PLoS ONE 6 (4): e19332. doi:10.1371/journal.pone.0019332. PMC 3081847. PMID 21547078.
- J. Kornhuber, EM Zenses, B Lenz, C Stoessel, P Bouna-Pyrrou, F Rehbein, S Kliem, T Mößle (2013): Low digit ratio 2D:4D associated with video game addiction. PLoS ONE 2013; Vol. 8, Nr. 11: e79539
- Evardone & Alexander, Milagros; Alexander, Gerianne M. (2009). "Anxiety, Sex-linked Behavior, and Digit Ratios". Arch Sex Behav. 38 (3): 442–55. doi:10.1007/s10508-007-9260-6. PMC 2768336. PMID 17943431.
- Manning JT, Taylor RP; Taylor (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; Gurnell; Rustichini (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. doi:10.1073/pnas.0810907106. PMC 2626753. PMID 19139402. Lay summary – Time (12 January 2009).
- Fink B, Manning JT, Neave N, Tan U; Manning; Neave; Tan (November 2004). "Second to fourth digit ratio and hand skill in Austrian children". Biological Psychology 67 (3): 375–84. doi:10.1016/j.biopsycho.2004.03.012. PMID 15294393.
- Titus-Ernstoff (2003). "Psychosexual Characteristics of Men and Women Exposed Prenatally to Diethylstilbestrol". CDC.
- Benderlioglu Z, Nelson RJ; Nelson (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.
- Beech, John R.; MacKintosh, Isla C. (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; Laing; Fink; Manning (October 2003). "Second to fourth digit ratio, testosterone and perceived male dominance". Proceedings of the Royal Society B 270 (1529): 2167–72. doi:10.1098/rspb.2003.2502. PMC 1691489. PMID 14561281.
- Burriss RP, Little AC, Nelson EC; Little; Nelson (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, Vanessa A.; Manning, John T. (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, John V. C.; Androuschak, Gregory; Desierto, Desirée; Jones, Garett; Yudkevich, Maria (2012). "2D:4D Asymmetry and Gender Differences in Academic Performance". PLoS ONE 7 (10): e46319. doi:10.1371/journal.pone.0046319. PMID 23056282. Retrieved 2014-09-20.
- "Finger Ratio Predicts Maths Ability?". http://ibmathsresources.com/. Retrieved 2014-09-20.
- Austin, Elizabeth J.; Manning, John T.; McInroy, Katherine; Mathews, Elizabeth (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, Marc F.; Buunk, Bram P. (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.
- 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.
- 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).
- Derval, Diana. The Right Sensory Mix: Targeting Consumer Product Development Scientifically. Springer, 2010, p. 129-130.
- Derval, Diana. The Right Sensory Mix: Targeting Consumer Product Development Scientifically. Springer, 2010, p. 129-135.
- Derval, Diana. The Right Sensory Mix: Targeting Consumer Product Development Scientifically. Springer, 2010, p. 62-67.
- Derval, Diana. The Right Sensory Mix: Targeting Consumer Product Development Scientifically. Springer, 2010, p. 112-122.
- Derval, Diana (2011). "Hormonal Quotient and tactile sensitivity: a segmentation model to understand and predict individuals' texture preferences based on prenatal exposure to hormones". Proceedings of Society for Behavioral Neuroendocrinology 15th Annual Meeting, Queretaro, Mexico, p.125.
- Csathó A, Osváth A, Bicsák E, Karádi K, Manning J, Kállai J; Osváth; Bicsák; Karádi; Manning; Kállai (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.
- Williams TJ; Pepitone ME; Christensen SE et al. (March 2000). "Finger-length ratios and sexual orientation". Nature 404 (6777): 455–6. doi:10.1038/35006555. PMID 10761903.
- Tortorice JL (2002). "Written on the body: butch vs. femme lesbian gender identity and biological correlates of low digit ratio". Rutgers University. OCLC 80234273.
- McFadden D, Shubel E; Shubel (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; Love (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; Wilson (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, David A.; Gaulin, Steven J. C.; Sporter, Robert J.; McBurney, Donald H. (May 2004). "Sex hormones and finger length: What does 2D:4D indicate?". Evolution and Human Behavior 25 (3): 182–99. 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; Noll; Delsignore; Milos; Schnyder; Hepp (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; Zucker; Steensma; Cohen-Kettenis (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; Dawood; Burriss; Zucker; Puts (2010). "Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women". Behav Neurosci 124 (2): 278–287. doi:10.1037/a0018764. PMID 20364887.
- Hirashi K, Sasaki S, Shikishima C, Ando J; Sasaki; Shikishima; Ando (Jun 2012). "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 41 (3): 711–24. doi:10.1007/s10508-011-9889-z. PMID 22270254.
- 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; Finn; Cooke; Breedlove (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.
- Hiraishi K, Sasaki S, Shikishima C, Ando J.; Sasaki; Shikishima; Ando (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.
- Churchchill AJG, Manning JT, Peters M; Manning; Reimers (2007). "The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D)". Archives of Sexual Behavior 36 (2): 251–260. doi:10.1007/s10508-006-9166-8. PMID 17394056.
- S.J. Robinson, J.T. Manning; Manning (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.
- M.V. Voracek, J.T. Manning & I. Ponocny; Manning; Ponocny (2005). "Digit ratio (2D:4D) in homosexual and heterosexual men from Austria". Archives of Sexual Behaviour 34 (3): 335–340. doi:10.1007/s10508-005-3122-x. PMID 15971016.
- Pappas, Stephanie. "Finger Length Linked to Penis Size". livescience.com. Retrieved 2014-09-20.
- Warner, Jennifer. "Penis Size Linked to Length of Fingers". WebMD.com. Retrieved 2014-09-20.
- Choi, Ho; Kim, Khae Hawn; Han, Jung; Yoon, Sang Jin; Kim, Soo Woong; Kim, Tae Beom. "Second to fourth digit ratio: a predictor of adult penile length". asiaandro.com. Asian Journal of Andrology. Retrieved 2014-09-20.
- Schneider HJ, Pickel J, Stalla GK; Pickel; Stalla (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 (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 et al. (October 2005). "Second to fourth digit ratio and face shape". Proceedings of the Royal Society B 272 (1576): 1995–2001. doi:10.1098/rspb.2005.3179. PMC 1559906. PMID 16191608.
- Fink, Bernhard; Manning, John T.; Neave, Nick; Grammer, Karl (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, Dean R. (2006). "Sexual dimorphism in Upper Palaeolithic hand stencils". Antiquity 80 (308): 390–404.
- Chazine, Jean-Michel; Noury, Arnaud (2006). "Sexual Determination of Hand Stencils at the Masri II Cave". Inora Newsletter 44: 21–6.
- Nelson, Emma C.; Manning, John T.; Sinclair, Anthony G. M. (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". Retrieved 29 October 2009.[dead link][self-published source?]
- Talarovičová A, Kršková L, Blažeková J; Krsková; Blazeková (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; Malisch; Hannon; Hurd; Garland Jr (2008). Svensson, Erik I., ed. "Selective Breeding for a Behavioral Trait Changes Digit Ratio". PLoS ONE 3 (9): e3216. doi:10.1371/journal.pone.0003216. PMC 2528935. PMID 18797502.
- 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.