Digit ratio

Short description: Ratio of lengths of fingers

For the observation about the frequencies of leading digits of numbers, see Benford's law.

Hand with index finger shorter than the ring finger, resulting in a small 2D:4D ratio. Image shows the measurement procedure of dorsal digit length using vernier calipers. The hand is placed on the edge of a table so that fingers are rested on table while making an angle of 90 degrees to the palm.

The digit ratio is the ratio taken of the lengths of different digits or fingers on a hand.

The most commonly studied digit ratio is that of the 2nd (index finger) and 4th (ring finger), also referred to as the 2D:4D ratio, measured on the palm side. It is proposed that the 2D:4D ratio indicates the degree to which an individual has been exposed to androgens during key stages of fetal development. A lower ratio (relatively shorter index finger) has been associated with higher androgen exposure, which would be the physiological norm for males but may also occur in some exceptional circumstances in females. The latter include developmental disorders such as congenital adrenal hyperplasia.

The 2D:4D ratio has been postulated to correlate with a range of physical and cognitive traits in childhood and adulthood, including personality traits such as assertiveness in women, aggressiveness in men, and cognitive abilities such as numerical skills.[1] It has also been shown to vary considerably between racial groups[2] with males having, on average, lower 2D:4D ratio than females.[3]

Studies in this field have drawn criticism over questionable statistical significance and difficulties in reproducing their findings[4][5][6] as well as lack of high quality research protocols.[7]

## Contents

* 1 Measurement
* 2 History of research
* 3 Sexual dimorphism and age
* 4 Effect of prenatal hormones
* 4.1 Developmental disorders
* 5 Geographic and ethnic variation
* 6 Cognitive and physical traits
* 6.1 Association with sexual orientation
* 7 Explanation of the digit ratio effect
* 8 Criticism
* 9 Other animals
* 10 See also
* 11 References
* 12 External links

## Measurement

The 2D:4D ratio is calculated by dividing the length of the index finger by the length of the ring finger of the same hand. Other digit ratios are also calculated similarly in the same hand.

The digit length is typically measured on the palmar (ventral, "palm-side") hand, from the midpoint of the bottom crease to the tip of the finger.[8] Measurement of the digits on the dorsal ("back-side") hand, from the tip of the finger to the proximal phalange-bone protrusion (which occurs when digits are bent at 90 degrees to the palm) has also recently gained acceptance.[9][10] A study has shown that, compared to the palmar digit ratio, the dorsal digit ratio is a better indicator of bone digit ratio.[10] Furthermore, the palmar digit ratio is affected by the differential positioning of flexion creases.[11]

It has been alleged that, because academics have accepted a variety of techniques and equipment (such as calipers, radiography and photocopy), researchers record multiple measurements and report only those which have significant findings,[7] a form of reporting bias.

## History of research

Various reports in the scientific literature of the late 1800s noted that a greater proportion of men have shorter index fingers than ring fingers as compared to women.[12] By 1930, statistically significant sex differences in digit ratio were established in a sample of 201 men and 109 women,[13] 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.[14] This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.[15] Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.[14]

In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children[16] 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.

2D:4D digit ratios have been used alongside other methods in an attempt to understand Palaeolithic hand stencils found in prehistoric European and Indonesian cave painting.[17][18][19]

## Sexual dimorphism and age

Studies measuring the 2D:4D of fetuses have found the ratio to be sexually dimorphic, being lower in males than females.[20][21]

However, when examining the extent of sexual dimorphism evident after birth, adults were found to demonstrate more dimorphism than children, who show the same extent of dimorphism as fetuses.[22] This has led the authors to suggest that digit ratio is also affected by postnatal development. Similarly, a study of people from Poland found statistically significant variation in 2D:4D between age groups (children, young adults and adults) for both males and females in the left hand, but only for women with the right hand.[23]

Sexual dimorphism is strongest in digit ratios involving digit 2 with palmar measurements.[24][25][26] However, when measured dorsally, digit ratios involving digit 5 showed strongest dimorphism, with women having shorter fifth digits on average.[9] Overall, dorsal digit ratios demonstrate stronger sexual dimorphism than palmar digit ratios,[9] including the palmar 2D:4D ratio that has been the basis of most study.[27] Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio.[10] Thus, while most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.

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",[28] which the authors claim may be the basis for the sex differences in 2D:4D and/or any putative hormonal influence on the ratios.

There has been no reliable correlation reported between 2D:4D and sex hormone levels in adulthood.[29][4] Marta Kowal has suggested that testosterone levels are correlated with other factors (such as smoking, body weight and diet) that would impact the relationship between 2D:4D and an adult's testosterone level.[4]

## Effect of prenatal hormones

It has been proposed that 2D:4D ratio is affected by fetal exposure to sex hormones, in particular to testosterone and other androgens. Lower 2D:4D is found to correlate with higher prenatal androgen exposure.[30][29][31][32][33][34][35][36] Therefore digit ratio could be considered a proxy variable (indirect measure) for prenatal androgen exposure. Various studies suggest that 2D:4D is also influenced by prenatal estrogen exposure, and that it correlates negatively, not with prenatal testosterone alone, but with the testosterone-to-estrogen ratio.[37][8][20][38]

Prenatal hormone exposure can be measured via amniocentesis (usually performed between 14th and 20th weeks of pregnancy), maternal serum sampling and umbilical cord sampling at birth, the latter being a measure of exposure in late gestation. A 2024 meta-analysis has suggested that digit ratio may be related to amniotic fluid testosterone levels, but not umbilical cord levels, requiring further validation.[39]

A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development".[30] 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.[40] If, during this period, the fetus is exposed to androgens (levels of which are usually far higher in male than female fetuses) the growth rate of the 4th digit is increased. In a 2006 study, digit ratio analysis of opposite-sex dizygotic twins found that the females in these pairings were born with significantly lower 2D:4D ratio, postulated to occur from exposure to excess androgens from their brothers in utero (the hormone-transfer theory).[41] However, an attempt to replicate these findings with a larger sample of dizygotic twins (867 individuals) found no differences in the variance or co-variance of same-sex and opposite-sex pairings to support the theory, though it did confirm female 2D:4D to be significantly higher than male as expected.[42]

Researchers have raised concerns that, although the general trend points towards a correlation between digit ratio and early androgen exposure, many results have not been statistically significant.[43]

Various findings have also challenged the general trend. One study of 66 children that attempted to replicate the findings of a frequently cited paper on the topic[37] found no association between prenatal testosterone and estrogen levels and 2D:4D in childhood.[44] Another paper also found no relationship between 2D:4D ratios and umbilical cord androgen and estrogen levels.[45] A large meta-analysis studying genomic correlations was unable to find evidence for 2D:4D being a marker for prenatal androgen exposure, but did not exclude the possibility given constraints in genomic knowledge (with 3.8% of the variance in 2D:4D ratio accounted for genetically).[46]

### Developmental disorders

Women with congenital adrenal hyperplasia (CAH), who have elevated androgen levels before birth, have lower (more masculine) 2D:4D on average[31][32][47] along with other possible physiological effects such as an enlarged clitoris and shallow vagina.[48] Males with CAH also express lower digit ratios than controls.[31][32] Amniocentesis samples in males with CAH show that prenatal levels of testosterone are in the high-normal range and levels of the weaker androgen androstenedione are several fold higher than in controls,[49][50][51] indicating that males with CAH are exposed to greater prenatal concentrations of total androgens.

A greater (more feminine) digit ratio occurs for men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to their fathers or to controls.[34]

Digit ratio in men may correlate with genetic variation in the androgen receptor gene.[52] Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater digit ratios, though there have also been reports of failure in replicating this finding.[53] Men with less sensitive androgen receptors may compensate for this by secreting more testosterone via reduced inhibitory feedback on gonadotropins.[54] 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 due to a dysfunctional gene for the androgen receptor present as women and have greater digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding suggests that the sex difference in digit ratios may be unrelated to the Y chromosome per se.[55]

## Geographic and ethnic variation

Several studies have presented evidence that digit ratios are heritable.[56][57]

Manning and colleagues have reported that 2D:4D ratios vary slightly between different ethnic groups. In a study of Han, Berber, Uygur and Jamaican children, 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).[2][58] 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."[59]

In 2007, Manning et al. reported further evidence of ethnic variation in mean 2D:4D, with higher ratios for Whites, non-Chinese Asians and Middle-Easterners, and lower ratios in Chinese and Black samples.[60] A 2008 study by Lu et al. found that the mean values of 2D:4D of the Hui and the Han in Ningxia were lower than those in European countries like Britain.[61]

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.[62][63]

Consanguineous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring,[64] 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.[65]

## Cognitive and physical traits

The 2D:4D ratio has been associated with a variety of traits including academic ability, personality as well as physical attributes such as health outcomes. Lower (masculinized) 2D:4D ratios have been associated with higher mathematical ability as well as personality traits such as aggressiveness (in males), assertiveness (in females) and reduced agreeableness.[1][66][67] Males with higher (feminized) ratios are reported to be more at risk of low sperm counts, heart disease, obesity, metabolic syndrome and depression than those with lower ratios.[68][69][70] Females with higher ratios are more at risk of eating disorders, anxiety and depression.[71]

Prenatal androgen exposure has been implicated as a determinant in left-handedness, though contradictory and inconclusive findings have been reported concerning an association between 2D:4D and left-hand dominance.[72][73][74] Kumar et al. have reported that hand preference is most strongly associated with 3D:4D digit ratios and that the effect is reversed between children and adults.[75]

Fink et al. (2004) found that men with low 2D:4D ratios (indicating high testosterone) and women with high 2D:4D ratios (indicating high estrogen) express greater levels of facial symmetry.[76] The prenatal sex hormone ratio (indicated by 2D:4D) was associated with similar alterations to both male and female face shapes in utero, though the effect was three times more pronounced in males than in females.[77]

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.[78] 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.[79]

In 2020, a paper by John T Manning and Bernhard Fink reported that at the national level, high mean male 2D:4D was associated with high case fatality rates with COVID-19, and suggested that this may be used to "identify for whom it would be advisable to exercise social distancing."[80] Subsequent researchers failed to replicate their findings,[81][6] and criticised Manning and Fink for publishing papers that fail under scrutiny despite the urgent need for high quality science during the pandemic that informs medical decisions.[6]

### Association with sexual orientation

2D:4D ratio is also correlated with sexual orientation. In a meta-analysis (Swift-Gallant et al.), digit ratios were more masculine in lesbians and more feminine in gay men. Gay men had more feminine digit ratios on both the right and left hands, contrary to previous analyses, such as Grimbos et al. which found “no association of 2D:4D with male sexual orientation”.[82] Although previous studies tended to group bisexual and homosexual individuals as "non-heterosexual", or left them out completely, the Swift-Gallant meta-analysis found that bisexual individuals were more similar to heterosexual individuals in their digit ratios.[83] ALSO SEE: McFadden, D., Loehlin, J.C., Breedlove, S.M., Lippa, R.A., Manning, J.T. and Rahman, Q. A reanalysis of five studies on sexual orientation and the relative length of the 2nd and 4th fingers (the 2D:4D ratio). Archives of Sexual Behavior, 2005, 34, 341-356.

## Explanation of the digit ratio effect

The mechanisms by which prenatal hormones influence digit ratio are not clearly understood. Traits such as otoacoustic emissions and arm-to-trunk length ratio are also influenced by prenatal hormones, and it is postulated that Hox genes responsible for both digit and penis development play an important role in affecting these multiple traits (pleiotropy).[84] 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.

Though it is unclear why the digit ratio on the right hand should be more sensitive to prenatal hormones than that on the left hand, as indicated by the greater sex difference on the right than the left,[85] these differences are not evident in dorsal measurements[86] nor bone digit ratios,[87][21][88] and it is possible that right-left differences in palmar 2D:4D may be attributed to differential placing of flexion creases.[11]

## Criticism

The credibility of research into the relationship between digit ratios and traits has been subject to criticism for various reasons.

1. Weaknesses in the statistical significance of numerous findings.[89][90][91]
2. Irreproducibility of findings in many instances, with some studies reporting contradictory results.[92][5][93][94][95]
3. Lack of a clear cause-and-effect relationship between digit ratio and traits.[96][90][7]

Concerning the evidence for causal relationships, the reliability of digit ratio as a proxy variable has come under scrutiny.[5][96][89] Some studies that investigate the relationship between a trait and an unobservable variable fail to consider possible confounding variables.[97] This is especially problematic in cases where the relationship is likely to be weak, such as with prenatal testosterone.[98][94][89] Furthermore, critics have pointed out that controlling for confounding variables is an impossible task due to the expanding list of confounding variables, including ethnicity, sex,[58][99] and physical, medical or behavioral conditions.[100]

A 2021 paper by James Smoliga, titled "Giving science the finger—is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study" followed similar research methodologies adopted by others in the field and came to conclude that low digit ratio is associated with good luck.[7] This result is attributed to chance and presented as an illustration of the reproducibility crisis. Smoliga would later go on to label the field as pseudoscience.[101] In a response, John T. Manning criticized the researchers for not controlling for ethnicity.[102]

Researcher Marta Kowal states: "One of the general limitations of studies on the digit ratios is the high number of degrees of freedom within the 2D:4D (the right, left, and right minus left 2D:4D). Running many analyses with different predictors increases the chances of finding allegedly significant results."[4]

## Other animals

* Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.[25] WRONG CITATION. SEE:

McFadden, D. and Bracht, M.S. Sex differences in the relative lengths of metacarpals and metatarsals in gorillas and chimpanzees. Hormones and Behavior, 2005, 47, 99-111. AND McFadden, D., and Bracht, M.S. The relative lengths and weights of metacarpals and metatarsals in baboons (Papio hamadryas). Hormones and Behavior, 2003, 43, 347-355.

* Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.[103]
* 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.[104]
* 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.[105]
* 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 in contradiction to the correlation between digit ratio and physical fitness in human beings, suggesting that 2D:4D may not be a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect the effect of glucocorticoids or other factors that regulate any of various genes.[106]
* 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.[107]
* Studies in mice indicate that prenatal androgen influences 2D:4D primarily by promoting growth of the fourth digit.[30]

## See also

* Anogenital distance
* Body mass index
* Chiromancy (palm reading)
* Dermatoglyphics
* Handedness and sexual orientation
* Waist–hip ratio

## References

1. ↑ 1.0 1.1 "2D:4D Ratio and its Implications in Medicine.". Journal of Clinical and Diagnostic Research 10 (12): CM01–CM03. 2016. doi:10.7860/JCDR/2016/21952.9000. PMID 28208851.
2. ↑ 2.0 2.1 "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. May 2000. doi:10.1016/S1090-5138(00)00029-5. PMID 10828555. Bibcode: 2000EHumB..21..163M.
3. ↑ Nayak, Satheesha B.; Nair, Dhiviah; Ravi, Vimal; Aithal, Ashwini P. (2018-09-20). "A comparative study on digit ratio and hand patterns of three ethnic races of Malaysia" (in en). Egyptian Journal of Forensic Sciences 8 (1): 56. doi:10.1186/s41935-018-0087-1. ISSN 2090-5939.
4. ↑ 4.0 4.1 4.2 4.3 "No relationship between the digit ratios (2D:4D) and salivary testosterone change: Study on men under an acute exercise". Scientific Reports 10 (1). June 2020. doi:10.1038/s41598-020-66915-9. PMID 32572079. Bibcode: 2020NatSR..1010068K.
5. ↑ 5.0 5.1 5.2 "Talk to the hand. Scientists try to debunk idea that finger length can reveal personality and health". Science. 2019. doi:10.1126/science.aay2735. https://www.science.org/news/2019/06/talk-hand-scientists-try-debunk-idea-finger-length-can-reveal-personality-and-health. Retrieved 1 October 2021.
6. ↑ 6.0 6.1 6.2 "(Mis-)understanding COVID-19 and digit ratio: Methodological and statistical issues in Manning and Fink (2020)". Early Human Development 148. September 2020. doi:10.1016/j.earlhumdev.2020.105095. PMID 32499091.
7. ↑ 7.0 7.1 7.2 7.3 "Giving science the finger-is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study". BMJ 375. December 2021. doi:10.1136/bmj-2021-067849. PMID 34911738.
8. ↑ 8.0 8.1 "Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle". Journal of Anatomy 211 (5): 630–638. November 2007. doi:10.1111/j.1469-7580.2007.00801.x. PMID 17764524.
9. ↑ 9.0 9.1 9.2 "Sexual Dimorphism in Digit Ratios Derived from Dorsal Digit Length among Adults and Children". Frontiers in Endocrinology 8: 41. 6 March 2017. doi:10.3389/fendo.2017.00041. PMID 28321205.
10. ↑ 10.0 10.1 10.2 "Second to fourth digit ratios, male genital development and reproductive health: a clinical study among fertile men and testis cancer patients". International Journal of Andrology 34 (4 Pt 2): e49–e58. August 2011. doi:10.1111/j.1365-2605.2010.01124.x. PMID 21091719.
11. ↑ 11.0 11.1 "Differential Placing of Flexion Creases Contributes to Sex Differences in the Second-to-Fourth Digit Ratio (2D:4D)". Frontiers in Endocrinology 10. 2 August 2019. doi:10.3389/fendo.2019.00537. PMID 31428058.
12. ↑ "Anthropological Notes on the Human Hand". American Anthropologist A1: 51–76. 1888. doi:10.1525/aa.1888.1.1.02a00040.
13. ↑ "Human finger types". The Anatomical Record 46 (2): 199–204. 1930. doi:10.1002/ar.1090460210.
14. ↑ 14.0 14.1 "Finger-length as an index of assertiveness in women". Personality and Individual Differences 4: 111–112. 1983. doi:10.1016/0191-8869(83)90061-2.
15. ↑ "Fingers to feminism: The rise of 2D:4D". Quarterly Review 4: 25–32. 2010. https://archive.org/details/sim_michigan-quarterly-review_winter-2010_49_1/page/25.
16. ↑ "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): 3000–3004. November 1998. doi:10.1093/humrep/13.11.3000. PMID 9853845.
17. ↑ "Sexual dimorphism in Upper Palaeolithic hand stencils". Antiquity 80 (308): 390–404. 2006. doi:10.1017/S0003598X00093704. https://archive.org/details/sim_antiquity_2006-06_80_308/page/390.
18. ↑ "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. 2008. https://hal.archives-ouvertes.fr/hal-00342551. Retrieved 30 January 2019.
19. ↑ "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. 2006. doi:10.3828/bfarm.2006.1.6.
20. ↑ 20.0 20.1 "Fetal development of the hand, digits and digit ratio (2D:4D)". Early Human Development 82 (7): 469–475. July 2006. doi:10.1016/j.earlhumdev.2005.12.002. PMID 16473482.
21. ↑ 21.0 21.1 "Sexual dimorphism in the prenatal digit ratio (2D:4D)". Archives of Sexual Behavior 39 (1): 57–62. February 2010. doi:10.1007/s10508-009-9485-7. PMID 19301112.
22. ↑ 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)" (in en). Archives of Sexual Behavior 39 (1): 57–62. doi:10.1007/s10508-009-9485-7. ISSN 0004-0002. PMID 19301112.
23. ↑ Pruszkowska-Przybylska, Paulina; Kobus, Magdalena; Żądzińska, Elżbieta; Rosset, Iwona; Pruszkowska, Milena; Kuczyński, Wojciech; Sitek, Aneta (2023-12-21). "The age difference in 2D:4D among the Polish population: An exploratory study" (in en). Anthropological Review 86 (4): 99–110. doi:10.18778/1898-6773.86.4.08. ISSN 2083-4594. https://czasopisma.uni.lodz.pl/ar/article/view/18751. Retrieved 25 December 2023.
24. ↑ "Relative finger lengths, sex differences, and psychological traits". Archives of Sexual Behavior 38 (2): 298–305. April 2009. doi:10.1007/s10508-007-9303-z. PMID 18340520. https://archive.org/details/sim_archives-of-sexual-behavior_2009-04_38_2/page/298.
25. ↑ 25.0 25.1 "Relative lengths of fingers and toes in human males and females". Hormones and Behavior 42 (4): 492–500. December 2002. doi:10.1006/hbeh.2002.1833. PMID 12488115. Bibcode: 2002HoBeh..42..492M.
26. ↑ "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. February 2009. doi:10.2466/pms.108.1.83-93. PMID 19425449.
27. ↑ "Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand". American Journal of Human Biology 22 (5): 619–630. September 2010. doi:10.1002/ajhb.21054. PMID 20737609.
28. ↑ "Differences in the 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line". Biology Letters 5 (5): 643–646. October 2009. doi:10.1098/rsbl.2009.0346. PMID 19553247.
29. ↑ 29.0 29.1 "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. May 2007. doi:10.1016/j.psyneuen.2007.01.007. PMID 17400395.
30. ↑ 30.0 30.1 30.2 "Developmental basis of sexually dimorphic digit ratios". Proceedings of the National Academy of Sciences of the United States of America 108 (39): 16289–16294. September 2011. doi:10.1073/pnas.1108312108. PMID 21896736.
31. ↑ 31.0 31.1 31.2 "Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia". Hormones and Behavior 42 (4): 380–386. December 2002. doi:10.1006/hbeh.2002.1830. PMID 12488105. Bibcode: 2002HoBeh..42..380B.
32. ↑ 32.0 32.1 32.2 "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. December 2002. doi:10.1016/s0378-3782(02)00073-7. PMID 12441204.
33. ↑ "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. 2014. doi:10.1002/ajhb.22545. PMID 24668932.
34. ↑ 34.0 34.1 "Digit ratio (2D:4D) in Klinefelter's syndrome". Andrology 1 (1): 94–99. January 2013. doi:10.1111/j.2047-2927.2012.00013.x. PMID 23258636.
35. ↑ "Digit ratio (2D:4D) in newborns: influences of prenatal testosterone and maternal environment". Early Human Development 89 (2): 107–112. February 2013. doi:10.1016/j.earlhumdev.2012.08.009. PMID 23017880.
36. ↑ "The use of digit ratios as markers for perinatal androgen action". Reproductive Biology and Endocrinology 4. February 2006. doi:10.1186/1477-7827-4-10. PMID 16504142.
37. ↑ 37.0 37.1 "2nd to 4th digit ratios, fetal testosterone and estradiol". Early Human Development 77 (1–2): 23–28. April 2004. doi:10.1016/j.earlhumdev.2003.12.002. PMID 15113628.
38. ↑ "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. June 2013. doi:10.1210/jc.2012-4057. PMID 23569219.
39. ↑ Sorokowski, Piotr; Kowal, Marta (5 October 2023). "Relationship between the 2D: 4D and prenatal testosterone, adult level testosterone, and testosterone change: Meta-analysis of 54 studies" (in en). American Journal of Biological Anthropology 183 (1): 20–38. doi:10.1002/ajpa.24852. ISSN 2692-7691. PMID 37795916. https://onlinelibrary.wiley.com/doi/10.1002/ajpa.24852.
40. ↑ "Early prenatal attainment of adult metacarpal-phalangeal rankings and proportions". American Journal of Physical Anthropology 43 (3): 327–332. November 1975. doi:10.1002/ajpa.1330430305. PMID 1211429. Bibcode: 1975AJPA...43..327G.
41. ↑ "Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins". Hormones and Behavior 49 (3): 315–319. March 2006. doi:10.1016/j.yhbeh.2005.08.003. PMID 16143332.
42. ↑ "No effects of prenatal hormone transfer on digit ratio in a large sample of same- and opposite-sex dizygotic twins" (in en). Personality and Individual Differences 44 (5): 1225–1234. 1 April 2008. doi:10.1016/j.paid.2007.11.017. ISSN 0191-8869. https://www.sciencedirect.com/science/article/pii/S0191886907004230. Retrieved 10 June 2023.
43. ↑ "Digit ratio (2D:4D) and prenatal/perinatal sex hormones: A response to Manning and Fink (2017)". Early Human Development 113: 75–76. October 2017. doi:10.1016/j.earlhumdev.2017.09.004. PMID 28917583. https://eprint.ncl.ac.uk/fulltext.aspx?url=249530/77FA418E-6576-4475-A6B5-EEE33F7CAF94.pdf&pub_id=249530.
44. ↑ "Digit ratio (2D:4D) and amniotic testosterone and estradiol: an attempted replication of Lutchmaya et al. (2004)". Journal of Developmental Origins of Health and Disease 12 (6): 859–864. December 2021. doi:10.1017/S2040174420001294. PMID 33472723. https://repository.uel.ac.uk/download/80146174549c10eed26a47263c60f382cfb6cac134adaa6fa3e297afd005044b/349843/Richards, Brown and Constantinescu.pdf. Retrieved 23 March 2022.
45. ↑ "Adult digit ratio (2D:4D) is not related to umbilical cord androgen or estrogen concentrations, their ratios or net bioactivity". Early Human Development 91 (2): 111–117. February 2015. doi:10.1016/j.earlhumdev.2014.12.011. PMID 25594498. https://research-repository.uwa.edu.au/en/publications/b3d44281-0fcf-499c-9743-7daac128c486.
46. ↑ "Genome-wide association study identifies nine novel loci for 2D:4D finger ratio, a putative retrospective biomarker of testosterone exposure in utero". Human Molecular Genetics 27 (11): 2025–2038. June 2018. doi:10.1093/hmg/ddy121. PMID 29659830.
47. ↑ "High fetal testosterone and sexually dimorphic cerebral networks in females". Cerebral Cortex 19 (5): 1167–1174. May 2009. doi:10.1093/cercor/bhn160. PMID 18854582.
48. ↑ Sex and sexuality. 1. Greenwood Publishing Group. 2006. p. 165. ISBN 978-0-313-04919-4. https://books.google.com/books?id=KBi9aG0pQAkC&pg=PA165.
49. ↑ "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): 223–229. August 1980. doi:10.1210/jcem-51-2-223. PMID 6447160.
50. ↑ "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): 117–120. January 1993. doi:10.1210/jcem.76.1.8421074. PMID 8421074.
51. ↑ "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): 2724–2728. August 1999. doi:10.1210/jcem.84.8.5870. PMID 10443667.
52. ↑ "The second to fourth digit ratio and variation in the androgen receptor gene". Evolution and Human Behavior 24 (6): 399–405. 2003. doi:10.1016/S1090-5138(03)00052-7. Bibcode: 2003EHumB..24..399M.
53. ↑ "Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D:4D ratio". Evol Hum Behav 33 (5): 557–561. 2012. doi:10.1016/j.evolhumbehav.2012.02.003. Bibcode: 2012EHumB..33..557H.
54. ↑ "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): 3604–3610. September 2007. doi:10.1210/jc.2007-0117. PMID 17579205.
55. ↑ "Fingers as a marker of prenatal androgen exposure". Endocrinology 150 (11): 5119–5124. November 2009. doi:10.1210/en.2009-0774. PMID 19819951.
56. ↑ "The big finger: the second to fourth digit ratio is a predictor of sporting ability in women". British Journal of Sports Medicine 40 (12): 981–983. December 2006. doi:10.1136/bjsm.2006.027193. PMID 17008344.
57. ↑ "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): 112–118. February 2008. doi:10.1007/s10508-007-9272-2. PMID 18074216. https://archive.org/details/sim_archives-of-sexual-behavior_2008-02_37_1/page/112.
58. ↑ 58.0 58.1 "Sex and ethnic differences in 2nd to 4th digit ratio of children". Early Human Development 80 (2): 161–168. November 2004. doi:10.1016/j.earlhumdev.2004.06.004. PMID 15500996.
59. ↑ "Finger-length ratios and sexual orientation". Nature 404 (6777): 455–456. March 2000. doi:10.1038/35006555. PMID 10761903. Bibcode: 2000Natur.404..455W.
60. ↑ "The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D)". Archives of Sexual Behavior 36 (2): 223–233. April 2007. doi:10.1007/s10508-007-9171-6. PMID 17373585. https://archive.org/details/sim_archives-of-sexual-behavior_2007-04_36_2/page/223.
61. ↑ "Study on the digit ratio of Hui and Han ethnic groups in Ningxia.". Acta Anatomica Sinica 39 (2): 267. 2008. http://en.cnki.com.cn/Article_en/CJFDTOTAL-JPXB200802033.htm. Retrieved 7 November 2016.
62. ↑ "Population differences in finger-length ratios: ethnicity or latitude?". Archives of Sexual Behavior 35 (6): 739–742. December 2006. doi:10.1007/s10508-006-9039-1. PMID 16688484. https://archive.org/details/sim_archives-of-sexual-behavior_2006-12_35_6/page/739.
63. ↑ "The digit ratio (2D:4D) in China: A meta-analysis". American Journal of Human Biology 27 (3): 304–309. 6 May 2015. doi:10.1002/ajhb.22639. PMID 25284473.
64. ↑ "Inbreeding is associated with lower 2D: 4D digit ratio". American Journal of Human Biology 26 (2): 183–188. 2014. doi:10.1002/ajhb.22491. PMID 24288299.
65. ↑ "Consanguinity and reproductive health among Arabs". Reproductive Health 6. October 2009. doi:10.1186/1742-4755-6-17. PMID 19811666.
66. ↑ "Second-to-fourth digit ratio related to verbal and numerical intelligence and the Big Five". Personality and Individual Differences 39 (5): 959–966. 2005. doi:10.1016/j.paid.2005.03.016.
67. ↑ "Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D: 4D digit ratio.". Evolution and Human Behavior 33 (5): 557–561. 2012. doi:10.1016/j.evolhumbehav.2012.02.003. Bibcode: 2012EHumB..33..557H.
68. ↑ "Sexual dimorphism in ratio of second and fourth digits and its relationship with metabolic syndrome indices and cardiovascular risk factors.". Journal of Research in Medical Sciences 19 (3): 234–239. 2014. PMID 24949031.
69. ↑ "Depression in men is associated with more feminine finger length ratios.". Personality and Individual Differences 39 (4): 829–836. 2005. doi:10.1016/j.paid.2004.12.017.
70. ↑ "Directional asymmetry (right–left differences) in digit ratio (2D: 4D) predict indirect aggression in women.". Personality and Individual Differences 43 (4): 865–872. 2007. doi:10.1016/j.paid.2007.02.010.
71. ↑ "Digit ratio (2D: 4D) in Klinefelter's syndrome". Andrology 1 (1): 94–99. 2013. doi:10.1111/j.2047-2927.2012.00013.x. PMID 23258636.
72. ↑ "Second to fourth digit ratio, handedness and testicular germ cell tumors.". Early Human Development 89 (7): 463–466. 2013. doi:10.1016/j.earlhumdev.2013.04.001. PMID 23623693.
73. ↑ "Differential effects of prenatal testosterone on lateralization of handedness and language.". Neuropsychology 25 (5): 581–589. 2011. doi:10.1037/a0023293. PMID 21500918. https://research.rug.nl/en/publications/3bef9c80-1698-44f9-90a1-d6454db544e5.
74. ↑ "Digit ratio (2D:4D) and hand preference for writing in the BBC Internet Study.". Laterality 14 (5): 528–540. 2009. doi:10.1080/13576500802637872. PMID 19177258.
75. ↑ "Effects of hand preference on digit lengths and digit ratios among children and adults". Early Human Development 151. December 2020. doi:10.1016/j.earlhumdev.2020.105204. PMID 33059164. https://epublications.marquette.edu/math_fac/56. Retrieved 23 March 2022.
76. ↑ "Second to fourth digit ratio and facial asymmetry". Evolution and Human Behavior 25 (2): 125–32. March 2004. doi:10.1016/S1090-5138(03)00084-9. Bibcode: 2004EHumB..25..125F.
77. ↑ "Second to fourth digit ratio and face shape". Proceedings. Biological Sciences 272 (1576): 1995–2001. October 2005. doi:10.1098/rspb.2005.3179. PMID 16191608.
78. ↑ "Stability of proposed biomarkers of prenatal androgen exposure over the menstrual cycle". Journal of Developmental Origins of Health and Disease 6 (2): 149–157. April 2015. doi:10.1017/S2040174414000646. PMID 25584807.
79. ↑ "The ratio of second to fourth digit lengths: a marker of impaired fetal growth?". Early Human Development 68 (1): 21–26. June 2002. doi:10.1016/s0378-3782(02)00009-9. PMID 12191526.
80. ↑ "Understanding COVID-19: Digit ratio (2D:4D) and sex differences in national case fatality rates". Early Human Development 146. July 2020. doi:10.1016/j.earlhumdev.2020.105074. PMID 32419720.
81. ↑ "No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities". Early Human Development 152. January 2021. doi:10.1016/j.earlhumdev.2020.105272. PMID 33227636.
82. ↑ Grimbos, Teresa; Dawood, Khytam; Burriss, Robert P.; Zucker, Kenneth J.; Puts, David A. (2010). "Sexual orientation and the second to fourth finger length ratio: A meta-analysis in men and women." (in en). Behavioral Neuroscience 124 (2): 278–287. doi:10.1037/a0018764. ISSN 1939-0084. PMID 20364887. https://doi.apa.org/doi/10.1037/a0018764.
83. ↑ Swift-Gallant, Ashlyn; Aung, Toe; Salia, Stephanie; Breedlove, S. Marc; Puts, David (2025-04-25). "Sexual orientation is associated with 2D:4D finger length ratios in both sexes: an updated and expanded meta-analysis". Frontiers in Psychology 16. doi:10.3389/fpsyg.2025.1559158. ISSN 1664-1078. PMID 40351575.
84. ↑ "HOX gene links limb, genital defects". Science 275 (5306): 1568. March 1997. doi:10.1126/science.275.5306.1568. PMID 9072822.
85. ↑ "Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand". American Journal of Human Biology 22 (5): 619–630. 2010. doi:10.1002/ajhb.21054. PMID 20737609.
86. ↑ "The effects of hand preference and sex on right-left asymmetry in dorsal digit lengths among adults and children". Early Human Development 153. February 2021. doi:10.1016/j.earlhumdev.2020.105293. PMID 33340946.
87. ↑ "A comparison of measurement methods and sexual dimorphism for digit ratio (2D:4D) in Han ethnicity". Archives of Sexual Behavior 43 (2): 329–333. February 2014. doi:10.1007/s10508-013-0179-9. PMID 24013635.
88. ↑ "Radiographic assessment of the index to ring finger ratio (2D:4D) in adults". Journal of Anatomy 212 (1): 42–48. January 2008. doi:10.1111/j.1469-7580.2007.00830.x. PMID 18005121.
89. ↑ 89.0 89.1 89.2 "Which Data to Meta-Analyze, and How?". Zeitschrift für Psychologie (Hogrefe Publishing Group) 227 (1): 64–82. 2019. doi:10.1027/2151-2604/a000357. ISSN 2190-8370.
90. ↑ 90.0 90.1 "No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities". Early Human Development (Elsevier BV) 152. January 2021. doi:10.1016/j.earlhumdev.2020.105272. PMID 33227636.
91. ↑ "Digit ratio (2D:4D) and sex-role orientation: Further evidence and meta-analysis". Personality and Individual Differences (Elsevier BV) 51 (4): 417–422. September 2011. doi:10.1016/j.paid.2010.06.009. ISSN 0191-8869. https://archive.org/details/sim_personality-and-individual-differences_2011-09_51_4/page/417.
92. ↑ "Null Effects of Game Violence, Game Difficulty, and 2D:4D Digit Ratio on Aggressive Behavior". Psychological Science (SAGE Publications) 30 (4): 606–616. April 2019. doi:10.1177/0956797619829688. PMID 30843758.
93. ↑ "A variant in LIN28B is associated with 2D:4D finger-length ratio, a putative retrospective biomarker of prenatal testosterone exposure". American Journal of Human Genetics 86 (4): 519–525. April 2010. doi:10.1016/j.ajhg.2010.02.017. PMID 20303062.
94. ↑ 94.0 94.1 "Interpreting digit ratio (2D:4D)-behavior correlations: 2D:4D sex difference, stability, and behavioral correlates and their replicability in young children". Hormones and Behavior (Elsevier BV) 78: 86–94. February 2016. doi:10.1016/j.yhbeh.2015.10.022. PMID 26542674.
95. ↑ "Revisiting the Criminological Consequences of Exposure to Fetal Testosterone: A Meta-Analysis of the 2D:4D Digit Ratio". Criminology (Wiley) 54 (4): 587–620. 22 September 2016. doi:10.1111/1745-9125.12115. ISSN 0011-1384.
96. ↑ 96.0 96.1 "Facing off with the phalangeal phenomenon and editorial policies: A commentary on Swift-Gallant, Johnson, Di Rita and Breedlove (2020)". Hormones and Behavior (Elsevier BV) 120. April 2020. doi:10.1016/j.yhbeh.2020.104710. PMID 32057822.
97. ↑ "A comprehensive allometric analysis of 2nd digit length to 4th digit length in humans". Proceedings. Biological Sciences 284 (1857). June 2017. doi:10.1098/rspb.2017.0356. PMID 28659446.
98. ↑ "Adult digit ratio (2D:4D) is not related to umbilical cord androgen or estrogen concentrations, their ratios or net bioactivity". Early Human Development (Elsevier BV) 91 (2): 111–117. February 2015. doi:10.1016/j.earlhumdev.2014.12.011. PMID 25594498. https://research-repository.uwa.edu.au/en/publications/b3d44281-0fcf-499c-9743-7daac128c486.
99. ↑ "Digit ratio (2D:4D) and Cattell's personality traits". European Journal of Personality (SAGE Publications) 22 (4): 347–356. June 2008. doi:10.1002/per.664. ISSN 0890-2070. https://archive.org/details/sim_european-journal-of-personality_2008-06_22_4/page/347.
100. ↑ "Does digit ratio (2D:4D) predict penile length?". Asian Journal of Andrology (Medknow) 13 (5): 667–668. September 2011. doi:10.1038/aja.2011.81. PMID 21725331.
101. ↑ "Faculty Spotlight: James M. Smoliga | Tufts University School of Medicine". https://medicine.tufts.edu/news-events/news/faculty-spotlight-james-m-smoliga.
102. ↑ Manning, John T. (11 June 2023). "The Big Finger Trumps the Lucky Jim Fallacy" (in en). The BMJ. https://www.bmj.com/content/375/bmj-2021-067849/rr. Retrieved 11 June 2023.
103. ↑ "Evolution of Primate Social Systems: Implications for Hominin Social Evolution". Lucy to Language: The Benchmark Papers. OUP Oxford. 2014. pp. 317–405. ISBN 978-0-19-965259-4. https://books.google.com/books?id=VEakAgAAQBAJ&pg=PA317.
104. ↑ "An eye for detail: selective sexual imprinting in zebra finches". Evolution; International Journal of Organic Evolution 60 (5): 1076–1085. May 2006. doi:10.1554/05-399.1. PMID 16817546. Bibcode: 2006Evolu..60.1076B.
105. ↑ "Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood". Hormones and Behavior 55 (1): 235–239. January 2009. doi:10.1016/j.yhbeh.2008.10.010. PMID 19022257.
106. ↑ "Selective breeding for a behavioral trait changes digit ratio". PLOS ONE 3 (9). September 2008. doi:10.1371/journal.pone.0003216. PMID 18797502. Bibcode: 2008PLoSO...3.3216Y.
107. ↑ "Examination marks of male university students positively correlate with finger length ratios (2D:4D)". Biological Psychology 71 (2): 175–182. February 2006. doi:10.1016/j.biopsycho.2005.03.006. PMID 15978716. https://archive.org/details/sim_biological-psychology_2006-02_71_2/page/175.

## External links

* PubMed listing of papers on digit ratios
* "Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning". Human Nature Review 2: 418–23. October 2002. http://human-nature.com/nibbs/02/manning.html.

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