Digit ratio

From Wikipedia, the free encyclopedia

Hand with index finger being shorter than the ring finger, resulting in a small 2D:4D ratio, pointing to a high exposure to testosterone in the uterus.
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. In modern sciences, the 2D:4D ratio has become the most commonly 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. On average, males have a lower 2D:4D ratio than females. [1]

Although studies have claimed to show a correlation between the 2D:4D digit ratio and various physical and behavioral traits, these studies have drawn criticism for irreproducible or contradictory findings,[2][3][4] exaggerated claims of usefulness,[5][6] and lack of high quality research protocols.[6]


The digit length is typically measured on the palmar (ventral, "front", "palm-side") hand, from the midpoint of the bottom crease to the tip of the finger.[7] 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 recently also gained acceptance.[8][9] A study has shown that, compared to the palmar digit ratio, the dorsal digit ratio is a better indicator of bone digit ratio.[9] Moreover, differential placing of flexion creases is a factor in the palmar digit ratio.[10]

Other digit ratios are also similarly calculated in the same hand.

It has been suggested that because academics have accepted a variety of techniques and equipment (such as calipers, radiography and photocopy), researchers deliberately record multiple measurements and report only those which have significant findings,[6] a form of reporting bias.


The ratio of two digits in particular – the palmar 2nd (index finger) and 4th (ring finger) – is supposedly affected by fetal exposure to hormones, in particular to testosterone, and other androgens; this 2D:4D ratio could therefore be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher prenatal androgen exposure.[11][12][13][14][15][16][17][18] 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).[19][7][20][21]

However, various studies have challenged the aforementioned findings. A meta analysis of 54 studies (with 8077 participants) found there was no correlation between digit ratio and adult testoserone levels (44 studies), testoserone change (6 studies) and prenatal testoserone (10 studies). The researchers conclude that due to the link being unsupported, it is concerningly common that papers use 2D:4D as a proxy for testoserone.[22] Another paper has also found no relationship between 2D:4D ratios and testosterone or estradiol levels, or even the testosterone-to-estradiol ratio.[23] The largest study on 2D:4D as of 2018 failed to find evidence for 2D:4D being a marker for prenatal androgen exposure,[24] and one frequently cited paper on the topic[19] has failed to be replicated.[25]

A study measuring the 2D:4D of deceased fetuses found the ratio to be sexually dimorphic, being lower in men than in women. However when comparing the extent of the sexual dimorphism to adults and children, the researchers found the extent was similar to children but lower than adults. This led the authors to suggest that the digit ratio is also affected by the postnatal development process.[26] In palmar digit ratios, strong sexual dimorphism occurs in those of digit 2.[27][28][29] In dorsal digit ratios, in contrast, strong sexual dimorphism occurs in those of digit 5, with women having shorter fifth digits on average.[8] Overall, the report of sexual dimorphism is much stronger in dorsal digit ratios[8] than in palmar digit ratios, especially as compared to the classic, palmar 2D:4D ratio.[30] Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio.[9] Thus, while most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.

Experimental studies of monkeys have shown prenatal testosterone injection produces male-typical changes in dermatoglyphics[31] and in palmar digit length, but not in bone digit length.[32] Moreover, this effect occurs in digit 2 but not in digit 4 in monkeys and frogs.[32][33] Additionally, human epidermal tissues have only androgen receptors and no estrogen receptors-α.[34] 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.[10]

History of research[edit]

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,[35] with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930,[36] 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.[37] This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.[38] Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.[37] In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children[39] 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.

Since 2006, 2D:4D digit ratios have been used alongside other methods in an attempt understand Palaeolithic hand stencils found in prehistoric European and Indonesian cave painting.[40][41][42]

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",[43] 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".[11] 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.[44] 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.[45]

Importantly, there has been no reliable correlation between the sex hormone levels of an adult and the individual's 2D:4D,[12] which implies that it is strictly the exposure in utero that causes this phenomenon.

Marta Kowal alongside other researchers in 2020 published a paper providing more evidence that there is no relationship between adult testosterone levels and 2D:4D ratio, highlighting that testosterone levels are correlated with other factors (such as smoking,[46] body weight,[47] and diet[48]) that would impact the relationship between 2D:4D and an adult's testosterone level.[2]

The paper also highlights an issue within the research field: "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."

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."[49] Subsequent researchers failed to replicate their findings,[50][4] 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.[4]

In 2021, a peer reviewed paper by Professor 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" heavily criticized the field of research. The researchers followed the same research methodology as other literature and were able to conclude that there is an association of low digit ratio with good luck. This result is attributed to chance, and an illustration of the reproducibility crisis. This is complemented by a satirical list of "pitfalls to avoid in research" which lists high quality research practices that authors hoping to publish false positive findings regarding 2D:4D digit ratios are likely to avoid, which are:

Smoliga also provides an algorithm for researchers to identify a correlation between 2D:4D digit ratio, and any outcome measure.[6]

In a response, John T. Manning criticized the researchers for not controlling for ethnicity.[51]

Evidence of androgen effect[edit]

The sex difference in 2D:4D is present before birth in humans.[20][52] The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlated with the child's subsequent 2D:4D ratio.[19] 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. However, this study has been criticised for its lack of scrutiny despite being frequently cited, and has failed to be replicated.[25]

The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, as researchers found no difference in estrogen levels between males and females.[19]

Researchers have highlighted that most research measuring amniotic fluid or umbilical cord blood to make conclusions regarding 2D:4D and early androgen exposure is tenuous, as although the research points towards a negative correlation, the results have not been statistically significant.[53]

Several studies present evidence that digit ratios are heritable.[54][55]

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.[56]

Disorders of sex development[edit]

Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average.[13][14][57] Other possible physiological effects include an enlarged clitoris and shallow vagina.[58] Males with CAH have smaller (more masculine) digit ratios than control males,[13][14] 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.[59][60][61] These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.

A greater digit ratio occurs for men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to control males, than in their fathers or control males.[16]

Digit ratio in men may correlate with genetic variation in the androgen receptor gene.[62] 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.[63] However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by secreting more testosterone,[64] 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.[65]

Explanation of the digit ratio effect[edit]

It is not clear why digit ratio might be influenced by prenatal hormones, with studies supporting the notion there is no relationhip between digit ratios and prenatal hormones.[53][66][23] 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[67] 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.[68] However, because no right–left difference has been found in sexual dimorphism of bone digit ratios (2D:4D[69][52][70]) and dorsal digit ratios[71] and because differential placing of flexion creases contributes to sex differences in palmar digit ratio,[10] right–left differences in the placing of flexion creases may be determining right–left difference in palmar 2D:4D ratio.

Geographic and ethnic variation in 2D:4D[edit]

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).[72][73] 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."[74]

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.[citation needed]

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.[75]

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.[76]

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.[77][78]

Consanguinous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring,[79] 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.[80]

Digit ratio and development[edit]

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.[81] Kumar et al.[82] have reported that a similar effect of hand preference on digit lengths and digit ratios occurs oppositely among children and adults. 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.[83] 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.[84]


The digit ratio has received attention within mainstream media because of supposed correlations with health and behavior,[6] however the credibility and usefulness of research into the relationship between digit ratios and traits has been subject to criticism.[85] Critics have found research to be lacking moderate to strong statistical significance,[86][87] consistently failing to be replicated, and useless as a proxy variable.[3]

Although research into digit ratios appears to find statistical significance, there has been criticism of researchers for failing to account for a majority of the variation within a dataset, with one of the most highly publicized papers on the topic failing to account for 96% of variation.[5] Furthermore, skepticism has also been attributed to an abundance of research lacking a cause-and-effect relationship between digit ratio and supposed traits, research that often lacks any theoretical motivation.[88][86] This criticism was demonstrated in the paper "Giving science the finger—is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study" by correlating low 2D:4D digit ratio with good luck, despite following established research methods.[6]

The utilization of digit ratio as a proxy variable has come under scrutiny,[88][85] as studies that investigate the relationship between a trait and some unobservable variable fail to consider other possible confounding variables.[89] This is especially problematic in cases where the relationship is likely to be weak, such as prenatal testosterone.[90][91][85] 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,[73][92] and physical, medical or behavioral conditions.[93]

Researchers have also pointed out that research into the digit ratio is illustrative of the ongoing replication crisis.[3] Not only have subsequent studies and meta-analysis failed to replicate previous research,[3][85][94] but have often found contradictory results,[95][91][96] or failed closer scrutiny.[97][86] Psychologist Martin Voracek has criticized the field for its irreproducible findings, stating it's "like a house of cards built on an unknown and uncertain base" and subsequently compared "the work on finger ratios to phrenology or physiognomy, the discredited ideas that people's head shape or facial features, respectively, reveal their personalities, character, and intelligence."[3]

Other animals[edit]

  • Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.[28]
  • Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.[98]
  • 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.[99]
  • 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.[100]
  • 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.[101]
  • 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.[102]
  • Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.[11]

See also[edit]


  1. ^ Zamani Sani, Seyed Hojjat; Sadeghi-Bahmani, Dena; Fathirezaie, Zahra; Aghdasi, Mohammad Taghi; Abbaspour, Kosar; Badicu, Georgian; Brand, Serge (5 June 2022). "Gender Differences and Relationship of 2D:4D-Ratio, Mental Toughness and Dark Triad Traits among Active Young Adults". Biology. 11 (6): 864. doi:10.3390/biology11060864. ISSN 2079-7737. PMC 9220407. PMID 35741385.
  2. ^ a b Kowal M, Sorokowski P, Żelaźniewicz A, Nowak J, Orzechowski S, Żurek G, et al. (June 2020). "No relationship between the digit ratios (2D:4D) and salivary testosterone change: Study on men under an acute exercise". Scientific Reports. 10 (1): 10068. Bibcode:2020NatSR..1010068K. doi:10.1038/s41598-020-66915-9. PMC 7308391. PMID 32572079.
  3. ^ a b c d e Leslie M (2019). "Talk to the hand. Scientists try to debunk idea that finger length can reveal personality and health". Science. doi:10.1126/science.aay2735. S2CID 241187298. Retrieved 1 October 2021.
  4. ^ a b c Jones AL, Satchell LP, Jaeger B, Schild C (September 2020). "(Mis-)understanding COVID-19 and digit ratio: Methodological and statistical issues in Manning and Fink (2020)". Early Human Development. 148: 105095. doi:10.1016/j.earlhumdev.2020.105095. PMC 7256504. PMID 32499091.
  5. ^ a b Joannides P (2011). "Male Genital Length Study Falls Short". Science. Retrieved 1 October 2021.
  6. ^ a b c d e f Smoliga JM, Fogaca LK, Siplon JS, Goldburt AA, Jakobs F (December 2021). "Giving science the finger-is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study". BMJ. 375: e067849. doi:10.1136/bmj-2021-067849. PMC 8672321. PMID 34911738.
  7. ^ a b 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.
  8. ^ a b c Kumar S, Voracek M, Singh M (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. PMC 5337486. PMID 28321205.
  9. ^ a b c 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". International Journal of Andrology. 34 (4 Pt 2): e49–e58. doi:10.1111/j.1365-2605.2010.01124.x. PMID 21091719.
  10. ^ a b c Kumar S, Voracek M, Singh M (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. PMC 6688535. PMID 31428058.
  11. ^ a b c 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.
  12. ^ a b 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): 313–321. doi:10.1016/j.psyneuen.2007.01.007. PMID 17400395. S2CID 44238240.
  13. ^ a b c 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): 380–386. doi:10.1006/hbeh.2002.1830. PMID 12488105. S2CID 8886238.
  14. ^ a b c 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.
  15. ^ 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. S2CID 7711519.
  16. ^ a b 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. S2CID 4503281.
  17. ^ 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.
  18. ^ 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.
  19. ^ a b c d 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.
  20. ^ a b 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.
  21. ^ 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.
  22. ^ 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". American Journal of Biological Anthropology. doi:10.1002/ajpa.24852. ISSN 2692-7691. PMID 37795916. S2CID 263670473.
  23. ^ a b Hollier LP, Keelan JA, Jamnadass ES, Maybery MT, Hickey M, Whitehouse AJ (February 2015). "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. doi:10.1016/j.earlhumdev.2014.12.011. PMID 25594498.
  24. ^ Warrington NM, Shevroja E, Hemani G, Hysi PG, Jiang Y, Auton A, et al. (June 2018). "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. doi:10.1093/hmg/ddy121. PMC 5961159. PMID 29659830.
  25. ^ a b Richards G, Browne WV, Constantinescu M (December 2021). "Digit ratio (2D:4D) and amniotic testosterone and estradiol: an attempted replication of Lutchmaya et al. (2004)" (PDF). Journal of Developmental Origins of Health and Disease. 12 (6): 859–864. doi:10.1017/S2040174420001294. PMID 33472723. S2CID 231664924.
  26. ^ 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.
  27. ^ Loehlin JC, Medland SE, Martin NG (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. PMID 18340520. S2CID 12340685.
  28. ^ a b 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. S2CID 1360679.
  29. ^ Voracek M (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. PMID 19425449. S2CID 10076143.
  30. ^ Hönekopp J, Watson S (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. S2CID 29503255.
  31. ^ Jamison CS, Jamison PL, Meier RJ (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. PMID 7943194.
  32. ^ a b Abbott AD, Colman RJ, Tiefenthaler R, Dumesic DA, Abbott DH (22 August 2012). Helle S (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. PMC 3425513. PMID 22927929.
  33. ^ Lofeu L, Brandt R, Kohlsdorf T (August 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. PMC 5541650. PMID 28768472.
  34. ^ 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. PMID 15024720.
  35. ^ Baker F (1888). "Anthropological Notes on the Human Hand". American Anthropologist. A1: 51–76. doi:10.1525/aa.1888.1.1.02a00040.
  36. ^ Ruggles G (1930). "Human finger types". The Anatomical Record. 46 (2): 199–204. doi:10.1002/ar.1090460210. S2CID 83821071.
  37. ^ a b 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.
  38. ^ Wilson G (2010). "Fingers to feminism: The rise of 2D:4D". Quarterly Review. 4: 25–32.
  39. ^ 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): 3000–3004. doi:10.1093/humrep/13.11.3000. PMID 9853845.
  40. ^ Snow DR (2006). "Sexual dimorphism in Upper Palaeolithic hand stencils". Antiquity. 80 (308): 390–404. doi:10.1017/S0003598X00093704. S2CID 16301395.
  41. ^ 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.
  42. ^ 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.
  43. ^ 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): 643–646. doi:10.1098/rsbl.2009.0346. PMC 2781964. PMID 19553247.
  44. ^ Garn SM, Burdi AR, Babler WJ, Stinson S (November 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. hdl:2027.42/37557. PMID 1211429.
  45. ^ 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): 315–319. doi:10.1016/j.yhbeh.2005.08.003. hdl:2027.42/83921. PMID 16143332. S2CID 17288420.
  46. ^ Zhao J, Leung JY, Lin SL, Mary Schooling C (April 2016). "Cigarette smoking and testosterone in men and women: A systematic review and meta-analysis of observational studies". Preventive Medicine. 85: 1–10. doi:10.1016/j.ypmed.2015.12.021. PMID 26763163.
  47. ^ Fui MN, Dupuis P, Grossmann M (March 2014). "Lowered testosterone in male obesity: mechanisms, morbidity and management". Asian Journal of Andrology. 16 (2): 223–231. doi:10.4103/1008-682X.122365. PMC 3955331. PMID 24407187.
  48. ^ Terrier JE, Isidori AM (September 2016). "How Food Intakes Modify Testosterone Level". The Journal of Sexual Medicine. 13 (9): 1292–1296. doi:10.1016/j.jsxm.2016.07.001. PMID 27555502.
  49. ^ Manning JT, Fink B (July 2020). "Understanding COVID-19: Digit ratio (2D:4D) and sex differences in national case fatality rates". Early Human Development. 146: 105074. doi:10.1016/j.earlhumdev.2020.105074. PMC 7224643. PMID 32419720.
  50. ^ Jones AL, Jaeger B, Schild C (January 2021). "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: 105272. doi:10.1016/j.earlhumdev.2020.105272. PMC 7670914. PMID 33227636.
  51. ^ Manning, John T. (11 June 2023). "The Big Finger Trumps the Lucky Jim Fallacy". The BMJ.
  52. ^ a b 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.
  53. ^ a b Richards G (October 2017). "Digit ratio (2D:4D) and prenatal/perinatal sex hormones: A response to Manning and Fink (2017)". Early Human Development. 113: 75–76. doi:10.1016/j.earlhumdev.2017.09.004. PMID 28917583.
  54. ^ 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): 981–983. doi:10.1136/bjsm.2006.027193. PMC 2577466. PMID 17008344.
  55. ^ 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): 112–118. doi:10.1007/s10508-007-9272-2. PMID 18074216. S2CID 41921265.
  56. ^ 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): 149–157. doi:10.1017/S2040174414000646. PMC 5119464. PMID 25584807.
  57. ^ Ciumas C, Lindén Hirschberg A, Savic I (May 2009). "High fetal testosterone and sexually dimorphic cerebral networks in females". Cerebral Cortex. 19 (5): 1167–1174. doi:10.1093/cercor/bhn160. PMID 18854582.
  58. ^ McAnulty RD, Burnette MM (2006). Sex and sexuality. Vol. 1. Greenwood Publishing Group. p. 165. ISBN 9780313049194.
  59. ^ Pang S, Levine LS, Cederqvist LL, Fuentes M, Riccardi VM, Holcombe JH, et al. (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): 223–229. doi:10.1210/jcem-51-2-223. PMID 6447160.
  60. ^ 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): 117–120. doi:10.1210/jcem.76.1.8421074. PMID 8421074.
  61. ^ 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): 2724–2728. doi:10.1210/jcem.84.8.5870. PMID 10443667.
  62. ^ 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.
  63. ^ 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.
  64. ^ 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): 3604–3610. doi:10.1210/jc.2007-0117. PMID 17579205. S2CID 33761349.
  65. ^ Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S (November 2009). "Fingers as a marker of prenatal androgen exposure". Endocrinology. 150 (11): 5119–5124. doi:10.1210/en.2009-0774. PMC 2775980. PMID 19819951.
  66. ^ Medland SE, Loehlin JC, Martin NG (1 April 2008). "No effects of prenatal hormone transfer on digit ratio in a large sample of same- and opposite-sex dizygotic twins". Personality and Individual Differences. 44 (5): 1225–1234. doi:10.1016/j.paid.2007.11.017. ISSN 0191-8869.
  67. ^ Dickman S (March 1997). "HOX gene links limb, genital defects". Science. 275 (5306): 1568. doi:10.1126/science.275.5306.1568. PMID 9072822. S2CID 33709778.
  68. ^ 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): 619–630. doi:10.1002/ajhb.21054. PMID 20737609. S2CID 29503255.
  69. ^ Xi H, Li M, Fan Y, Zhao L (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. PMC 3890058. PMID 24013635.
  70. ^ Robertson J, Zhang W, Liu JJ, Muir KR, Maciewicz RA, Doherty M (January 2008). "Radiographic assessment of the index to ring finger ratio (2D:4D) in adults". Journal of Anatomy. 212 (1): 42–48. doi:10.1111/j.1469-7580.2007.00830.x. PMC 2423386. PMID 18005121.
  71. ^ Kumar S, Voracek M, Singh M (February 2021). "The effects of hand preference and sex on right-left asymmetry in dorsal digit lengths among adults and children". Early Human Development. 153: 105293. doi:10.1016/j.earlhumdev.2020.105293. PMID 33340946. S2CID 229333079.
  72. ^ Manning JT, Barley L, Walton J, Lewis-Jones DI, Trivers RL, Singh D, 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.
  73. ^ a b 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): 161–168. doi:10.1016/j.earlhumdev.2004.06.004. PMID 15500996.
  74. ^ Williams TJ, Pepitone ME, Christensen SE, Cooke BM, Huberman AD, Breedlove NJ, et al. (March 2000). "Finger-length ratios and sexual orientation". Nature. 404 (6777): 455–456. Bibcode:2000Natur.404..455W. doi:10.1038/35006555. PMID 10761903. S2CID 205005405.
  75. ^ Lu H, Huo ZH, Shi ZY, Peng L, Dang J, Jiao HY, et al. (2008). "Study on the digit ratio of Hui and Han ethnic groups in Ningxia". Acta Anatomica Sinica. 39 (2): 267.
  76. ^ 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): 223–233. doi:10.1007/s10508-007-9171-6. PMID 17373585. S2CID 2042042.
  77. ^ 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): 739–742. CiteSeerX doi:10.1007/s10508-006-9039-1. PMID 16688484. S2CID 4763807.
  78. ^ Xu Y, Zheng Y (6 May 2015). "The digit ratio (2D:4D) in China: A meta-analysis". American Journal of Human Biology. 27 (3): 304–309. doi:10.1002/ajhb.22639. PMID 25284473. S2CID 40725875.
  79. ^ Ozener B, Hurd PL, Duyar I (2014). "Inbreeding is associated with lower 2D: 4D digit ratio". American Journal of Human Biology. 26 (2): 183–188. doi:10.1002/ajhb.22491. PMID 24288299. S2CID 23618195.
  80. ^ 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.
  81. ^ 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.
  82. ^ Kumar S, Singh M, Voracek M (December 2020). "Effects of hand preference on digit lengths and digit ratios among children and adults". Early Human Development. 151: 105204. doi:10.1016/j.earlhumdev.2020.105204. PMID 33059164. S2CID 222829822.
  83. ^ 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.
  84. ^ 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.
  85. ^ a b c d Voracek M, Kossmeier M, Tran US (2019). "Which Data to Meta-Analyze, and How?". Zeitschrift für Psychologie. Hogrefe Publishing Group. 227 (1): 64–82. doi:10.1027/2151-2604/a000357. ISSN 2190-8370. S2CID 86788835.
  86. ^ a b c Jones AL, Jaeger B, Schild C (January 2021). "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: 105272. doi:10.1016/j.earlhumdev.2020.105272. PMC 7670914. PMID 33227636.
  87. ^ Voracek M, Pietschnig J, Nader IW, Stieger S (September 2011). "Digit ratio (2D:4D) and sex-role orientation: Further evidence and meta-analysis". Personality and Individual Differences. Elsevier BV. 51 (4): 417–422. doi:10.1016/j.paid.2010.06.009. ISSN 0191-8869.
  88. ^ a b McCormick CM, Carré JM (April 2020). "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: 104710. doi:10.1016/j.yhbeh.2020.104710. PMID 32057822. S2CID 211112860.
  89. ^ Lolli L, Batterham AM, Kratochvíl L, Flegr J, Weston KL, Atkinson G (June 2017). "A comprehensive allometric analysis of 2nd digit length to 4th digit length in humans". Proceedings. Biological Sciences. 284 (1857): 20170356. doi:10.1098/rspb.2017.0356. PMC 5489719. PMID 28659446.
  90. ^ Hollier LP, Keelan JA, Jamnadass ES, Maybery MT, Hickey M, Whitehouse AJ (February 2015). "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. doi:10.1016/j.earlhumdev.2014.12.011. hdl:20.500.11937/48405. PMID 25594498.
  91. ^ a b Wong WI, Hines M (February 2016). "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. doi:10.1016/j.yhbeh.2015.10.022. PMID 26542674. S2CID 26592548.
  92. ^ Lindová J, Hrušková M, Pivoňková V, Kuběna A, Flegr J (June 2008). "Digit ratio (2D:4D) and Cattell's personality traits". European Journal of Personality. SAGE Publications. 22 (4): 347–356. doi:10.1002/per.664. ISSN 0890-2070. S2CID 18205251.
  93. ^ McQuade DB (September 2011). "Does digit ratio (2D:4D) predict penile length?". Asian Journal of Andrology. Medknow. 13 (5): 667–668. doi:10.1038/aja.2011.81. PMC 3739583. PMID 21725331.
  94. ^ Fossen FM, Neyse L, Johannesson M, Dreber A (5 January 2021). "2D:4D and Self-Employment: A Preregistered Replication Study in a Large General Population Sample". Entrepreneurship Theory and Practice. 46 (1): 21–43. doi:10.1177/1042258720985478. hdl:10419/235855. ISSN 1042-2587. S2CID 234247887.
  95. ^ Medland SE, Zayats T, Glaser B, Nyholt DR, Gordon SD, Wright MJ, et al. (April 2010). "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. doi:10.1016/j.ajhg.2010.02.017. PMC 2850436. PMID 20303062.
  96. ^ Pratt TC, Turanovic JJ, Cullen FT (22 September 2016). "Revisiting the Criminological Consequences of Exposure to Fetal Testosterone: A Meta-Analysis of the 2D:4D Digit Ratio". Criminology. Wiley. 54 (4): 587–620. doi:10.1111/1745-9125.12115. ISSN 0011-1384.
  97. ^ Hilgard J, Engelhardt CR, Rouder JN, Segert IL, Bartholow BD (April 2019). "Null Effects of Game Violence, Game Difficulty, and 2D:4D Digit Ratio on Aggressive Behavior". Psychological Science. SAGE Publications. 30 (4): 606–616. doi:10.1177/0956797619829688. PMID 30843758. S2CID 73496727.
  98. ^ 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. OUP Oxford. pp. 317–405. ISBN 978-0-19-965259-4.
  99. ^ Burley NT (May 2006). "An eye for detail: selective sexual imprinting in zebra finches". Evolution; International Journal of Organic Evolution. 60 (5): 1076–1085. doi:10.1554/05-399.1. JSTOR 4095408. PMID 16817546. S2CID 28869542.
  100. ^ 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): 235–239. doi:10.1016/j.yhbeh.2008.10.010. PMID 19022257. S2CID 23727819.
  101. ^ 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.
  102. ^ 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): 175–182. doi:10.1016/j.biopsycho.2005.03.006. PMID 15978716. S2CID 39777964.

External links[edit]