Finger Second Digit

[Terresa Cherrie]

Themost 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 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]

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.

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.

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]

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]

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.

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]

Manning and colleagues have reported that 2D:4D ratios vary greatly 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 (0.9540.032). They were followed by the Berbers (0.9500.033), then the Uygurs (0.9460.037) and the Jamaican children had the lowest mean 2D:4D (0.9350.035).[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]

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