The digit ratio is the ratio of the lengths of different digits or fingers. The digit length is typically measured on palmar (ventral) hand, from the midpoint of bottom crease (where the finger joins the hand) to the tip of the finger. However, recently measurement of digit on dorsal hand, from tip of finger to proximal phalange-bone protrusion (which occurs when digits are bent at 90 degree angle to the palm), has also gained acceptance. A study has shown that, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio. Moreover, differential placing of flexion creases is a factor in palmar digit ratio.
The 2D:4D ratio is the most studied digit ratio and is calculated by dividing the length of the index finger of a given hand by the length of the ring finger of the same hand. Other digit ratios are also similarly calculated in the same hand.
The ratio of two digits in particular, the palmar 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens, e.g., testosterone while in the uterus and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher prenatal androgen exposure.
There are also studies that suggest that the palmar 2D:4D ratio is also 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).
Experimental studies have shown prenatal testosterone injection produces male-typical changes in dermatoglyphics and in palmar digit length, but not in bone digit length. Moreover, this effect occurs in digit 2 but not in digit 4. Additionally, human epidermal tissues only have androgen receptors and no estrogen receptors-α. Thus, it is likely that dermatoglyphic tissues in fingers may be more sensitive to prenatal testosterone effect whereas, as reported above, bone digit ratios may be sensitive to testosterone to estrogen ratio. Hence, the palmar 2D:4D ratio reflects combination of two different hormonal sensitivities. In support, recent study has shown that differential placing of flexion creases contributes to sex differences in palmar 2D:4D ratio.
The digit ratios are sexually dimorphic; i.e., most digit ratios are lower in men than in women. In palmar digit ratios strong sexual dimorphism occurs in ratios comprising digit 2. In contrast, strong sexual dimorphism occurs in ratios comprising digit 5 in dorsal digit ratios. Overall, the report of sexual dimorphism is much stronger in dorsal digit ratios than in palmar digit ratios, especially as compared to the classic, palmar 2D:4D ratio. Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio. Thus, whereas most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
- 1 History of research
- 2 Distribution
- 3 Evidence of androgen effect
- 4 Explanation of the digit ratio effect
- 5 Geographic and ethnic variation in 2D:4D
- 6 Correlation with traits
- 7 Palaeolithic hand stencils
- 8 Other animals
- 9 See also
- 10 References
- 11 External links
History of research
That a greater proportion of men have shorter index fingers than ring fingers than do women was noted in the scientific literature several times through the late 1800s, with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930, after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr Glenn Wilson of King's College, London, published a study examining the correlation between assertiveness in women and their digit ratio. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero. In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children and further developed the idea that the index was a marker of prenatal sex hormones. Since then research on the topic has burgeoned around the world.
A 2009 study in Biology Letters argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women," which may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios.
A 2011 paper by Zhengui Zheng and Martin J. Cohn reports "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development". The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood. During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio.
Importantly, there has been no correlation between the sex hormone levels of an adult and the individual's 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon.
A major problem with the research on this topic comes from the contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio.
From a study of 136 males and 137 females at the University of Alberta:
- Males: mean 0.947, standard deviation 0.029
- Females: mean 0.965, standard deviation 0.026
Assuming a normal distribution, the above lead to 95% prediction intervals for 2D:4D ratio of 0.889–1.005 for males and 0.913–1.017 for females.
- Males: mean 0.951, standard deviation 0.035
- Females: mean 0.968, standard deviation 0.028
Evidence of androgen effect
Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average. Other possible physiological effects include an enlarged clitoris and shallow vagina.
Males with CAH have more smaller (more masculine) digit ratios than control males, suggesting that prenatal androgens affect digit ratios. Amniocentesis samples show that prenatal levels of testosterone are in the high normal range in males with CAH, while levels of the weaker androgen androstenedione are several fold higher than in control males. These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
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.
Digit ratio in men may correlate with genetic variation in the androgen receptor gene. Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater, i.e. more feminine, digit ratios. There are reports of a failure to replicate this finding. However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by secreting more testosterone, probably as a result of reduced negative feedback on gonadotropins. Thus, it is not clear that 2D:4D would be expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen.
XY individuals with androgen insensitivity syndrome (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratios is unrelated to the Y chromosome per se.
Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, and when examined researchers found no difference in estrogen levels between males and females.
Explanation of the digit ratio effect
It is not clear why digit ratio is influenced by prenatal hormones. There is evidence of other similar traits, e.g. otoacoustic emissions and arm-to-trunk length ratio, which show similar effects. Hox genes responsible for both digit and penis development have been implicated in affecting these multiple traits (pleiotropy). Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes. Likewise, it is unclear why digit ratio on the right hand should be more responsive than that on the left hand, as is indicated by the greater sex difference on the right than the left. However, because no right-left difference has been found in sexual dimorphism of bone digit ratios (2D:4D;) and because differential placing of flexion creases contributes to sex differences in palmar digit ratio, the study of right-left difference in placing of flexion creases may clarify this right-left difference in palmar 2D:4D ratio.
One study on mice from 2011 suggests that the 2D:4D ratio correlates with prenatal sex hormone levels because the androgen receptor and estrogen receptor activity is higher in digit 4 than in digit 2. Inactivation of AR decreases growth of digit 4, which causes a higher 2D:4D ratio, whereas inactivation of ER-α increases growth of digit 4, which leads to a lower 2D:4D ratio.
Geographic and ethnic variation in 2D:4D
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups. In a study with Han, Berber, Uygur and Jamaican children as subjects, Manning et al. found that Han children had the highest mean values of 2D:4D (0.954±−0.032), they were followed by the Berbers (0.950±0.033), then the Uygurs (0.946±0.037), and the Jamaican children had the lowest mean 2D:4D (0.935±0.035). This variation is far larger than the differences between sexes; in Manning's words, "There's more difference between a Pole and a Finn, than a man and a woman."
It should be noted, however, that 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.
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.
In 2007 Manning et al. also found that mean 2D:4D varied across ethnic groups with higher ratios for Whites, Non-Chinese Asians, and Mid-Easterners and lower ratios in Chinese and Black samples.
Two studies explored the question of whether geographical differences in 2D:4D ratios were caused by gene pool differences, or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). The conclusions were that geographical differences in 2D:4D ratio were caused by genetic pool differences, not by geographical latitude.
Consanguinous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring, which may account for some of the geographical and ethnic variation in 2D:4D ratios, as consanguinity rates depend among others on religion, culture, and geography.
Correlation with traits
Some authors suggest that digit ratio correlates with health, behavior, and even sexuality in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with either high or low digit ratio.
Low digit ratio
High digit ratio
|Physiology and disease||
|Physical and competitive behavior||Increased aggressive behavior in sports.|
|Cognition and personality||
Male-to-female transgender women
A study in Germany has found a correlation between digit ratio and male-to-female transgender women. Trans women were found to have a higher digit ratio than males. This was not true for trans men, however, who were within the average range for biological females.
Digit ratio and development
There is some evidence that 2D:4D ratio may also be indicative for human development and growth. Ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life. Moreover, studies about 2D:4D correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development. Prenatal sex steroid ratios (in terms of 2D:4D) and actual chromosomal sex dimorphism were found to operate differently on human faces, but affect male and female face shape by similar patterns. Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express greater levels of facial symmetry.
Palaeolithic hand stencils
- Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.
- Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.
- Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.
- Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.
- Peter L. Hurd, Theodore Garland Jr., and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolution). These high-runner mice exhibit increased 2D:4D. This apparent "feminization" is opposite to the relation seen between 2D:4D and physical fitness in human beings, and is difficult to reconcile with the idea that 2D:4D is a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes.
- Anogenital distance
- Body mass index
- Chiromancy—hand analysis
- Handedness and sexual orientation
- Waist–hip ratio
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|Wikimedia Commons has media related to Digit ratio.|
- PubMed listing of papers on digit ratios
- Mills ME (October 2002). "Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning". Human Nature Review. 2: 418–23.