Digit ratio

The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens e.g. testosterone while in the uterus and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher androgen exposure.

2D:4D is sexually dimorphic: in males, the second digit tends to be shorter than the fourth, and in females the second tends to be the same size or slightly longer than the fourth. However, homosexual men tends to have a higher 2D:4D digit ratios than heterosexual men. This trait may be better considered 'sexually differentiated' rather than 'sexually dimorphic' in recognition of the fact that the effect size is fairly small (2D:4D distributions of the two sexes overlap to a great degree), especially compared to other sexually dimorphic traits e.g. height.

History of digit ratio research
That a greater proportion of men have shorter index fingers than ring fingers than do women was noted in the scientific literature several times through the late 1800s, with the statistically significant sex difference in a large sample (201 men, 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. Digit ratio research has since exploded, as perusal of citations at the end of this article will attest. In 2002 Manning of the University of Liverpool published a book summarizing all such research on the topic to that point, asserting that prenatal testosterone affects digit ratios and their psychological correlates.

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.

Evidence of androgen effect on digit ratio

 * Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average.
 * Males with CAH have more masculine (smaller) digit ratios than control males, which also suggests that prenatal androgens affect digit ratios, since amniocentesis samples show that prenatal levels of testosterone are in the high normal range in males with CAH, while levels of the weaker androgen androstenedione are several fold higher than in control males   .  These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
 * Digit ratio in men correlates with genetic variation in the androgen receptor gene. Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios.
 * 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 is unrelated to the Y chromosome per se.
 * The sex difference in 2D:4D is present before birth in humans, which rules out any social influences that might affect digit growth differentially in the two sexes. Because all somatic sex differences in mammals to date have been found to be due to either androgenic masculinization or effects of the sex chromosomes, and as the AIS finding rules out a role for sex chromosomes in the sex difference in digit ratios, the prenatal sexual dimorphism also indicates that androgens act before birth to affect digit ratios.
 * The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlates with the child's subsequent 2D:4D ratio. The effect of a child's sex is confusingly controlled for in this study.
 * 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.

There is evidence that this reflects fetal exposure to the hormones testosterone and estrogen.

Several studies present evidence that digit ratios are heritable.

Explanation of the digit ratio effect
It is not clear why digit ratio ought to be influenced by prenatal hormones. There is evidence of other similar traits, e.g. otoacoustic emissions and arm-to-trunk length ratio, which show similar effects. Hox genes responsible for both digit and penis development have been implicated in this 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.

Geographic/Ethnic variation in 2D:4D
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups. This variation is far larger than the differences between sexes, in Manning's words "There’s more difference between a Pole and a Finn than a man and a woman." The variation appears to be correlated with latitude, such that more northerly populations have higher digit ratios.

Correlation between digit ratio and traits
Some authors suggest that digit ratio correlates with health, behavior, and even sexuality in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with digit ratio.

Physiology and disease

 * Sperm counts
 * Heart disease
 * Obesity & Metabolic syndrome

Psychological disorders

 * Autism
 * Depression
 * Schizophrenia
 * ADHD

Physical and competitive ability

 * Skiing
 * Soccer ability
 * Sporting ability in females
 * Financial trading

Cognition and personality

 * Assertiveness in women
 * Aggression
 * Masculinity of Handwriting
 * Perceived 'dominance' and masculinity of man's face
 * Personality
 * Exam scores: a higher ratio is correlated with higher exam scores among male students
 * Musical ability

Sexual orientation

 * Bem sex role score in women; erotic role preference in men.
 * Lesbians vs. straight women;        butch vs. femme lesbians.
 * Gay vs straight men and the very odd Europe vs. North American straight man effect.
 * Difference in digit ratio between identical female twins discordant for sexual orientation
 * Fraternal birth order effect on digit ratio.

Transsexualism

 * A recent study in Germany has found a correlation between digit ratio and male to female transsexualism. Male to female transsexuals (Transwomen) were found to have a higher digit ratio than control males, but one that was comparable to control females.

Digit ratio and handedness, autism, other immune diseases
There is some evidence that testosterone facilitates the differentiation of the brain both prenatally and postnatally. There have been many extensions of this, such as the Geschwind–Galaburda hypothesis, that immune diseases and autism are related to prenatal testosterone, this also explaining why more men are left-handed, autistic, etc. than women. Prenatal exposure to testosterone is thought to promote the development of the right-hemisphere and increase the incidence of sinistrality. As such low 2D:4D was found to be associated with improved left-hand performance.

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. However, exposure to very high levels of testosterone and/or estrogen in the womb may have also negative effects. Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express lower levels of facial symmetry.

Digit ratio and palaeolithic hand stencils
It is generally assumed that creating cave art was a male behavior. 2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves.

Digit ratio research in animals

 * Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.
 * Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.
 * Sexual dimorphism in hind limb 2D:4D has been demonstrated in mice by two studies by both John Manning and Marc Breedlove's research groups. There is some evidence to suggest that this effect is not seen in all mouse strains.
 * Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.
 * Front limb D2:D3 has shown to be influenced by prenatal alcohol exposure in female rats
 * Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.
 * Peter L. Hurd, Theodore Garland, Jr., and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolution). These high-runner mice exhibit increased 2D:4D. This apparent "feminization" is opposite to the relation seen between 2D:4D and physical fitness in human beings, and is difficult to reconcile with the idea that 2D:4D is a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes.