Digit ratio

The digit ratio is the ratio of the lengths of different digits, fingers or toes, typically as 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 such as testosterone while in the uterus and that this 2D:4D ratio can be used as a crude measure for prenatal androgen exposure.

2D:4D is sexually dimorphic: in men, 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. Some would prefer to say that this trait is '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 as compared to other sexually dimorphic traits such as height.

Digit ratio research often meets with a considerable degree of skepticism due to the obvious parallels to palmistry, phrenology and other discredited traditions within the field of anthropometry.

History of digit ratio research
That a greater proportion of men have shorter index fingers than ring fingers than do women has been noted in the scientific literature several times through the 1800s. 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. Digit ratio research has since exploded with a very active and ongoing programme of research by John Manning in Liverpool and Marc Breedlove in California. In 2002 Manning published a book summarizing all such research on the topic to that point, confirming the role of prenatal testosterone in digit ratios and their psychological correlates.

Evidence of androgen effect on digit ratio

 * Women with congenital adrenal hyperplasia (CAH) have lower, more masculinized 2D:4D (Brown et al 2002, Okten et al 2002). CAH leads to greatly elevated androgen concentrations in utero.
 * The ratio of testosterone to estradiol measured in amniocentesis samples correlates with the child's subsequent 2D:4D ratio (Lutchmaya et al 2004). The effect of a child's sex is confusingly controlled for in this study.
 * Digit ratio in men correlates with genetic variation in the androgen receptor gene (Manning et al 2003). Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios.
 * 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 (Romano et al 2005).

There is evidence that this reflects fetal exposure to the hormones testosterone (McIntyre 2006) and estrogen.

Another study has evidence to show that the ratio is inherited. (Paul 2006)

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 eg otoacoustic emissions, arm to trunk length ratio, which show similar effects. Hox genes responsible for both digit and gonad growth have been implicated in this pleiotropy. Alternatively, direct effects of sex hormones on bone growth might be responsible.

Geographic/Ethnic variation in 2D:4D
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups (Manning et al 2000, Manning et al 2004). This variation is far larger than the differences between sexes, as Manning puts it “There’s more difference between a Pole and a Finn than a man and a woman.” The variation appears to be related to latitude, such that more northerly populations have higher digit ratios.

Correlation between digit ratio and psychological traits
Some authors suggest that digit ratio correlated to health, behavior, and even sexuality, in later life. What follows is a non-exhaustive list of some traits which have been either demonstrated or suggested (some of these studies are questionable) to correlate with digit ratio.

Physiology and disease

 * Sperm counts (Manning et al 1998)
 * Heart disease (Manning & Bundred 2001)
 * Obesity & Metabolic syndrome (Fink, Manning, Neave 2005)

Psychological Disorders

 * Autism (Manning et al 2001)
 * Depression (Bailey & Hurd, 2005b)
 * Schizophrenia (Arato et al 2004)

Sporting and Physical Ability

 * Skiing (Manning 2002b)
 * Soccer ability (Manning & Taylor 2001)
 * Sporting ability in females (Paul et al 2006)

Cognition and personality.

 * Assertiveness in women (Wilson 1983)
 * Spatial ability (van Anders & Hampson 2005)
 * Aggression (Benderlioglu & Nelson, 2004, Bailey & Hurd 2005a)
 * Masculinity of Handwriting (Beech and Macintosh 2004)
 * Perceived 'dominance' and masculinity of man's face (Neave et al 2003; Burriss et al. 2006)
 * Personality (Austin et al 2002, Fink et al 2004, Luxen & Buunk 2005)
 * Exam scores: a higher ratio is correlated with higher exam scores among male university students
 * Musical ability (Sluming et al 2000)

Sexual orientation

 * Bem sex role score in women (Csatho et al 2003), erotic role preference in men (McIntyre 2003)
 * Lesbians vs. straight women, butch vs. femme lesbians (Brown et al 2002)
 * Gay vs straight men and the very odd Europe vs. North American straight man effect (reviewed in McFadden et al 2005).
 * Difference in digit ratio between identical female twins discordant for sexual orientation (Hall & Love 2003)
 * Fraternal birth order effect on digit ratio (Williams et al 2000).

See also biology and sexual orientation.

Transsexualism

 * A recent study in Germany has found a correlation between digit ratio and male to female transsexualism. Male to female transsexuals were found to have a higher digit ratio than control males, but one that was comparable to control females. (Transmen showed ratio comparable to biological females.) (Schneider, Pickel & Stalla 2005)

Digit ratio and handedness, autism, other immune diseases
There is some evidence that testosterone facilitates the differentiation of the brain at prenatally and postnatally. There have been many extensions of this, such as the Geschwind-Galaburda Hypothesis, that immune diseases (Geschwind and Galaburda, 1985) and autism (Baron-Cohen et al., 2004) 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 (Manning et al 2000, Fink et al 2004).

Digit Ratio & 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 2005). 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 & Palaeolithic Hand Stencils
It is generally assumed that creating cave art was a male behaviour even though there is little evidence to substantiate this. 2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves (Snow, 2006; Chazine and Noury, 2006; also see Nelson et al. 2006).

Digit ratio research in non-human 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. (http://www.digitratio.com)
 * 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