Digit ratio: Difference between revisions

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. 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.

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 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.

However, a 2009 study in Biology Letters has cast doubt on previous finger ratio assumptions, arguing that, "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."^[1]

• 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.
• XY individuals with androgen insensitivity syndrome due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios, 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 (Berenbaum et al. 2009).
• 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.
• 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.
• 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).

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 gonad growth have been implicated in this pleiotropy. Alternatively, direct effects of sex hormones on bone growth might be responsible.

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, 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.

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.

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

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

• Skiing (Manning 2002b)
• Soccer ability (Manning & Taylor 2001)
• Sporting ability in females (Paul et al. 2006)
• Financial trading (Coates et al. 2009)[1]

• 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 (Romano et al. 2006)
• Musical ability (Sluming et al. 2000)

• Bem sex role score in women (Csatho et al. 2003), erotic role preference in men (McIntyre 2003)
• Lesbians vs. straight women ^{[2] [3][4][5][6][7][8][9][10][11]}, 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 [2]).
• 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).

• 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)

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 (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).

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 .

It is generally assumed that creating cave art was a male behavior even though there is little evidence to substantiate this.^{[citation needed]} 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; Nelson et al. 2006).

• 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 (Yan et al. 2009) 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. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes.

• Dermatoglyphics
• Chiromancy (Handanalysis)

• Book review - Digit Ratio: A Pointer to Fertility, Behavior and Health
• abstract of The 2nd to 4th digit ratio and autism.
• Digit ratio in non-human primates[7]
• Finger Length Predicts Aggression in Men
• Finger Length Predicts SAT Performance