Digit ratio

The term digit ratio is used refers to 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 scientits 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 womb 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 longer. 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 scepticism 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 1800's. In 1975 Wilson published a study examining the correaltion betwen 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. Digit ratio research has exploded since John Manning proposed that digit ratio reflects prenatal androgen exposure, launching a very active and on going area of research. In 2002 Manning published a book summarizing all such research on the topic to that point.

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 childs' sex is confusingly controlled for in this study.
Digit ratio in men correlates with genetic variationin 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 and estrogen.

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.

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 lattitude, 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 and behavior (including 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

Psychological Disorders

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

Sporting and Physical Ability

Skiing (Manning 2002b)
Sprinting
Soccer ability (Manning & Taylor 2001)

Cognition etc.

Spatial ability
Aggression (Benderlioglu & Nelson, 2004 , Bailey & Hurd 2005a)
Masculinity of Handwriting (Beech and Macintosh 2004)
Perceived 'dominance' of man's face (Neave et al 2003)
Pesonality (Luxen & Buunk 2005)
Exam scores (Romano et al 2005b)
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 staight man effect (reviewed in McFadden et al 2005). Difference in digit ratio between identical twins discordant for sexual orientation (Hall & Love 2003)
Fraternal birth order effect on digit ratio (Williams et al 2000).

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.
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

References

Brown, W. M., Finn, C. J., Cooke, B. M., & Breedlove, S. M. (2002). Differences in finger length between self-identified "butch" and "femme" lesbians. Archives of Sexual Behavior, 31, 123-128.
Csathó, A., Osváth, A., Bicsák, E., Karádi, Manning, J., Kállai, J. (2003) Sex role identity related to the ratio of second to fourth digit length in women. Biological Psychology, 62, 147-156
Lutchmaya, S., Baron-Cohen, S., Raggatt, P., Knickmeyer, R. & Manning, J.T. (2004). '2nd to 4th digit ratios, fetal testosterone and estradiol' Early Human Development 77, 23-28.
Manning, J.T., Wilson, D.J. and Lewis-Jones, D.I. (1998). The ration of 2nd to 4th digit length: a predictor of sperm numbers and concentration to testosterone, leteinizing hormone and oestrogen, Human Reproduction, 13 (11), 3000-3004
Manning, J.T., Barley, L., Walton, J., Lewis-Jones, D.I., Trivers, R.L., Singh, D., Thornhill, R., Rohde, P., Bereckei, T., Henzi, P., Soler, M. & Sved, A. (2000). The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success: evidence for sexually antagonistic genes. Evolution and Human Behavior. 21, 163-183
Manning, J.T." (2002). Digit ratio: a pointer to fertility, behaviour, and health. Rutgers U. Press. New Brunswick, NJ.
Manning, J.T., Bundred, P.E., Newton, D.J., & Flanigan, B.F. (2003). 'The second to fourth digit ratio and variation in the androgen receptor gene' Evolution and Human Behavior 24, 399-405.
Manning, J.T., Stewart, A., Bundred, P.E. & Trivers, R.L. (2004). Sex and ethnic differences in 2nd to 4th digit ratio of children. Early Human Development 80, 161-168.
Williams, T. J., Pepitone, M. E., Christensen, S. E., Cooke, B. M., Huberman, A. D., Breedlove, N. J., Breedlove, T. J., Jordan, C. L., & Breedlove, S. M. (2000). Finger-length ratios and sexual orientation. Nature, 404, 455-456.
Wilson, G.D. (1983). Finger-length as an index of assertiveness in women. Personality and Individual Differences 4, 111-112.

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