Digit Ratio

Prenatal hormones

The digit ratio (the ratio of the lengths of different fingers) is possibly affected by exposure to androgen hormones (e.g. testosterone) while in the uterus. The index to ring finger ratio (2D:4D) in particular has been suggested as a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher androgen exposure.

A longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio of less than 1. The 2D:4D digit ratio is sexually dimorphic: while the second digit is typically shorter in both females and males, the difference between the lengths of the two digits is greater in males than females.

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 that ‘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 utero, is thought to occur at 14 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, but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation with the 2D:4D. So there should either be a correlation with one or the other but not both.

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 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.

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 (when a single gene affects a number of traits in the same organism). 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. 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.’

Some authors suggest that digit ratio correlates with health, behavior, and even sexuality in later life. High digit ratio has been linked to right handedness, lowered sperm counts, reduced birth size, reduced performance in sports, reduced financial trading ability, as well as increased risk for heart disease, obesity, metabolic syndrome, anxiety disorders, depression, and schizophrenia in males (and an increased rate of psychopathy in females). It has also been associated with a reduced risk for prostate cancer, reduced rate of autism, and reduced risk of alcohol dependency, and higher exam scores (among male students) and better color, smell, and tactile perception (e.g. supertasters).

By contrast, low digit ratio is associated with increased risk of ADHD and aggression in males and a reduced risk in males and females for eating disorders. Low digit ratio is also linked to assertiveness in females, masculinity of handwriting, perceived ‘dominance’ and masculinity of man’s face, musical ability in males, and leadership and innovation. Sexuality is also reputedly correlated to digit ratio. High digit ratio for women is associated with femininity. Low digit ratio is associated with a ‘feminine’ erotic sex preference in gay men. A recent study in Germany has found a correlation between digit ratio and male to female transsexualism. Male to female transsexuals (trans women) were found to have a higher digit ratio than control males, but one that was comparable to control females.

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