Digit ratio - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Digit_ratio
Digit ratio
From Wikipedia, the free encyclopedia
The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the
midpoint of bottom crease where the finger joins the hand to the tip of the finger.[1] 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
prenatal androgen exposure. Writing in the Proceedings of the National Academy of Sciences,
developmental biologists Martin Cohn, Ph.D., and Zhengui Zheng, Ph.D., of the Howard Hughes
Medical Institute and the department of molecular genetics and microbiology at the UF College of
Medicine, show that male and female digit proportions are determined by the balance of sex hormones
during early embryonic development. The 2D:4D ratio is calculated by dividing the length of the index
finger of the right hand by the length of the ring finger of the right hand. 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 in females.
Hand with index finger being shorter
than the ring finger, resulting in a
small 2D:4D ratio, pointing to a high
exposure to testosterone in the uterus.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
Contents
1 History of digit ratio research
2 Digit ratio distribution
3 Evidence of androgen effect on digit ratio
4 Explanation of the digit ratio effect
5 Geographic and ethnic variation in 2D:4D
6 Correlation between digit ratio and traits
6.1 Male-to-Female Transsexual People
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6.2 Digit ratio and development
7 Digit ratio and palaeolithic hand stencils
8 Digit ratio research in animals
9 See also
10 References
11 External links
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,[2][3] with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930,[4] 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.[5] This was the first study to examine the correlation
between digit ratio and a psychological trait within members of the same sex.[6] Wilson proposed that skeletal structure and personality were
simultaneously affected by sex hormone levels in utero.[5] In 1998, John T. Manning and colleagues reported the sex difference in digit ratios
was present in two-year-old children[7] 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,"[8] 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 "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen
signaling during a narrow window of digit development."[9] The formation of the digits in humans, in utero, is thought to occur by 13 weeks,
and the bone-to-bone ratio is consistent from this point into an individual’s adulthood.[10] 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.[11]
Importantly, there has been no correlation between the sex hormone levels of an adult and the individual’s 2D:4D,[12] which implies that it is
strictly the exposure in utero that causes this phenomenon.
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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,[12] but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation
with the 2D:4D. So there should be a correlation with one or the other but not both.
Digit ratio distribution
From a study of 136 males and 137 females:[13]
Males: mean 0.947, standard deviation 0.029.
Females: mean 0.965, standard deviation 0.026.
Assuming a normal distribution, the 95% confidence interval for average 2D:4D ratio is 0.889-1.005 for
males and 0.913-1.017 for females.
Evidence of androgen effect on digit ratio
A visualization of the distributions:
Men (blue), women (green), and the
whole population (red).
Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized
2D:4D on average.[14][15][16] Other possible physiological effects include an enlarged clitoris and shallow vagina.[17]
Males with CAH have more masculine (smaller) digit ratios than control males,[14][15] 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.[18][19][20] These measures indicate that males with CAH are
exposed to greater prenatal concentrations of total androgens than are control males.
Digit ratio in men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to control males, are greater
(i.e., more feminine) than in their fathers or control males.[21]
Digit ratio in men correlates with genetic variation in the androgen receptor gene.[22] Men with genes that produce androgen receptors that are
less sensitive to testosterone (because they have more CAG repeats) have greater, more feminine, digit ratios. There are reports of a failure to
replicate this finding.[23] However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by
secreting more testosterone,[24] probably as a result of reduced negative feedback on gonadotropins. Thus, it is not clear that 2D:4D would be
expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen.
XY individuals with androgen insensitivity syndrome (AIS) due to a dysfunctional gene for the androgen receptor present as women and have
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feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex
difference in digit ratios is unrelated to the Y chromosome per se.[25]
The sex difference in 2D:4D is present before birth in humans,[26][27] 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.[28]
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.[29]
Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.[30]
There is evidence that this reflects fetal exposure to the hormones testosterone[31] and estrogen.
Several studies present evidence that digit ratios are heritable.[32][33]
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, and when examined researchers found no difference in estrogen
levels between males and females.[34]
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[35] have been implicated
in affecting these multiple traits (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. 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.[36]
Geographic and ethnic variation in 2D:4D
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups.[37][38] 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."[39]
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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 either high or low digit ratio.
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Low digit ratio
High digit ratio
Lowered sperm counts [40]
Increased risk for heart disease in males[41]
Physiology and
disease
Increased risk of obesity and metabolic syndrome in males[42]
Reduced risk for prostate cancer[43]
Reduced birth size in males[44][45]
Reduced rate of autism and Asperger syndrome[52]
Psychological
disorders
Increased rate of ADHD in males[46]
Increased risk for depression in males[53]
[47][48][49]
Increased rate of schizophrenia[54]
Reduced risk in females for anorexia
Increased rate of psychopathy in females [55]
nervosa[50] and in males for eating
Reduced risk of alcohol dependency[56]
disorders[51]
Reduced risk of video game addiction[57]
Increased Anxiety in males[58]
Reduced performance in sports[59]
Physical and
competitive
behavior
Reduced financial trading ability[60]
Right handedness Skills [61] (inconclusive)[62]
Assertiveness in females[5]
Cognition and
personality
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Aggression in
males[13][63]
Personality traits correlated with digit ratio, higher being more
feminized[70][71][72]
Masculinity of Handwriting[64]
Paranormal and superstitious beliefs among men with a higher digit
Perceived 'dominance' and masculinity
ratio[73]
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of man's face[65][66]
In an orchestral context, rank and
musical ability in males[67]
Higher exam scores among male students[29][74]
Academic Performance[68]
Maths ability[69]
Management
Leadership[75]
Innovation[76]
Smell perception[77]
Sensory
Perception
Color perception[78]
Tactile perception[79]
Sexual preference for more masculine men among women[80] and
gay men[92] with high digit ratio; a preference for a masculine facial
Lesbians have a lower digit ratio, on
Sexual orientation
average, than heterosexual women;
[80][81][82][83][84][85][86][87][88][89]
[90][91]
type means a more "feminized" mindset.
Lesbians are more likely to be femme and less likely to be butch with
a high digit ratio.[82][93] Identical female twins discordant for sexual
orientation still show the difference (lesbian less than straight, on
average) in digit ratio.[84][94]
Homosexuality for men,[83][95] but this is disputed,[90][96] and subject
to geographic variations [97]
Physical
Development
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Penile Length[98][99][100]
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Male-to-Female Transsexual People
A study in Germany has found a correlation between digit ratio and male-to-female transsexualism. Trans women were found to have a higher
digit ratio than cisgender males, but one that was comparable to control females.[101]
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.[102] 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.[103] Fink et al. (2004) found that men with low (indicating high testosterone) and women
with high (indicating high estrogen) 2D:4D ratios express greater levels of facial symmetry.[104] However, exposure to very high levels of
testosterone and/or estrogen in the womb may have negative effects as well.
Digit ratio and palaeolithic hand stencils
2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in prehistoric European and Indonesian cave painting.
[105][106][107]
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.[83]
Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human
sociality.[108]
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.
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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.[109]
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.[110]
See also
Anogenital distance
Waist–hip ratio
Dermatoglyphics
Body mass index
Handedness and sexual orientation
Chiromancy — hand analysis
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External links
PubMed listing of papers on digit ratios (http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&
term=%22digit%20ratio%22or%22digit%20ratios%22or%222D%3A4D%22)
Mills, Michael E. (October 2002). "Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning"
(http://human-nature.com/nibbs/02/manning.html). Human Nature Review 2: 418–23.
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Digit ratio - Wikipedia, the free encyclopedia
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