Digit ratio
Encyclopedia
The digit ratio is the ratio of the lengths of different digit
s 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 androgen
s 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. The 2D:4D ratio is calculated by measuring the length of the index finger of the right hand, then that of the ring finger, and dividing the former by the latter. 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: in males, the second digit tends to be shorter than the fourth, and in females the second digit tends to be the same size or slightly longer than the fourth.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
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.
Assuming a normal distribution, the 10th and 90th percentiles for males are 0.910 and 0.984. For females: 0.932 and 0.998.
There is evidence that this reflects fetal
exposure to the hormone
s testosterone
and estrogen
.
Several studies present evidence that digit ratios are heritable.
. 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.
in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with digit ratio.
.
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
s 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
Index finger
The index finger, , is the first finger and the second digit of a human hand. It is located between the first and third digits, between the thumb and the middle finger...
) and 4th (ring finger
Ring finger
The ring finger is the fourth digit of the human hand, and the second most ulnar finger, located between the middle finger and the little finger. It is also called digitus medicinalis, the fourth finger, digitus annularis, digitus quartus, or digitus IV in anatomy.- Etymology :According to László A...
), is affected by exposure to androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
s e.g. testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
while in the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
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. The 2D:4D ratio is calculated by measuring the length of the index finger of the right hand, then that of the ring finger, and dividing the former by the latter. 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: in males, the second digit tends to be shorter than the fourth, and in females the second digit tends to be the same size or slightly longer than the fourth.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
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, with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930, after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr Glenn WilsonGlenn Wilson (psychologist)
Glenn Daniel Wilson is a psychologist best known for his work on attitude and personality measurement, sexual attraction, deviation and dysfunction, partner compatibility, and psychology applied to performing arts.In 2001, Wilson was ranked among the 10 most frequently cited British psychologists...
of King's College, London
King's College London
King's College London is a public research university located in London, United Kingdom and a constituent college of the federal University of London. King's has a claim to being the third oldest university in England, having been founded by King George IV and the Duke of Wellington in 1829, and...
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.
Digit ratio distribution
From a study of 136 males and 137 females:- Males: mean 0.947, standard deviationStandard deviationStandard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
0.029. - Females: mean 0.965, standard deviation 0.026.
Assuming a normal distribution, the 10th and 90th percentiles for males are 0.910 and 0.984. For females: 0.932 and 0.998.
Evidence of androgen effect on digit ratio
- Women with congenital adrenal hyperplasiaCongenital adrenal hyperplasiaCongenital adrenal hyperplasia refers to any of several autosomal recessive diseases resulting from mutations of genes for enzymes mediating the biochemical steps of production of cortisol from cholesterol by the adrenal glands ....
(CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average. Other possible physiological effects include an enlarged clitorisClitorisThe clitoris is a sexual organ that is present only in female mammals. In humans, the visible button-like portion is located near the anterior junction of the labia minora, above the opening of the urethra and vagina. Unlike the penis, which is homologous to the clitoris, the clitoris does not...
and shallow vaginaVaginaThe vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
. - 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 androstenedioneAndrostenedioneAndrostenedione is a 19-carbon steroid hormone produced in the adrenal glands and the gonads as an intermediate step in the biochemical pathway that produces the androgen testosterone and the estrogens estrone and estradiol.-Synthesis:Androstenedione is the common precursor of male and female sex...
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. - Digit ratio in men correlates with genetic variation in the androgen receptorAndrogen receptorThe androgen receptor , also known as NR3C4 , is a type of nuclear receptor that is activated by binding of either of the androgenic hormones testosterone or dihydrotestosterone in the cytoplasm and then translocating into the nucleus...
gene. Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios. - XY individuals with androgen insensitivity syndromeAndrogen insensitivity syndromeAndrogen insensitivity syndrome is a condition that results in the partial or complete inability of the cell to respond to androgens. The unresponsiveness of the cell to the presence of androgenic hormones can impair or prevent the masculinization of male genitalia in the developing fetus, as...
(AIS) due to a dysfunctional gene for the androgen receptor present as women and have 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. - The sex difference in 2D:4D is present before birth in humans, 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 estradiolEstradiolEstradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...
measured in 33 amniocentesisAmniocentesisAmniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
samples correlates with the child's subsequent 2D:4D ratio. - In pheasantPheasantPheasants refer to some members of the Phasianinae subfamily of Phasianidae in the order Galliformes.Pheasants are characterised by strong sexual dimorphism, males being highly ornate with bright colours and adornments such as wattles and long tails. Males are usually larger than females and have...
s, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the eggEgg (biology)An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
.
There is evidence that this reflects fetal
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
exposure to the hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
and estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
.
Several studies present evidence that digit ratios are heritable.
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 have been implicated in this pleiotropyPleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
. 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.
Geographic/Ethnic variation in 2D:4D
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."Correlation between digit ratio and traits
Some authors suggest that digit ratio correlates with health, behavior, and even sexualityHuman sexuality
Human sexuality is the awareness of gender differences, and the capacity to have erotic experiences and responses. Human sexuality can also be described as the way someone is sexually attracted to another person whether it is to opposite sexes , to the same sex , to either sexes , or not being...
in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with digit ratio.
Physiology and disease
- Lowered sperm counts
- Increase risk for heart diseaseHeart diseaseHeart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
- Increased risk of obesity and the metabolic syndromeMetabolic syndromeMetabolic syndrome is a combination of medical disorders that, when occurring together, increase the risk of developing cardiovascular disease and diabetes. It affects one in five people in the United States and prevalence increases with age...
- Reduced risk for prostate cancer
- Reduced penis lengthHuman penis sizeHuman penis size is the measured length and width of the human penis. The most accurate measurement of the human penis comes from several measurements at different times, as there is natural variability in size due to arousal level, time of day, room temperature, frequency of sexual activity, and...
Psychological disorders
- Reduced rate of autism
- Increased risk for depression
- Increase rate of schizophrenia
- Increased rate of ADHD
- Reduced risk for eating disorders in females and males
- Reduced rate of psychopathyPsychopathyPsychopathy is a mental disorder characterized primarily by a lack of empathy and remorse, shallow emotions, egocentricity, and deceptiveness. Psychopaths are highly prone to antisocial behavior and abusive treatment of others, and are very disproportionately responsible for violent crime...
- Reduced risk of alcohol dependency
Physical and competitive behavior
- Skiing
- Soccer playing
- Sporting in females
- Financial trading
- Hand Skills
Cognition and personality
- Assertiveness in women
- Aggression
- Masculinity of Handwriting
- Perceived 'dominance' and masculinity of man's face
- Personality
- Exam scores: a higher ratio is correlated with higher exam scores among male students
- Musical ability
Sexual orientation
- Bem sex role score in women; erotic role preferences in gay men.
- Lesbians vs. straight women; butch vs. femme lesbians.
- Gay vs straight men, but most studies find no differences in digit ratio between gay and straight men. Some studies correlate male homosexuality and 2D:4D positively, others negatively
- Difference in digit ratio between identical female twins discordant for sexual orientation
- Fraternal birth order effect on digit ratio.
- ParaphilicParaphiliaParaphilia is a biomedical term used to describe sexual arousal to objects, situations, or individuals that are not part of normative stimulation and that may cause distress or serious problems for the paraphiliac or persons associated with him or her...
(or fetish) sexual interest, including pedophiliaPedophiliaAs a medical diagnosis, pedophilia is defined as a psychiatric disorder in adults or late adolescents typically characterized by a primary or exclusive sexual interest in prepubescent children...
.
Transsexualism
- A recent study in Germany has found a correlation between digit ratio and male to female transsexualismTranssexualismTranssexualism is an individual's identification with a gender inconsistent or not culturally associated with their biological sex. Simply put, it defines a person whose biological birth sex conflicts with their psychological gender...
. Male to female transsexuals (Transwomen) were found to have a higher digit ratio than control males, but one that was comparable to control females.
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. 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. 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 symmetryFacial symmetry
Facial symmetry, is one element of bodily symmetry, including fluctuating asymmetry. Along with traits such as averageness and youthfulness it influences judgements of aesthetic traits of physical attractiveness and beauty, and is associated with fitness-linked traits including health, It is also...
.
Digit ratio and palaeolithic hand stencils
It is generally assumed that creating cave art was a male behavior. 2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves.Digit ratio research in animals
- Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apesGreat ApesGreat Apes may refer to*Great apes, species in the biological family Hominidae, including humans, chimpanzees, gorillas, and orangutans*Great Apes , a 1997 novel by Will Self...
, 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 BreedloveMarc BreedloveStephen Marc Breedlove is currently the Barnett Rosenberg professor of Neuroscience at Michigan State University in East Lansing, Michigan. He was born and raised in the Ozarks of southwestern Missouri. After graduating from Central High School in 1972, he earned a bachelor's degree in...
'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. HurdPeter L. HurdPeter L. Hurd is an academic specialising in biology. He is currently an Associate Professor aligned to both the Department of Psychology's Biocognition Unit, and the University's Centre for Neuroscience at the University of Alberta...
, Theodore Garland, Jr.Theodore Garland, Jr.Theodore Garland, Jr. is a biologist specializing in evolutionary physiology. He was on the faculty at the University of Wisconsin–Madison for 14 years, served at the National Science Foundation for one year, and is currently Professor of Biology at the University of California, Riverside. He...
, and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolutionExperimental evolutionIn evolutionary and experimental biology, the field of experimental evolution is concerned with testing hypotheses and theories of evolution by use of controlled experiments. Evolution may be observed in the laboratory as populations adapt to new environmental conditions and/or change by such...
). 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.
See also
- Anogenital distanceAnogenital distanceAnogenital distance is the distance from the anus to the genitalia, the base of the penis or vagina. It is considered medically significant for a number of reasons, in both humans and animals. It is regulated by dihydrotestosterone, which can be disrupted by phthalates common in plastics...
- Waist–hip ratio
- DermatoglyphicsDermatoglyphicsDermatoglyphics is the scientific study of fingerprints. The term was coined by Dr. Harold Cummins, the father of American fingerprint analysis, even though the process of fingerprint identification had already been in use for several hundred years. All primates have ridged skin...
- ChiromancyChiromancyPalmistry or chiromancy , is the art of characterization and foretelling the future through the study of the palm, also known as palm reading, or chirology. The practice is found all over the world, with numerous cultural variations...
(Pseudoscientific Hand analysis)