1595
Refers to 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.
The ratio of the length of the index finger to the length of the ring finger in adults is affected by the level of exposure to male sex hormones of the embryo in utero.
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.
It has been suggested that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens while in the uterus.
The 2D:4D ratio can be considered a crude measure for prenatal androgen exposure.
Lower 2D:4D ratios pointing to higher prenatal androgen exposure.
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 lower than 1.
The 2D:4D digit ratio is sexually dimorphic
Thee 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.
Studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
A greater proportion of men have shorter index fingers than ring fingers than do women.
Sex difference in digit ratios is present in two-year-old children.
The index is a marker of prenatal sex hormones.
2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women, may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios.
The formation of the digits in humans, in utero, is thought to occur by 13 weeks.
Bone-to-bone ratio is consistent 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.
There is 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.
There is contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio.
Male sexual traits that are attributed to testosterone levels and correlation with the 2D:4D.
Males: mean 0.947, standard deviation 0.029.
Females: mean 0.965, standard deviation 0.026.
Women with congenital adrenal hyperplasia (CAH), and elevated androgen levels before birth, have lower, more masculinized 2D:4D on average.
Effects of elevated in utero androgens may possibly result an enlarged clitoris and shallow vagina.
Males with congenital adrenal hyperplasia have more masculine (smaller) digit ratios than control males.
Digit ratio in men with Klinefelter’s syndrome, who have reduced testosterone secretion throughout life compared to control males, are greater, that is more feminine, than in their fathers or control males.
Digit ratio in men correlates with genetic variation in the androgen receptor gene.
Men with genes that produce androgen receptors that are less sensitive to testosterone have greater, more feminine, digit ratios.
XY individuals with androgen insensitivity syndrome (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios, demonstrating 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, ruling out any social influences that might affect digit growth differentially in the two sexes.
Androgens act before birth to affect digit ratios.
The testosterone to estradiol ratio measured in amniocentesis samples correlates with the child’s subsequent 2D:4D ratio.
Studies suggest digit ratios are heritable.
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D.
There is no difference in estrogen levels in amniotic fluid between males and females.
It is not clear why digit ratio is influenced by prenatal hormones.
2D:4D ratios vary greatly between different ethnic groups.
It is suggested that digit ratio correlates with health, behavior, and even sexuality in later life.
Some traits that have been either demonstrated or suggested to correlate with either high or low digit ratio and include:
Increased risk of prostate cancer and prostate diseases in males.
Slower utero fetal development in both sexes.
Increased reproductive success in males.
Lowered sperm counts
Increased risk for heart disease in males
Increased risk of obesity and metabolic syndrome in males
Reduced risk for prostate cancer
Reduced birth size in males
Increased reproductive success in females.
Psychological disorders
Increased rate of ADHD in males
Increased rate of Asperger syndrome and other autism spectrum disorders
Increased risk in females for anorexia nervosa.
Increased psychopathy in men with low digit ratio and high adult testosterone levels.
Increased rate of alcohol dependency.
Increased risk for depression in males.
Increased rate of schizophrenia.
Increased rate of psychopathy in females.
Reduced risk of alcohol dependency.
Reduced risk of video game addiction.
Increased anxiety in males.
Increased risk in females for bulimia.
Physical and competitive behavior
Reduced performance in sports
Reduced financial trading ability
Right handedness skills
Cognition and personality
Assertiveness in females
Aggression in males
Masculinized handwriting in females
Perceived ‘dominance’ and masculinity of man’s face.
Musical ability in males.
Right hand low digit ratio predicts academic performance.
Mathematical ability.
Decreased empathy in response to adult testosterone levels.
Personality traits correlated with digit ratio, higher being more feminized.
Higher exam scores among male students.
Higher neuroticism in both sexes with higher right hand digit ratio.
Higher left hand digit ratio in response to high adult testosterone levels predicts
musical orchestra rank in females.
Higher verbal fluency in both sexes.
Higher visual recall in females.
Lesbians have a lower digit ratio, on average, than heterosexual women.
Bisexual men have a lower digit ratio than exclusively homosexual men and community volunteers recruited regardless of sexual orientation.
Sexual preference for more masculine men among women and gay men with high digit ratio.
Lesbians are more likely to be femme and less likely to be butch with a high digit ratio.
Identical female twins discordant for sexual orientation still show the difference lesbian less than straight, on average in digit ratio.
Homosexuality for men, according to some studies have shown that the digit ratio in homosexual men is similar to that of heterosexual men.
Meta-analysis concluded that there are no significant sexual orientation differences in digit ratio in men.
2D:4D ratio may also be indicative for human development and growth.
Men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.
2D:4D correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development.
Men with low, indicating high testosterone, and women with high, indicating high estrogen, 2D:4D ratios express greater levels of facial symmetry
Digit formation, in utero, is thought to occur by 13 weeks.
During this period if the fetus is exposed to androgens, and the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins.
In dizygotic twins, the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio.
There is no correlation between the sex hormone levels of an adult and the individual’s 2D:4D, suggesting it is the exposure in utero that causes this phenomenon.
Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have more masculinized 2D:4D on average.
Males with congenital adrenal hyperplasia have more masculine digit ratios than control males,
Digit ratio in men with Klinefelter’s syndrome, who have reduced testosterone secretion throughout life compared to control males, are greater, more feminine, than in their fathers or control males.
Digit ratio in men correlates with genetic variation in the androgen receptor gene.
Men with genes that produce androgen receptors that are less sensitive to testosterone have greater, more feminine, digit ratios.
The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlated with the child’s subsequent 2D:4D ratio.
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, as there is no difference in estrogen levels between males and females.
Digit ratio on the right hand is more responsive than that on the left hand, as is indicated by the greater sex difference on the right than the left.
2D:4D ratios vary greatly between different ethnic groups.
Lesbians have a lower digit ratio, on average, than heterosexual women.
Bisexual men have a lower digit ratio than exclusively homosexual men.
Lesbians are more likely to be feminine and less likely to be butch with a high digit ratio.
Identical female twins discordant for sexual orientation still show differences in digit ratio.
Other studies have shown that the digit ratio in homosexual men is similar to or lower than that of heterosexual men.
A meta-analysis concluded that there are no significant sexual orientation differences in digit ratio exist in men.
2D:4D ratio may also be indicative for human development and growth.
Men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.