The term "digit ratio" can refer to the relative lengths of any two fingers or toes, but is almost always used to refer specifically to the relative length of a person's ring and index fingers. This is commonly called the 2D:4D ratio, in reference to the index and ring fingers' respective positions as the second and fourth digits on the hand. The ratio is calculated by the dividing the length of the right index finger by the length of the right ring finger, so a ratio less than 1 indicates that the ring finger is longer, and a ratio higher than 1 indicates the reverse. Scientific research has shown a correlation between differences in digit ratio and a number of physical and psychological traits.
This digit ratio is an example of human sexual dimorphism, physical differences between men and women. The majority of men have ring fingers that are longer than their index fingers, and the majority of women have index fingers that are equal in length to their ring fingers, or longer. The development of a fetus in the womb is guided by the type of hormones to which it is exposed, which influence the development of the fetus' genitals and other sex-specific structures and traits.
Masculine anatomy is developed in response to the presence of higher levels of what are called androgenizing hormones, such as testosterone. This includes differences in the relative sizes of different areas of the brain, compared to the average female. One of the effects of androgenizing hormones is relatively longer ring fingers and relatively shorter index fingers. Why this should be the case is not yet known, though sexual dimorphism in the relative lengths of different digits has been observed in other species such as gorillas and chimpanzees. Male-female differences in digit ratio are one of the few physical sex differences that is apparent from birth rather and don't arise from hormonal changes during puberty.
A number of personal traits tend to correlate with digit ratio, though the correspondence between a particular digit ratio and particular mental and physical characteristics is by no means absolute. Average digit lengths and ratios also vary between different ethnic groups for reasons of heredity rather than in utero testosterone exposure, which further complicates attempts to use digit ratios as a predictor. Longer ring fingers and a more masculine digit ratio have been shown in studies to be positively correlated with greater aggression, assertiveness and competitive behavior. Masculine digit ratios are also disproportionately common in people who are autistic or left-handed.
More masculine digit ratios in men positively correlate with traits such as more masculine facial features, higher sperm count and musical ability. Masculine digit ratios in women are positively correlated with traits such as high achievement in competitive sports and health problems that are more common in males than females, such as heart disease. More masculine digit ratios also are positively correlated with homosexuality in females, but most research has shown no difference in digit ratios between heterosexual and homosexual men. More masculine digit ratios are also particularly common in men who have prostate cancer, and more feminine digit ratios are more common among people who suffer from eating disorders and schizophrenia.