# Digit ratio

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The digit ratio is the ratio of the lengths of different digits 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 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 androgen exposure. The 2D:4D ratio is calculated by measuring the index finger of the right hand, then the ring finger, and dividing the former by the latter. A longer ring finger will result in a ratio of less than 1, a longer index finger will result in a ratio higher 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 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.

## Contents

### 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,[1][2] with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930,[3] 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.[4] This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.[5] Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.[4] In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children [6] 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.