The 2D:4D ratio is the ratio of length between a person's index finger and ring finger - the second digit and fourth digit counting from the thumb with the thumb as the 1st digit.

Most males have an index finger that is noticeably shorter than the ring finger- - a low 2D:4D ratio. Most females have an index finger that is the same size or longer than the ring finger -- or a high 2D:4D ratio. This difference is attributed to fetal exposure to androgens -- especially testosterone. It also appears that the right hand is affected more by these hormonal levels than the left hand such that length differences are more pronounced on the right hand.

This ratio has a statistically significant correlation to human behavior. Studies of human sexual orientation indicate that lesbians, as a group, have ratios similar to the male general population and that homosexual men have ratios that are closer to the female average -- though the correlation is stronger in females than in males.

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