Although the reason is not yet understood, earlier studies have shown that finger-length ratios are a robust marker of how much of that hormone a baby has been exposed to in utero the more testosterone, the longer theringfinger.
They worked out the ratio of the length of the index finger to the length of theringfinger for each child, and compared it with those of 34 of the children's healthy siblings, 88 of their fathers, 88 of their mothers, and a number of unrelated controls that were matched for sex and age.