[Dr. Peter] Hurd and his graduate student Allison Bailey have shown that a man's index finger length relative to ring finger length can predict how inclined that man is to be physically aggressive. Women do not show a similar effect.
A psychologist at the University of Alberta, Hurd said that it has been known for more than a century that the length of the index finger relative to the ring finger differs between men and women. More recently, researchers have found a direct correlation between finger lengths and the amount of testosterone that a fetus is exposed to in the womb. The shorter the index finger relative to the ring finger, the higher the amount of prenatal testosterone, and--as Hurd and Bailey have now shown--the more likely he will be physically aggressive throughout his life.
I expect there will be some sort of finger measuring device keychain on sale any day now. It will be called a Creepalizer, or maybe Thugalizer.