Finger length

There are now many studies that show how the relative length of the ring and index fingers, known as the 2D:4D ratio, predict a variety of outcomes including susceptability to prostate cancer, sporting prowess, longevity and sexual orientation. It is believed that it reflects exposure to testosterone shortly after conception.
This newspaper article discusses how it predicts earnings amongst traders in the City. It would be interesting to see how it fares in a human capital model on a representative sample of the population. Since it is relatively easy to measure (as biomarkers go), hopefully it will be incorporated into datasets in the future.