Some of the backgound:
testosterone levels normally surge during the middle of a pregnancy. This not only shapes the brain and sex organs of the child, but also affects the way its fingers grow. High levels of the stuff extend the ring fingers, making them longer than the index fingers. In general, men have relatively longer fourth fingers than women. Previous research has shown that men with significantly elongated ring fingers excel at competitive sports.
Finger length ratios are so easy to measure - just have the respondent trace his hand on a sheet of paper, that I am surprised it is not routinely done in surveys like the Health and Retirement Study and the Panel Study of Income Dynamics.
For the curious, my index and ring fingers are of approximately equal length.
Hat tip: Dan Black