Here's a study I saw a few weeks ago that I've been meaning to post about. Researchers hypothesized that prenatal testosterone exposure would lead to differences in physical development, which would then lead to differences in adult endurance performance.
The study used a digit ratio, the ratio of the length of the second and fourth fingers, abbreviated as 2D:4D. This 2D:4D ratio has been shown to be predictive of performance in other activities, such as soccer, skiing, and basketball.
The authors measured the digit ratio in men and women and then tested them over a series of 1 and 4-mile cross-country runs. After analyzing the data, it turned out that the digit ratio explained 25% of the variation in running performance.
This is quite a large amount of variation that's attributed to a single factor. The authors believe that endurance running was an important part of human evolution, and that there were strong selection pressures on endurance performance and prenatal testosterone.