Twin studies (http://en.wikipedia.org/wiki/Twin_study http://en.wikipedia.org/wiki/Twin_study) aim to examine the differences due to genetics vs environment. On

Twin studies (http://en.wikipedia.org/wiki/Twin_study http://en.wikipedia.org/wiki/Twin_study) aim to examine the differences due to genetics vs environment. One naive method is to look at identical twins who have been separated, and see if there is a difference in exercise levels.