[lorenzsell]

so what you're saying is that people who exercise are genetically predisposed to exercise, and those who don't, aren't. therefore any study that suggests the health benefits of exercising should take this into account.

how would you measure for something like that?

[xel02]

Twin studies (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.