Sorry, maybe wasn't very clear. What I meant is that they picked couples of twins which both used to exercise regularly - that seems to be a good indicator of

Sorry, maybe wasn't very clear. What I meant is that they picked couples of twins which both used to exercise regularly - that seems to be a good indicator of the fact they felt a need for it and got a reward from it. They also hint at the fact that it was difficult to find a couple of twins in which one of the two had stopped exercising, and they say that this is usually in response to a pressure of some kind, or in other words, it is a forced behaviour. So in the end they're comparing somebody who apparently feels the need to exercise, with an identical twin who feels the same need but can't anymore. So, bottom line(s): 1) they'not simply comparing one that exercises with one that does not. They're comparing someone who needs exercising with someone who has the same need but can't anymore. But they would be wrong in assuming that everybody needs exercising as much as these twins do. 2) again, they're comparing couples of twins in which one keeps doing what he likes, so he's probably more happy and fulfilled, with another one who can't anymore, so is probably more stressed. Would the same work for golfing, singing or painting?