The New York Times published a story about a study that was done with identical twins. One twin worked out regularly; the other did not.
This study is interesting because it talks not only about the affect our genetic make up has on our bodies, but about the affects exercise itself has on the body AND brain. It's got to be hard to conduct a study about how exercise can affect a person regardless of genes because, who are you going to compare them to? No two people are exactly alike, except for twins. This study even went far enough to try and use twins that had a similar fitness background and activity level growing up.
The findings, in my opinion, just go to show how crucial staying active is. Both for physical, and mental health and strength.
Take a look for yourself. You decide...