[gela]

I get your point, but I think your underestimating scientists.

They know more then we do. They probably have asked all those questions at one point or another; plus hundreds more.

What you are concerned about it activity 'z' that I described in my post. The activity that wasn't asked about nor examined. What I am saying is that because of the abundance and diversity of studies done, every possible z is bound to have been covered. If a single study was done including 'z' and it came up with results pointing to 'z'; then more studies would have been done with z as the subject. If z continued to give results, then even more studies would be done - it would start to become public, scientists would start debating about it - and eventualy the scientific community would come to a conclusion about z.

People can be stressed without smoking, and people can smoke without being stressed.

Two favorite topics of studies are cancer and stress. If there was a definitive link between the two, it would have already been found.