Do any of these people know anything for certain?
No, they don't, actually. Many "studies" are just observational, in which they try to compare groups of people and find a correlation between various characteristics. But those studies don't explain anything, and too often, only confirm whatever the "researcher's" initial bias was. Most MDs who do these sorts of studies have not, in fact, had any formal training as researchers.
As a formally trained medical researcher, it drives me nuts to see the results of so many "studies" used as gospel truth.