Who decides what is "common knowledge" when it comes to marketing claims?

Watching the movie Supersize Me, I was struck by something McDonald's said in response to a lawsuit against it by two obese young women.

Apparently McD's defended itself in part by saying it's "common knowledge" that processed foods are less healthy than unprocessed foods.

Actually in my experience this assertion is far from true. Just the opposite sometimes - there is the thought that if someone has "cooked the hell out of it," processed and pasteurized it and sealed it up tight in plastic, then it's much more safe than "unprotected" natural food.

I myself did not know there was a difference between chicken nuggets and regular chicken, actually, because so often you see things like "100% white meat" on the package.

Similarly I really thought Papa John's tagline, "Better Ingredients. Better Pizza" was a statement of fact.

I am no rocket scientist but I do have a Ph.D. And I have been working in and studying marketing for decades.

Are you going to tell me that most people are savvy disbelievers by nature?

Come on!

I think what's common knowledge is that people are frequently easily fooled.

And even if they are cynical - are you then going to argue that the entire marketing industry is built on open lies that people can easily unravel? Or that it should be?

It's common knowledge that nobody trusts a liar.