People whose work involves behavior change, often use theories that incorporate the concept of social norms. 

However, because a person believes something to be normal (common or routine) doesn't mean that it is.

We often base our ideas about behavior outcomes and behavior frequency on what is happening in our personal  world (our view or frame).

Consider the 16 year old smoker.  He may have started smoking because his friends smoke or chose his friends because they smoke.  Either way, he is surrounded by 16 year olds who smoke.

Now I ask him, "What percent of 16 year olds smoke cigarettes?"  and his answer is, "60 to 70%"
The true number is closer to 20%.  The teenager is basing his truth about the world based on what he sees in his circle of friends. He thinks almost ALL teenagers smoke.   He is mistaken.

So when you hear about heart attacks, dementia cases and other diseases that are related to poor diets and being overweight and say, "well -  my grandmother ate fried foods every day and lived to be a 100."  or "I know people who are overweight and eat whatever they want and they are just fine."  You should recall the mistaken perceptions of the teenage smoker.  Just because the three or four people you know did the wrong things and didn't get sick, doesn't mean those things don't make people sick.