[BillyTheAdult]

I think it is 100% self selecting, the problem is that we aren't usually consciously aware of our decisions, or don't feel powerful enough to override decisions we are aware of but do not like. So by the time you are a teenager you may already be to the point that you are severely overweight or something, and you have formed a deeply ingrained belief "I am overweight" and as a result "I am unattractive." It's not that you were born unattractive it's that by that time you have already unconsciously not cultivated attractiveness. Also teenagers (and adults) tend to largely form their identities from social structure i.e. you believe what others tell you about yourself. So if others tell you yo are attractive you believe it and if they tell you you aren't you believe that too. Or you may even tell yourself because you are getting some payoff from it that you are not even consciously aware of.

As my beloved Dr. Hawkins would say "The human mind is incapable of telling truth from falsehood" so it doesn't take a lot to make a mind believe in something that's false.