There are so many pre-conceived notions that we have come across that we have sometimes assumed in our life. Here are some of them which come to mind...
People who are loud are clean hearted and good and the quiet ones are arrogant .
Serious people are matured and jovial ones are immature.
People who speak their mind have no class and the reverse "good natured and dignified"
Children of rich people are spoilt and the ones poor or from middle class are really good natured and well raised.
These are the ones I can come up with, you can add some more if you want and post your thoughts on whether you actually any such notion and if you actually believe in this theory.