Why some people are stupid

There're different levels of stupidity. Here're how they may be classified (in ascending order):

1. silly
2. stupid
3. dumb
4. fool
5. moron

As to why people are stupid, there may be more than one possibilty. it could be due to:

1. sad genes
2. food/drinks they eat
3. the air they breathe
4. the people they mixed with
5. plain stupidity (w/o reason)

i need to figure out how to live with them. Hopefully i can provide the answers soon. Bless me!

Suggestions, anybody?