Seriously. If there is one thing most smart people have in common, it's that they are not in Mensa.
I went to an Ivy League college. I work in an elite university. There are two Nobel laureates on my floor. I spend my days surrounded by certifiably Very Smart People.
I don't know a single person who belongs to Mensa. Most of the people I know would laugh at the idea of joining Mensa. They don't need to join an organization to show off how smart they are. Everyone already knows they're smart. They don't need to join an organization to find other smart people. They spend their days surrounded by smart people, because they work in fields that require intelligence.