The natural inclination to do "manly" things around your buddies. This includes, but is not limited to: scratching your balls around your buddies, burping, farting, picking your nose, etc.
Guy 1: Hey dude! Is John's girlfriend going to watch the game with us?
Guy 2: Don't know, why?
Guy 1: Think about it man...we won't be able to be ourselves...our bropensity got's to be in check!