I think people divide themselves into one of two categories. Either they have decided that they own the arm rest and some distance on the other side, or they keep their arms pinned to their sides (so to speak!) and are careful to not inconvenience their seatmate.

It may be a genetic thing because each seems predisposed to one of the categories.

Then, there are a few souls who will wait to see if the armrest is available and carefully put their arm on their half of it allowing an equally tiny space for the other. Interesting dance of the arms!

Linda in SW Ontario