Pulling the ladder up behind you is something that happens pretty often in human history.

That’s not to say it’s intrinsic to us or destined to happen. But “hurt people hurt people” is probably what causes that to manifest.

In certain circumstances I bet most if not all of us would do it, too. A full life raft will swamp and kill everyone aboard if one more person gets on, for example. If there’s a guy trying to force his way on, what should the people on the boat do?

There’s a fair amount of that misguided belief among some anti immigrant movements: “we’re full” or equivalent.