I think some studies have shown that the same set of people (or, let's say, people randomly assigned to one group or the other) can have very different behavior depending on the situation they're in. It seems at least possible that a forum could maintain its rules and norms while people join... though received wisdom says that if too many join at once, then the norms can be broken.

Yea, but HackerNews is really only going to appeal to a select group of people. Computer illiterate folk aren't going to be talking about compilers, lisp, APL, Smalltalk, blockchain, or the other 1000 compsci things that come up on here daily.