> My question is not terribly complex or dependent on unusual or unrealistic situations.
No, usually the psychological arguments are needed to hand-wave away crucial but obvious problems.
> Well, that's a big weakness of these theories
No, it's not. It's perhaps their most important insight.
I mean, IMO they're wrong, but the insight about the need for a psychological aspect is important and correct.
Libertarians and especially anarcho-capitalists need similar psychological arguments (the fact that they often hand-wave over this need not-withstanding).