A common response from those who seem to dislike the idea that people aren't rational, self-interested maximisers is to focus on the way that people learn.
For example, I've seen someone somewhere use this as an argument against the endowment effect. Their argument was that, yes, maybe people do put a higher value on something once they own it, but that may change with experience. So second-hand car salesman won't experience the endowment effect in the same way (if at all?) in respect of a car they've just bought at auction compared to a regular punter. The point seems to be that people learn to value things more rationally the more they trade.
That all sounds ok. But I wonder what is really going on here. Is it that rational self-interested 'core' is exposed through repeated transactions? All that cranky irrational stuff gets worn away with experience. Or could it also be that to some extent we internalise a new norm and act in accordance with it?
Clearly I'm not discounting the idea that people do act in a rational and self-interested way. But to what extent is that actually resulting from an internal drive which might be uncovered through experience, and to what extent is it a norm that people behave in line with?
I reckon there's something worth thinking about here, and having googled 'Self-interest as a norm' I found this paper, which looks like it might be worth a read.
UPDATE: That paper is well worth a read. Lots of interesting stuff and useful refs in there.