One problem, surely, in real life is not knowing what the other person's
"utility function" is? So someone may behave apparently irrationally - e.g.
giving away money - because their utility function involves making
themselves feel good, or getting a reward in heaven, or they want to show
off how generous they are to impress someone, or something else we don't
know. So in practice it isn't even theoretically possible to know if
someone else is behaving rationally a lot of the time.

Personally, I think anyone without brain damage or mental illness will
normally behave rationally according to their own lights. We call it
cognitive dissonance when someone is unable to justify their beliefs or
actions - they have found some contradiction within themselves - but they
usually quickly act to reduce this, by changing their beliefs or doing
something different. And it doesn't seem to happen very often, as far as I
know, so it seems to me that most people are acting rationally according to
their own utility functions most of the time.

By the way, I don't see how a random decision can be considered "irrational
by definition". To say something is rational surely means there is a reason
for doing it which "attempts to maximise the person's utility function" -
so making a random decision is rational so long as there is a good reason
to do so - e.g. neither option seemed better, or a "leftfield" move
confounded an opponent, or it was more important to make *some* decision
quickly than to work out the best decision (as in the chess clock example)

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