Economists' Assumptions

One related item I've always not seen satisfactorily addressed is that self-interest can be defined in many different ways, and usually in ways that contradict the simple-minded materialism of economists. There often is some function being optimized, it is just as often _not_ the gluttonous acquisition of goods and services at the cheapest prices that so often starts off the analysis. Charity, ease of the transaction, etc. play a significant part.

Your purported weaker version of self-interest is actually the stronger version. Many Economists admit to this strength by continual referral to households in analysis, instead of the concept of the individual. Power in economic action comes from organizational structure. lgl

Actually, it is incorrect to say that the underlying assumption is that individuals simply seek to maximize their personal financial well-being. As Boudreaux points out, the assumptions are that people have preferences that satisfy some simple conditions.

But these preferences can be lots of things. To take a simple example, consider an individual who chooses to go home at five rather than work overtime. Even though this behavior does not maximize financial well-being, there is nothing about it that defies any economic assumption.