Rational beings systematically maximizing their utility

Yes, that has been the fundamental economic assumption for the past 50 years or so. But over 300 years of bubbles, manias and irrational exuberance argues against the basic assumption. Prof. Robert Schiller of Yale, who famously warned against the U.S. real estate bubble well before it burst,writes about the new field of neuroeconomics. Worth reading.
The neuroeconomic revolution