[W]e always believe people are rational. ... [W]hen it comes to the physical domain, we see very clearly our limitations.  We know we're not supermen; we know how high we can jump and how fast we can run and see how limited we are.  And interestingly enough, when we design the physical world, we take this into consideration, right?  Everything we design, or almost everything, fits out physical abilities. ... But when we move into the mental domain, we assume people are infinitely capable. ... It's like we're supermen of the mind.  And we design the world with this in mind, which is incredibly baffling. ... [W]e really don't take the mind's limitations into account when we design things that are complex, like savings plans, annuities, insurance, healthcare.  For all of those things, we assume perfect rationality.