Decision theory seems, by its name, to be a fairly straightforward concept. At first blush, it would seem to be the theory of making decisions. While that is technically correct, these waters are deceptively deep and even a bit turbid. The notion of what actually constitutes a decision is sometimes called into question—which doesn’t make it any simpler to advance a theory about it.
Regardless of what we settle on as a definition of decision, there are plenty of theories about not only the “right” way to come to one, but also theories on why humans and other such decision-making entities often fail miserably to make that right decision.