Carried to extreme this could lead a behavioral economist to report, "the subject made the perfectly rational decision not to peform any mental effort, and so w

Carried to extreme this could lead a behavioral economist to report, "the subject made the perfectly rational decision not to peform any mental effort, and so was eaten by a shark". You say, "Many times they are simply not accounting for cognitive or longterm costs." How do distinguish those times from the other times when a person is being rational?