Behavioral economist Dan Ariely doesn't buy-into the notion of immutable human rationality, upon which economic study is traditionally based. In fact, he says, we are often predictably irrational. We understand our preferences less well than we imagine; and all-too-frequently our decision-making is manipulated by the way in which alternatives are presented. We know our physical limitations, and we design for them. If we better appreciated the limitations of our cognative abilities, we might redesign our decision-points as well.