Even economics is based on the notion of “revealed preferences.” What people “think” is not relevant—you want to avoid entering the mushy-soft and self-looping discipline of psychology. People’s “explanations” for what they do are just words, stories they tell themselves, not the business of proper science. What they do, on the other hand, is tangible and measurable and that’s what we should focus on. This axiom, perhaps even principle, is very powerful but is not followed too much by researchers. Revelation of preferences is best understood by the betrothed: a diamond, particularly when it is onerous to the buyer, is vastly more convincing a commitment (and much less reversible) than a verbal promise.