I am starting a new series entitled “Dumb and Dumber.” First, I want to make a distinction between dumb and stupid. Dumb is when a person makes a wrong choice out of ignorance of the facts. Stupid is when a person hasn’t the sense to act in their own self-interest, often because their minds are captured by slogans and myths. An example of the latter is when low and modest income people support tax breaks for the rich. That's stupid.
The first example of dumb is the large number of people who buy gluten-free foods when only two percent of the population is allergic to gluten. Gluten-free is now a fad food, perhaps confused in popular minds with carb-free and thus good for those on diets. That’s dumb.
Please add your favorite example as a comment.