Just a small summary of Caplan's argument (I bet my language is different: I've never studied public choice seriously, I'm Austrian, and I read the book "The myth of the rational voter" more than one year ago).
- If I believe that gravity doesn't exist and I jump off a bridge, I die. That's the cost of being irrational, which is particularly strong in real "private" life.
- Where's this feedback mechanism in politics? Each single voter has a 0.0000001% probability of being relevant at the margin at the elections. The benefits of informed and rational choices are public, the costs are private: reasonableness is a public good. Rationality is systematically underproduced, because rational politics from the point of view of the voters is a tragedy of the common.
The objections raised in the comments are:
- Organized political groups are competitive and will invest in "rationalizing" the voters.
- Economists don't invest in laymen's education: when they did it, voters were more rational.