Perhaps a majority could be taught to be rational, I am not denying this possibility, but I do not think that it is economically feasible.

I'd doubt the feasibility without brain modification tech.

There is very little to rationality. All it takes is to be committed to take consequent actions that are implied by two basic questions:

• What do I want?
• How do I achieve what I want?

If you ask those questions, everything else will follow naturally. The very first implication is to ask,

• How do I figure out what I want?

Rationality, in its broadest sense, is a collection of heuristics that help you to answer those questions. In that respect rational decision making is already implied by our preference for world states that satisfy our utility-function.

This means that brain modifications, if necessary, are not a precondition but a possible consequence of rationality.

I think that most healthy humans could be taught to ask those questions and pursue follow-up actions. The problem are the circumstances in which they reside.