Must or Should?

One of the things we teach in introductory economics is the distinction between positive and normative statements. It is useful when reading (or writing) op-eds to keep the distinction in mind.

For example, in today's NY Times, Cornell economics professor Robert Frank writes:

Top earners have captured the big share of all income and wealth gains during the last three decades. They’re where the money is. If we’re to pay for public services they and others want, they must carry a disproportionate share of the tax burden.

The first two sentences are correct statements of fact. The third sentence appears to draw a positive inference from them. Interpreted as such, the sentence is just wrong. Is there any reason to think it is impossible for the government to raise adequate revenue with a proportional tax? Not that I know of, and the article gives no indication of why Frank might think otherwise.

Maybe Frank meant to write "should" rather than "must." In that case, the sentence would have conveyed a personal political opinion, rather than suggesting (incorrectly) a conclusion of economic science. It would have been more clearly labeled as a normative statement.