The Romney fundraiser in the Hamptons continues to inspire much justified hilarity. Matt Yglesias has fun with whining rich people complaining that they are the engine of the economy, pointing out that quite a few of the whiners make their money in ways that arguably does very little for growth — say, by running funds that collect so much in fees that they leave investors worse off.

There is, however, an even broader critique of the whole keep taxes low on jobcreatorsenginesoftheeconomy thing — it doesn’t make sense even when the rich really earn their money. I’ve tried to make this point before, with regard to optimal top tax rates, but without as much success as I’d like; so let’s try it again.

So, imagine a Romney supporter named John Q. Wheelerdealer, who works 3000 hours a year and makes $30 million. And let’s suppose that he really does contribute that much to the economy, that his marginal product per hour — the amount he adds to national income by working an extra hour — really is $10,000. This is, by the way, standard textbook microeconomics: in a perfectly competitive economy, factors of production are supposedly paid precisely their marginal product.

Now suppose that President Obama has reduced Mr. Wheelerdealer to despair; not only does the president waste money by doing things like feeding children, he says mean things about some rich people, which is just like the Nazis invading Poland, or something. So Wheelerdealer decides to go Galt. Well, actually just one-third Galt, reducing his working time to just 2000 hours a year so he can spend more time with his wife and mistress.

According to marginal productivity theory, this does in fact shrink the economy: Wheelerdealer adds $10,000 worth of production for every hour he works, so his semi-withdrawal reduces GDP by $10 million. Bad!

But what is the impact on the incomes of Americans other than Wheelerdealer? GDP is down by $10 million — but payments to Wheelerdealer are also down by $10 million. So the impact on the incomes of non-Wheelerdealer America is … zero. Enjoy your leisure, John!

OK, there’s a catch: Wheelerdealer pays taxes, and when he chooses to work less, he pays less taxes. So there is in fact a welfare loss to the rest of America, but it comes only through the fact that revenue falls.

This is the basis of the result from optimal taxation theory that says that from the point of view of everyone except the very rich the optimal top tax rate is the rate that maximizes revenue — full stop. And since the rich already make so much money, their marginal utility from an extra dollar is very small, so the revenue-maximizing tax also maximizes welfare for society as a whole.

Oh, and we have pretty good evidence on the (small) actual incentive effects of changes in top tax rates, enough to suggest that the optimal rate is in the 70-80 percent range — which is where it was in the 1960s, a decade of very good economic performance.

Now, obviously the Romney fans have a very different view. They seem to believe that John Q. Wheelerdealer somehow adds much more to the economy than even the lavish amount he gets paid; he’s an engine of growth! Oddly, though, they also claim to believe in the perfection of markets — and the only way to justify their belief in their own special role is to claim that there is some kind of massive market failure.

So there’s a huge contradiction in the whole position of the self-regarding rich — a contradiction that I’m quite sure bothers them not at all. More champagne?