Did the IGM Reject Laffer Optimism or Old-School Keynesianism?

I think you are right. The question asks for opinions on the effect 5 years out. Fiscal stimulus has been pretty well shown to peter out well before then, so this question seems to be a subset of the larger question whether fiscal stimulus results in greater tax revenue 5 years out than if no stimulus had been put in place. I think even pro-Keynesian models would say not.

Unlikely. Most would say the economy would be larger, but not the tax revenues for it takes a much larger economy to make up for even somewhat lower taxes, but it does bring to fore that the empirical data is largely based on periods of growth rather than recession/depression. Still likely not sufficient to cover the economy/revenue spread, but less so.

Agree with Lord. Those seem like two very different propositions.

On the supply-siders issue, it seems like there's a problem here with treating supply-side economics and Keynesian economics as opposites. They're not.

Do low tax rates have supply side benefits? I can't imagine answering "no" to that. Do low tax rates stimulate demand? I can't imagine answering "no" to that either.

Real business cycle theory, it seems to me, is a different story. That is more opposed to Keynesianism.

Does anybody really think of "supply side economics" as an "economic theory", though? I think of that as a political movement that had a lot of things to say about economic policy, but then that was - if not before my time - at least before my formative years.

Although I should be careful with my comment on RBC... people are often too strenuous on these points. There's absolutely no reason why real shocks and demand shocks can't both contribute to recessions. Indeed, there's no reason they can't both contribute simultaneously! They're only contradictory insofar as you allege that one operates exclusively.

Which is right is, of course, an empirical question.

Well given that the IGM topic is the “Laffer Curve,” I think we should assume that the question refers to cuts in marginal income tax rates for only top earners. Isn’t “old style Keynesian” essentially the belief that the only tax cuts that spur economic growth are the ones that stimulate consumption by “putting money in people’s pockets?” I don’t think these economists would disagree, though at this point it seems that such cuts would be ineffective as many people do not pay federal income taxes.

It’s also important to emphasize that supply-side economics isn’t only about tax cuts. It’s a policy mix. If government fails to cut spending and crowds out resources, or if the central bank fails to adopt a rules-based monetary policy, you may not have any beneficial supply-side effects.