Greg Mankiw pooh-poohs the data using one of his favorite tactics:
Princeton economist Alan Blinder says Democratic Presidents are better for economic equality between rich and poor.
Chicago economist Casey Mulligan says Republican Presidents are better for economic equality between men and women.
My take: These articles are completely persuasive, as long as you buy into the axiom that correlation=causation. Otherwise, not so much.
Post hoc ergo propter hoc is the eternal comeback in macroeconomics, because we can't replay the world a different way and see what "would have" happened.
It's a very effective counter when you're looking at brief periods (let's call it micromacro) with no comparative or control element.
1. That comeback is much less convincing when comparing outcomes from two sets of policies
A. Over a long period
B. When those policies are consistently and systematically different
2. Data—especially long-term data—can quite effectively disprove a theory. (Mankiw stipulates to this in "Growth of Nations.")
i.e.: Growth in Europe and the US have been the same over decades, while tax rates have been massively different (40% versus 28% of GDP).
i.e. 2: Since WWII, growth under Democratic presidents has been far greater.
No post-hoc argument here: These facts quite effectively disprove the theory that Republican/supply-side/trickle-down policies cause greater/faster economic growth.
On the two "equality" studies that Mankiw pooh-poohs.
Dems deliver less rich/poor inequality: Blinder's point is not that Dems deliver more equality. It's that everyone is better off under Democratic policies. (Bartels demonstrates this in spades.) The equality is a wonderful (causally connected?) bonus.
Pubs deliver less male/female inequality: women appear to have done relatively better under 'pubs because working-class men have done so poorly under same.
Update: Casey Mulligan—author of the Republicans-promote-gender-equality article—responds to Mankiw in his new blog.