The one I'd like to see "tested" isn't crime. I always assumed that both the improved economy in the 80's and 90's, and that the tail-end of the baby boomers aging out of "crime prime years" of males aged 15-30 or whatever, is what dropped our crime rate so precipitously from it's peak. I never bought the Freakanomics argument on Abortion and Crime, when there were other demographic changes that were so much larger that easily swallow it's impact.
Also, I'll note after reading the article, that the author admits the other economists and statisticians merely found one flaw in the controls Donohue and Levitt applied. They did not prove anything in the opposite direction.
I haven't seen anybody yet debunk the other big assertion made in Freakanomics, namely that abortion depopulates the Democrat voting base. Of course, that just begs another "the ends don't justify the means" argument I guess. Even if completely banning abortion now, could it be done, my gut instinct is that it would likely mean a Leftist majority would just re-legalize it in 18-20 years again, and we'd lose the Constitution, Capitalism, and RKBA along the way (or at least faster than we would otherwise). But of course abortion is one messy debate, with most all sides arguing at cross purposes and not really addressing each other's ideological basis or concerns.
As an aside, between Roe V. Wade, and our Welfare State keeping our "native underclass" out of low-level and entry-level service sector, manufacturing, agricultural, and construction jobs, that's why I am guessing we have the unholy alliance between the Democrats who want to poll pack, and the "Chamber of Commerce" pressures on the GOP to open the floodgates to illegal imigration.
