the best way to put egalitarianism to an empirical test is to realize as complete a social equality as possible and see what happens.

Indeed! And one can't even say "racial and sex discrimination is illegal in some countries, so we can assume that the remaining differences are biological", because stereotype threat is present in even the most liberal modern cultures.

I'm having a hard time imagining how a good social scientist a hundred years ago would go about measuring the effect of race and sex on, say, math skills, and not be surprised by the progress we've made nowadays.