Why did some societies have serfs or slaves and others not? You could talk about culture and national character and climate and changing mores and heroes and revolts and the history of agriculture and the Romans and the Christians and the Middle Ages and all the rest of it; or, like Krugman’s economics teacher Evsey Domar, you could argue that if peasants are barely surviving there’s no point in enslaving them, because they have nothing to give you, but if good new land becomes available it makes sense to enslave them, because you can skim off the difference between their output and what it takes to keep them alive.

-Larissa Macfarquhar, from her profile of Paul Krugman in the New Yorker