The further we get away from the age of slavery, the more angry people seem to be getting about it.I wonder how many lives the "institution of slavery" (a phrase that is probably more obfuscating than it is clarifying, since the variables have, well, varied a lot across time and place) has saved throughout anatomically modern human history.
Well, some people. You know … black people. Slavery was a nearly universal feature of human society until the early-modern period, and was no respecter of race or nationality. A few years ago I reviewed Robert Davis's fine book Christian Slaves, Muslim Masters. Davis is a professor of history at Ohio State. In his book he tells the story of Muslim slave-raiding across the Mediterranean.
Lots of slaves became as much as a result of capture by an opposing military force engaged in campaigns in which human booty was not the official (or primary) objective. It's often safer and almost always easier for a fighter to kill his potential captive than it is to subdue him, keep him alive, and get him to a market or trade caravan--especially civilians inside a town that has been successfully sieged--but because that captive tended to be worth more alive than dead, there was a strong monetary incentive for the fighter not to kill or maim him. It's not as though slavery came without risks--volcanic ash wasn't the only thing to explode out of Mt. Vesuvius; Nat Turner's rebellion brought slave owners' worst nightmares to life; under the Yuan dynasty, Chinese slaves apparently often targeted their direct co-ethnic owners rather than their Mongol overlords during uprisings and rebellions.
Net-net, are there more people alive today than there would otherwise be had no person ever been held in involuntary bondage to another? Presumably it's an impossible question to answer and of course it's an evil one that only an evil person would even think to ask.