I'm not asking anyone to put forward any arguments for slavery, I think the moral arguments against it are obvious enough.

Nonetheless my question more or less has 2 parts:

A) What was greatest % of the human population that was enslaved at any one point in history? and B) Did societies that had more slaves have an economic advantage over those that had less? If there was a strong advantage, then surely every nation would seek to acquire as many as possible in an endless quest to gain an competitive edge over their neighbors.

Thank you for your help :)