It should be pointed out that the decision to base legislative representation on population, but to count slaves at a discount, was an attempt to limit the legislative power of the southern states where slave holding was prevalent, not a comment on the essential humanity of slaves themselves.  The latter was clearly dealt with when slavery itself was allowed to continue.  

Sixty percent was the result of a compromise between northern interests who held that, if slaves were property, they should not be counted at all (except  for purposes of assessing the wealth, and thence the taxes owed, of the states) and the southern states that held that they should be fully counted for representation purposes (even though they --  like women-- could not vote), but not at all for taxation purposes.  

Regardless of his other points, Caplan seems to me to be correct when he identifies black/white race relations as the defining thread in U.S. domestic history.