More Slaves Now Than in the 18th Century?

The American Interest says

The oldest human rights movement is the anti-slavery movement dating from the 18th century. There are more slaves now than there were when the movement was founded.

In this day and age, we should be able to get basic facts like this in five minute or less, right? So I tried to verify the claim in five minutes or less.

Here's a screencast of my attempt:


My rating scale, from best to worst, is VERIFIED TRUE, UNVERIFIED, VERIFIED UNSUPPORTED, VERIFIED FALSE.

I'd chalk this one up as VERIFIED UNSUPPORTED. I found the book that is the likely source, read that part of the book on Amazon, and it looks like the author has not provided any particular back-up for his claim.

If I am wrong, I'd love to hear about it! Please let me know.