I have often claimed in my critique of late capitalism that there are more slaves in the world today than at any other time in history. Is this claim true? What measure am I using to test the claim? Is it measured in absolute numbers or on a per capita basis? The claim is almost certainly true in total numbers. But it also appears to be likely true on a per capita basis, as well.

First, let’s define slavery. Benjamin Skinner, author of A Crime So Monstrous: Face to Face with Modern-Day Slavery, defines slaves as “those forced to work, held through fraud, under threat of violence, for no pay beyond subsistence.” This includes those held in debt bondage, as well those held as chattel slaves.
Now, let’s get a comparison point. In 1850, there were 3,204,313 slaves in the United States (this speaks to the importance of ensuring an accurate census). To be sure, there were slaves elsewhere in the world; however, the United States had the greatest number as defined above. The world’s population in 1850 was 1.171 billion persons. Taking the US slave number, we can estimate a rate of 270 slaves per 100,000 persons. I stress tat this is an undercount.
According to Kevin Bales, in Disposable People: New Slavery in the Global Economy, the number of slaves in the world today is estimated to be 27 million human beings. In 2010, the world’s population was estimated at 6.8 billion. Using these figures, we can calculate a rate of 390 slaves per 100,000 persons. Using these figures, the number of slaves in the world today is not only greater in total numbers than ever before, but also greater on a per capita rate basis that in times previous.
It is unfortunate that we don’t have a solid estimate for the world slave population in 1850. If we take a guess and estimate 5 million slaves in the world in 1850, we can calculate a rate of 426 slaves per 100,000 persons, which is greater than the present day rate. On the other hand, there are many slaves not counted in the 2010 figure.
There are at least 8 million prisoners in the world, most of who are in prison for nonviolent offenses, and many of them are forced to work. This form of slavery is called “penal slavery” and, in many places, including the United States, it is legal. If we include half of the world’s prisoners in our calculations, we produce a number of 456 slaves per 100,000 persons. Including just one-quarter of them in our calculations will result in a rate of 441 slaves per 100,000 persons.