Slavery is not a thing of the past. A report released Tuesday by the U.N.-affiliated International Labor Office (ILO) and the Walk Free Foundation estimates that there were 40.3 million people in some form of modern slavery around the world on any given day last year. But by its very nature, the accuracy of that figure is hard to gauge. Slavery tends to be a hidden, illegal practice — one in which the victim's ability to speak out is limited. The authors of the Global Estimates of Modern Slavery study admit there are gaps in the available information: Although extensive...