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 United Nations data has been used in the study, some countries and sub-national regions are missing.

“It's difficult or even impossible to do research in areas of high conflict,” said Fiona David, Walk Free Foundation's executive director of global research, pointing to areas such as Syria or northern Nigeria that had to be excluded from the study. Because of this, David said, the estimate of 40.3 million is probably conservative.

Walk Free hopes that disconcerting detail could prompt global action. “This is truly historic,” said Andrew Forrest, an Australian mining magnate and founder of the anti-slavery nonprofit organization. “We know we're dealing with a major problem.”

The Global Estimates of Modern Slavery study found that slavery occurs in every region — and probably every country — of the world. In terms of sheer numbers, the majority of modern slaves probably live in Asia and the Pacific region. Meanwhile, slavery as a practice is thought to be most prevalent in Africa. The report cautions, however, that with more accurate data, these rankings could well change.

Notably, the study does not break down its figures country by country, like the separate Global Slavery Index published annually since 2013 by Walk Free. Some experts, such as prominent human trafficking scholar Anne Gallagher, have criticized the methodology of Walk Free's indexes, while noting that the aim of providing an accurate estimate of modern slavery is a noble one.

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