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Trump says human trafficking ‘is worse than it’s ever been in the history of the world.’ Where to begin?

Trump promises to solve 'human trafficking epidemic' in White House meeting (Video: The Washington Post)

This post has been updated.

President Trump is not a historian. In fact, as The Washington Post's Jenna Johnson documented so well Wednesday, he often seems to be learning basic historical facts before our very eyes while boldly claiming nobody previously knew those basic facts. Other times, he makes huge historical claims without any real basis.

But even by Trump's standards, he let loose a doozy Thursday.

Appearing at an event in Florida, Trump declared that “human trafficking is worse than it’s ever been in the history of the world.” Trump has said before that human trafficking “may be” worse than ever before; now, it seems, he's become convinced of it.

But how?

Estimates peg the number of slaves in the world at between 20 million and 40 million, with one going as high as 45.8 million. That's between about one-quarter and one-half of 1 percent of every human on Earth.

It seems logical to assume that these numbers were significantly higher when slavery was prevalent. While there aren't great data on worldwide slavery over time, there are a few data points from which we can glean some conclusions.

In 1974, Robert Fogel and Stanley Engerman wrote a groundbreaking book on the economics of slavery, titled “Time on the Cross,” that in part sought to characterize the scope of the slavery over time. While there aren't any graphs or firm numbers, they do note that some estimates indicate that during the early Roman Empire, as many as 3 out of every 4 people on the Italian peninsula were enslaved — a total of 21 million people.

A more recent estimate of Roman slavery by Stanford University's Walter Scheidel in 2007 hypothesized that slaves accounted for “between 7 and 13% of the imperial population,” with between 5 million and 8.5 million people enslaved at any given point in Italy, Egypt and elsewhere. That would be at least 12 times more pervasive than today.

And slavery thrived for centuries. As recently as 1762, Adam Smith in a famous quote noted that slavery was still pervasive across the world. “We are apt to imagine that slavery is entirely abolished at this time, without considering that this is the case in only a small part of Europe; not remembering that all over Moscovy and all the eastern parts of Europe, and the whole of Asia, that is, from Bohemia to the Indian Ocean, all over Africa, and the greatest part of America, it is still in use.”

And shortly before slavery was abolished in the United States, the 1860 census found 3.9 million slaves in a U.S. population of 31 million people — meaning 1 out of 8 people were slaves, or 12.5 percent.

Some argued that perhaps Trump was referring to the raw number of slaves. The population during the early Roman Empire, after all, was only about 200 million, so even if 10 percent of all people worldwide were enslaved, that would have been 20 million -- and probably fewer total people than today.

But even if you go by that metric -- and believe that pervasive slavery in a world with a smaller population is better than much more limited slavery in a much-larger population -- Trump's claim is suspect. The worldwide population was about 1 billion around the turn of the 19th Century, shortly after Smith noted that slavery was still prominent across the developed world. That would required only 3 to 5 percent of the world to have been enslaved at that time for there to have been more slaves than today.

One estimate highlighted by the New York Times in 2013 put the modern number of slaves at 27 million, versus 25 million in 1860. So even at that arbitrary juncture -- and with a much smaller worldwide population -- the raw numbers were basically the same.

Another way you could possibly make an argument for Trump's claim is if you don't consider slavery to be human trafficking. Perhaps Trump was thinking about sex trafficking or something narrower? But slavery is in fact the very definition of human trafficking.

It seems this is one of those facts that nobody else knew — because it's not true.