The United Nations’ International Labor Organization (ILO) estimates that there are over 12.3 million forced laborers worldwide.
Forced labor takes different forms, including debt bondage, trafficking and other forms of modern slavery. The victims are often women and girls forced into prostitution, migrants trapped by so-called debt, and sweatshop or farm workers forced to work by clearly illegal tactics and paid little or nothing.
A lot of forced labor is related to migration: workers seeking employment abroad often find that upon arrival their passports are taken away from them and they are forced to stay and work in order to pay off “debt”, meaning the money they own for accommodation and food. International sex trafficking is similar: this involves the use of force, fraud or coercion to transport unwilling victims into sexual exploitation. Many women start their journey voluntarily, and some even intend to work in the sex industry abroad, but upon arrival they are forced into practices that are coercive and inhuman. Like in the case of other forced laborers, traffickers steal the women’s passports and exploit their status of illegal immigrants to force them into unpaid prostitution.
It’s incredibly hard to have good numbers on any type of forced labor since you want to exclude willing professionals who are simply traveling in search of a better income. How do you know which prostitute is willing and which isn’t? It’s estimated that 10,000 sex slaves are brought into the U.S. annually, but that number could be wrong (source). Usually, these numbers are extrapolated on the basis of the numbers of known cases, i.e. cases that have ended up with the police or in court. As victims of sex trafficking are particularly well hidden (it’s a type of crime that’s vastly underreported since the victims are under total control by their bosses) you need to multiply the known cases by a certain number, but that number is by definition a very rough guess.
The UNODC estimates the number of victims of human trafficking at any one time at 2.5 million, 80% of those are women trafficked as sex slaves (the rest as slaves in homes or sweatshops).
Similar caveats are valid for estimates of the numbers of victim of other types of forced labor.
There’s an interesting database here.

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