Yumi played her role robotically. On one occasion, Yumi was arrested for prostitution, and she says the police asked her if she had been trafficked.

“I said no,” she recalled. “I was really afraid that if I hinted that I was a victim, the gang would send the video to my family.”

Then one day Yumi’s closest friend in the brothel was handcuffed by a customer, abused and strangled almost to death. Yumi rescued her and took her to the hospital. She said that in her rage, she then confronted the pimps and threatened to go public.

At that point, the gang hurriedly moved offices and changed phone numbers. The pimps never mailed the video or claimed the homes in China; those may have been bluffs all along. As for Yumi and her friend, they found help with Restore NYC, a nonprofit that helps human trafficking victims in the city.

I can’t be sure of elements of Yumi’s story, but it mostly rings true to me and to the social workers who have worked with her. There’s no doubt that while some women come to the United States voluntarily to seek their fortunes in the sex trade, many others are coerced  and still others start out forced but eventually continue voluntarily. And it’s not just foreign women. The worst cases of forced prostitution, especially of children, often involve home-grown teenage runaways.

No one has a clear idea of the scale of the problem, and estimates vary hugely. Some think the problem is getting worse; others believe that Internet services reduce the role of pimps and lead to commercial sex that is more consensual. What is clear is that forced prostitution should be a national scandal. Just this month, authorities indicted 29 people, mostly people of Somali origin from the Minneapolis area, on charges of running a human trafficking ring that allegedly sold many girls into prostitution  one at the age of 12.