The business model of forced prostitution is remarkably similar from Pakistan to Vietnam — and, sometimes, in the United States as well. Pimps use violence, humiliation and narcotics to shatter girls’ self-esteem and terrorize them into unquestioning, instantaneous obedience.

One girl working with Pross was beaten to death after she tried to escape. The brothels figure that occasional losses to torture are more than made up by the increased productivity of the remaining inventory.

After my last column, I heard from skeptical readers doubting that conditions are truly so abusive. It’s true that prostitutes work voluntarily in many brothels in Cambodia and elsewhere. But there are also many brothels where teenage girls are slave laborers.

Young girls and foreigners without legal papers are particularly vulnerable. In Thailand’s brothels, for example, Thai girls usually work voluntarily, while Burmese and Cambodian girls are regularly imprisoned. The career trajectory is often for a girl in her early teens to be trafficked into prostitution by force, but eventually to resign herself and stay in the brothel even when she is given the freedom to leave. In my blog, www.nytimes.com/ontheground, I respond to the skeptics and offer some ideas for readers who want to help.

Pross herself was never paid, and she had no right to insist on condoms (she has not yet been tested for HIV, because the results might be too much for her fragile emotional state). Twice she became pregnant and was subjected to crude abortions.

Image Nicholas D. Kristof Credit... Fred R. Conrad/The New York Times

The second abortion left Pross in great pain, and she pleaded with her owner for time to recuperate. “I was begging, hanging on to her feet, and asking for rest,” Pross remembered. “She got mad.”