The survey created "scientifically rigorous estimates" for prostitution in 20.
With the exception of Seattle and Atlanta, all the underground sex economies shrunk in that four-year period.
Researchers gleaned information from interviews with 73 convicted pimps and hundreds of law enforcement officials.
In Dallas, researchers interviewed 15 pimps, seven child pornographers and six sex workers.
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There are no accounting records to trace, no receipts to scrutinize, and no legal records to analyze.
Simply, it is difficult to grasp the size of this economy.
For the first time, researchers have attempted to estimate the size of underground sex economies in eight major cities.
According to the Urban Institute, Dallas' sex industry is worth about million.
Prostitution is said to be the world’s oldest profession, but understanding the size and scope of this economy, and the methods and actors involved in this trade, is still a murky endeavor.