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Vanessa Bouche is a professor at Texas Christian University.
She and her colleague step up cameras outside 32 known massage parlors.
Bouche says an online forum helped them hone in on those locations out of hundreds in the city.
"It was a geographically representative sample and it was also representative in terms of the number of reviews that it received on the website," she says.
Bouche says they also found a trend in where these hidden commercial sex services are happening.
"They are significantly less likely to be in neighborhoods that are largely African American, so like in east Houston there's not nearly as many as there are in west Houston," she says.
This is because customers for these services tend to have more disposable income.
As for the workers performing these services, Bouche draws a connection to sex trafficking.
"The women that are there came to the United States," she says. "The trafficker paid for their visa and their documentation and now they are quote-unquote working off that immigration debt."
Bouche says they estimate massage parlors have 3,000 male customers a day across Houston.