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An estimated 31,282 Portland men -- 3.7 percent of the adult male population -- solicit prostitutes from online sex advertisements, according to a novel study of 15 U.S. cities by

.

The Rose City fell in the middle of the pack in per capita solicitations, lagging far behind Houston (21.4 percent), Kansas City (14.5 percent) and Las Vegas (13.5 percent), according to the study.

Researchers posted decoy ads, featuring a close-up photo of a woman's derrière in a thong, on the "escorts" section of www.backpage.com sites in all 15 cities.

They kept the ads open during two 24-hour stretches beginning at 2 p.m. on back-to-back Fridays in June, and documented the number of calls and texts that poured in, according to the authors of "Invisible Offenders: A Study Estimating Online Sex Customers."

The research team also posted the ads on Craigslist, in hopes of collecting similar data. But Craigslist quickly blocked the ads, forcing researchers to abandon the numbers for the purposes of their study.

Their Craigslist ad in Las Vegas stayed open just 27 minutes but generated 14 calls, said

, director of Arizona State's Office of Sex Trafficking Intervention Research.

"We were really surprised by the rapid fire," she said.

Portland's callers proved to be persistent, Roe-Sepowitz said.

She noted that during the running of both Friday ads on www.backpage.com, 49 men placed 79 calls or texts to the Portland number -- meaning that many of them phoned more than once. But they didn't quit there. Some men apparently jotted down the number and, after the ad went down, kept calling.

"What we found in Portland was the calls came over a long period," Roe-Sepowitz said.

"They came after the week was done."

Researchers also documented the number of sex ads placed on www.backpage.com during those Fridays, which Roe-Sepowitz acknowledged were often paydays for the men lurking online for prostitutes. She pointed out that the average number of sex ads posted on the "escorts" site in Portland was145.5, ranking the city 10th out of the 15 in the study.

U.S. Attorney

, Oregon's top federal prosecutor, applauded the study.

"This is an issue of supply and demand," she said. "This gives us data in terms of buyers. It's not the whole universe of buyers -- it's buyers on the Internet (and) doesn't count those trolling the streets."

Marshall has committed a team of prosecutors to combat sex trafficking of children in Oregon, which ranked fifth of all U.S. judicial districts in the number of prosecutions for the crime, according to the

at Syracuse University.

A

showed that at least 469 children in the metropolitan area were exploited as commercial sex workers from 2009 to 2013, nearly half of them connected to gangs that put them on the street.

"Now we can see the other side of the coin," Marshall said, "the significant number of men in our community buying sex online."

-- Bryan Denson