Updated

Roughly 5 percent of men over the age of 18 in U.S. cities are potentially soliciting online sex ads, according to a study.

The Arizona State University study, “Invisible Offenders: A Study of Online Sex Customers,” used decoy online sex ads in 15 U.S. cities, including Houston, New York, Chicago and others, and found that on average 1 out of every 20 males is potentially soliciting posts on Backpage.com.

The ads received 677 contacts, calls and texts and the researchers collected distinct phone numbers of 451 online sex ad customers. Population estimates were created for all 15 cities with Phoenix having an estimated 78,412 online sex ad buyers with a range from Houston with 169,920 to San Francisco with 9,594, according to the study.

The findings ranged from roughly one out of every five males in Houston, or 21.4 percent, followed by Kansas City (14.5 percent) and Las Vegas (13.5 percent). On the other end of the spectrum, the study tallied less than one of 166 males in San Francisco, or 0.6 percent, the study found.

"We currently know very little about the scope of the demand for sexual services online,” said Lauren Martin, director of research at the Urban Research Outreach Engagement Center of the University of Minnesota. “This study represents a much needed effort in using quantitative and statistical methods for estimating this population.”

The cities, all of whose police departments were made aware of the study included: Atlantic City, N.J., Boston, Baltimore, Chicago, Houston, Kansas City, Las Vegas, Miami, Minneapolis, New York City, Phoenix, Portland, Ore., San Diego, San Francisco and Salt Lake City.