A new report on Chicago prostitution comes out tomorrow. The report says most men who purchase sex see the behavior as an addiction.

The Chicago Alliance Against Sexual Exploitation put ads in Craigslist offering $40 to any local john who sat down for a two-hour interview. The alliance says it interviewed 113 respondents.

The average age of when they said they first paid for sex was 21. Three-quarters said seeing a prostitute hurts a marriage, even if the wife doesn’t know about it. And 83 percent said buying sex is an addiction.

The report on johns closely follows a study of Chicago prostitutes. In January, University of Chicago economist Steven D. Levitt and Columbia University sociologist Sudhir Venkatesh reported that the prostitutes in their study averaged about $340 week and got beaten about once a month.