THE self portraits, some on pink paper decorated with colourful glitter and pinned to the bare walls of a meeting room of Cook County jail, America’s largest, resemble the drawings of a ten-year-old. A few have oversize teardrops rolling down their cheeks. “I lost my childhood,” says Patrice, an African-American inmate, who is being held on charges of identity theft. Patrice was raped at the age 12, started to work in a strip club on the West side of the city when she was 14 and was lured into prostitution in California by the man she then thought of as her boyfriend at the age of 16.

Patrice is one of about 150 women in Division 17, a part of the jail for mostly young women, many of them mothers or pregnant, held on relatively minor charges. Most of them were prostitutes, many have mental-health problems and nearly all are recovering addicts. Katie, a woman in her 20s of Puerto Rican descent, dropped out of school when she was 14, got pregnant by a gang member, had an abortion and started to drink heavily. A few years later she was hooked on crack and heroin and worked as a prostitute for “Josh and John”, who put her up in a hotel room, took her earnings (up to $1,700 a day) and supplied her with drugs in exchange.

Katie gave birth to three children, two of whom were taken away by social workers because of her addictions. Her oldest, a nine-year-old boy, is living with his father. And then there is Hannah, who comes from a prosperous, white family on Chicago’s North side. Hannah has been on drugs since she was 15, though she still managed to graduate with honours from New Trier, one of the county’s most prestigious high schools, and subsequently got a degree from Tulane University. To feed her own heroin habit and that of her addicted boyfriend, she sold herself for sex, sometimes even taking clients into her parents’ home.

Patrice, Katie and Hannah have all been advertised on the adult section of Backpage.com, a classified advertising website, which includes subdivisions that range from “escort”, “domination & fetish” to “body rubs”. Prostitution is illegal in the United States, in all but a few counties in Nevada, but Backpage, which is run by two former journalists, Jim Larkin and Michael Lacey, is by most estimates the biggest online marketplace for buying and selling sex in America. It has been under attack for several years from activists and politicians, who accuse the site of facilitating the pimping out of under-age girls. In 2011, 46 attorneys-general sent the firm a letter demanding that it do more to fight sex trafficking on its website.

Backpage has defended itself with reference to the Communications Decency Act, a federal law passed in 1996 that says that internet-service providers are merely hosts and not publishers, which means they cannot be held liable for whatever is posted on them by a third party. The site’s supporters make a less legalistic defence: women who advertise themselves on the website do not have to walk the street, with all the dangers that brings. Sites like Backpage may also make it easier for law-enforcement officers to identify those who are under-age, or have been coerced, and to track down those responsible.

Thomas Dart, the sheriff of Cook County, does not see it that way. He says that he has tried for years to work with Backpage to help the company clean up its adult section, which is where around 70% of all sex adverts are posted. (The site became the market leader after Craigslist.com, another classified ad site, was pressed into shutting down its adult division in 2010.) In just one recent month in spring Backpage.com posted more than 1.4m ads in its “adult escort” section, of which perhaps less than 1% are for something other than sex.

After his appeals for co-operation were ignored, the sheriff went on the offensive. On June 29th he sent a letter to MasterCard and Visa asking the two credit-card companies to stop immediately the use of their cards to place ads on the adult section of Backpage. To the sheriff’s surprise, both companies reacted almost instantly and 48 hours after Mr Dart’s letter was made public MasterCard and Visa had stopped processing ad purchases on Backpage. (American Express, another big credit-card company, stopped doing business with Backpage earlier this year.)

The firm is not responding in public to its ostracism by the credit-card behemoths, but on July 7th it gave them the finger by making all advertising on its adult section free. Yet this is almost certainly only a temporary solution while Backpage works out how it can be paid easily in cash or with Bitcoin, a digital currency, because the sex ads are so lucrative. According to conservative estimates, Backpage rakes in $9m-10m every month from its adult section. Mr Dart says he is not so naive as to imagine that he can end prostitution. “This is not a moral crusade,” he says. But he wants to stop sex trafficking, especially when it involves children.

His next move will be to target men who buy sex from prostitutes. In 2011 he launched an annual “National Day of Johns Arrests”, a two-week operation across America targeting the clients of prostitutes and pimps. The sheriff would like to shift attention further to the demand side of prostitution. Cook County now arrests more buyers of sex than sellers. In America as a whole, by contrast, more than 90% of those arrested for taking part in the business are prostitutes.