Take HBO's hit documentary series, Cathouse, which features the most famous of the Nevadan brothels, the Moonlight Bunny Ranch. Tune in and you'd be forgiven for thinking that all prostitutes in Nevada are on to a good thing. The women speak coyly about loving their work, their customers, their bosses. "The series sheds light not only on the numerous joys and challenges of working at a legal brothel," says the HBO website, "but on the therapeutic benefits that customers take with them after a stint at the Ranch."

Given such great PR, a new book - Prostitution and Trafficking in Nevada: Making the Connections - makes interesting reading. During a two-year investigation, the author, Melissa Farley, visited eight legal brothels in Nevada, interviewing 45 women and a number of brothel owners. Far from enjoying better conditions than those who work illegally, the prostitutes she spoke to are often subject to slave-like conditions.

Described as "pussy penitentiaries" by one interviewee, the brothels tend to be in the middle of nowhere, out of sight of ordinary Nevadans. (Brothels are officially allowed only in counties with populations of fewer than 400,000, so prostitution remains an illegal - though vast - trade in conurbations such as Las Vegas.) The brothel prostitutes often live in prison-like conditions, locked in or forbidden to leave.

"The physical appearance of these buildings is shocking," says Farley. "They look like wide trailers with barbed wire around them - little jails." The rooms all have panic buttons, but many women told her that they had experienced violent and sexual abuse from the customers and pimps.

"I saw a grated iron door in one brothel," says Farley. "The women's food was shoved through the door's steel bars between the kitchen and the brothel area. One pimp starved a woman he considered too fat. She made a friend outside the brothel who would throw food over the fence for her." Another pimp told Farley matter-of-factly that many of the women working for him had histories of sexual abuse and mental ill-health. "Most," he said, "have been sexually abused as kids. Some are bipolar, some are schizophrenic."

Then there is the fact that legal prostitutes seem to lose the rights ordinary citizens enjoy. From 1987, prostitutes in Nevada have been legally required to be tested once a week for sexually transmitted diseases and monthly for HIV. Customers are not required to be tested. The women must present their medical clearance to the police station and be finger-printed, even though such registration is detrimental: if a woman is known to work as a prostitute, she may be refused health insurance, face discrimination in housing or future employment, or endure accusations of unfit motherhood. In addition, there are countries that will not permit registered prostitutes to settle, so their movement is severely restricted.

Those who support the system claim that the regulations may help prevent pimping, which they see as a worse form of exploitation to that which occurs in brothels. According to Farley's research though, most women in legal brothels have pimps outside anyway, be they husbands or boyfriends. And, as Chong Kim, a survivor of prostitution who has worked with Farley, says, some of the legal brothel owners "are worse than any pimp. They abuse and imprison women and are fully protected by the state."

The women are expected to live in the brothels and to work 12- to 14-hour shifts. Mary, a prostitute in a legal brothel for three years, outlines the restrictions. "You are not allowed to have your own car," she notes. "It's like [the pimp's] own little police state." When a customer arrives, a bell rings, and the women immediately have to present themselves in a line-up, so he can choose who to buy.

Sheriffs in some counties of Nevada also enforce practices that are illegal. In one city, for example, prostitutes are not allowed to leave the brothel after 5pm, are not permitted in bars, and, if entering a restaurant, must use a back door and be accompanied by a man.

So how did Farley gain access to her interviewees? Those in control of the women were confident that they would not be honest about the conditions, she says. "Pimps love to brag, and I know how to listen," she adds. Although left alone with the women during interviews, Farley noted that they were all very nervous, constantly looking out for the brothel owners.

Investigating the sex industry - even the legal part - can be dangerous. During one visit to a brothel, Farley asked the owner what the women thought of their work. "I was polite," she writes in her book, "as he condescendingly explained what a satisfying and lucrative business prostitution was for his 'ladies'. I tried to keep my facial muscles expressionless, but I didn't succeed. He whipped a revolver out of his waistband, aimed it at my head and said: 'You don't know nothing about Nevada prostitution, lady. You don't even know whether I will kill you in the next five minutes.'"

Farley found that the brothel owners typically pocket half of the women's earnings. Additionally, the women must pay tips and other fees to the staff of the brothel, as well as finders' fees to the cab drivers who bring the customers. They are also expected to pay for their own condoms, wet wipes, and use of sheets and towels. It is rare, the women told Farley, to refuse a customer. One former Nevada brothel worker wrote on a website: "After your airline tickets, clothing, full-price drinks and other miscellaneous fees you leave with little. To top it off, you are ... fined for just about everything. Fall asleep on your 14-hour shift and get $100 [£50] fine, late for a line-up, $100-500 in fines." (The women generally negotiate directly with the men over the money; what they get depends on the quality of the brothel. It can be anything from $50 for oral sex to $1,000 for the night, but that doesn't take account of the brothel's cut.)

Farley found a "shocking" lack of services for women in Nevada wishing to leave prostitution. "When prostitution is considered a legal job instead of a human rights violation," says Farley, "why should the state offer services for escape?" More than 80% of those interviewed told Farley they wanted to leave prostitution.

The effect of all this on the women in the brothels is "negative and profound," according to Farley. "Many were suffering what I'd describe as the traumatic effects of ongoing sexual assaults, and those that had been in the brothels for some time were institutionalised. That is, they were passive, timid, compliant, and deeply resigned."

"No one really enjoys getting sold," says Angie, who Farley interviewed. "It's like you sign a contract to be raped."

Meanwhile, illegal brothels are on the increase in Nevada, as they are in other parts of the world where brothels are legalised. Nevada's illegal prostitution industry is already nine times greater than the state's legal brothels. "Legalising this industry does not result in the closing down of illegal sex establishments," says Farley, "it merely gives them further permission to exist."

Farley found evidence, for example, that the existence of state-sanctioned brothels can have a direct effect on attitudes to women and sexual violence. Her survey of 131 young men at the University of Nevada found the majority viewed prostitution as normal, assumed that it was not possible to rape a prostitute, and were more likely than young men in other states to use women in both legal and illegal prostitution.

The solution, Farley believes, is to educate people about the realities of legalised abuse of women. "Once the people of Nevada learn of [prostitutes'] suffering and emotional distress, and their lack of human rights, they, like me, will be persuaded that legal prostitution is an institution that just can't be fixed up or made a little better. It has to be abolished." The prevailing attitude in Nevada remains as it was a few centuries back though - that men have sexual "needs" that they have a right to fulfil. Outside one of the legal brothels a sign reads: "He who hesitates, masturbates."

· Some names have been changed.