It is always refreshing to read a book that turns an issue on its head. Laura María Agustín's trenchant and controversial critique of the anti-trafficking crusade goes a step further: it lays out the matter - in this case, "human trafficking" - on the operating table, dissects it, unravels its innards, and shows the reader, in gory, sometimes eye-watering detail, why everything we think about it is Wrong with a capital W. It's a jarring read; I imagine that those who make a living from campaigning against the scourge of human trafficking will throw it violently across the room, if not into an incinerator. Yet it may also be one of the most important books on migration published in recent years.

Most of us recognise the ideological under pinnings of old-style baiting of migrants. When newspaper hacks or populist politicians talk about evil Johnny Foreigners coming here and stealing our jobs or eating our swans, it does not take much effort to sniff out their xenophobic leanings. Agustín's contention is that the new "discourse" on migrants (in which many of them, especially the women and children, are seen as "victims of trafficking" in need of rescue) is also built on ideological foundations. Like its demented cousin - tabloid hysteria about foreign scroungers - the trafficking scare is based on a deeply patronising view of migrants, rather than any hard statistical evidence that human trafficking is rife.

Agustín begins by challenging the idea that there is a "new slave trade" in which hundreds of thousands of women and children are sold like chattels across borders. The US state department claims that between 600,000 and 800,000 people are trafficked for forced labour or sex worldwide every year; Unicef says a million children and young people are trafficked each year. Upmarket newspapers - which have embraced the seemingly PC "trafficking discourse" with the same fervour as the tabloid newspapers screech about fence-leaping job-stealers from Sangatte - tell us that "thousands" of women and children have been trafficked into Britain and "traded for tawdry sex", and that some of them (the African ones) "live under fear of voodoo".

Agustín says the numbers are "mostly fantasies". She does not doubt that there are instances of forced migration, or that, in a world where freedom of movement is restricted by stiff laws and stringent border controls, many aspiring migrants have little choice but to seek assistance from dodgy middlemen. Yet, having researched trafficking and sex workers' experiences for the past five years, both academically and through fieldwork in Latin America and Asia, she concludes that the figures are based on "sweeping generalisations" and frequently on "wild speculation". "Most of the writing and activism [on trafficking] does not seem to be based on empirical research, even when produced by academics," she notes. Many of the authors rely on "media reports" and "statistics published with little explanation of methodology or clarity about definitions".

Agustín points out that some anti-trafficking activists depend on numbers produced by the CIA (not normally considered a reliable or neutral font of information when it comes to inter national issues), even though the CIA refuses to "divulge its research methods". The reason why the "new slavery" statistics are so high is, in part, that the category of trafficking is promiscuously defined, sometimes disingenuously so. Some researchers automatically label migrant women who work as prostitutes "trafficked persons", basing their rationale on the notion that no woman could seriously want to work in the sex industry. The Coalition Against Trafficking in Women argues that "all children and the majority of women in the sex trade" should be considered "victims of trafficking". As Agustín says, such an approach "infantilises" migrant women, "eliminating any notion that women who sell sex can consent". Ironically, it objectifies them, treating them as unthinking things that are moved around the world against their will.

The reality is very different, the author says. Most migrant women, including those who end up in the sex industry, have made a clear decision to leave home and take their chances overseas. They are not "passive victims" who must be "saved" by anti-trafficking campaigners and returned to their country of origin. Rather, frequently, they are headstrong and ambitious women who migrate in order to escape "small-town prejudices, dead-end jobs, dangerous streets and suffocating families". Shocking as it might seem to the feminist social workers, caring police people and campaigning journalists who make up what Agustín refers to as the "rescue industry", she has discovered that some poor migrant women "like the idea of being found beautiful or exotic abroad, exciting desire in others". I told you it was controversial.

One of Agustín's chief concerns is that the anti-trafficking crusade is restricting international freedom of movement. What presents itself as a campaign to protect migrants from harm is actually making their efforts to flee home, to find work, to make the most of their lives in often difficult and unforgiving circumstances, that much harder. She writes about the "rescue raids" carried out by police and non-governmental organisations, in which even women who vociferously deny having been trafficked may be arrested, imprisoned in detention centres and sent back home - for the benefit of their own mental stability, of course. It used to be called repatriation; now, dolled up in therapeutic lingo, it is called "rescue".

For all its poisonous prejudices, the old racist view of migrants as portents of crime and social instability at least treated them as autonomous, sentient, albeit "morally depraved", adults. By contrast, as the author illustrates, the anti-trafficking lobby robs migrants of agency and their individual differences, and views them as a helpless, swaying mass of thousands who must be saved by the more savvy and intelligent women of the west and by western authorities.

Agustín reserves her most cutting comments for the flourishing "rescue industry", arguing convincingly that it is driven by a colonial-style, maternalistic attitude to foreign women. In its world, "victims become passive receptacles and mute sufferers who must be saved, and helpers become saviours - a colonialist operation". Bitingly, she compares today's anti-trafficking feminists with the "bourgeois women" of the 19th century who considered it a moral virtue to save poor prostitutes, who were "mistaken, misled, deviant". Like them, anti-trafficking crusaders see women as weak, easily victimised, and in need of guidance from a caring chaperone.

In truth, poor women - and men and children - migrate for many different reasons and have many different experiences, some good, some bad, some tragic. Such migrants are wise and wily, says Agustín; they have gumption, ambition and hope; they are often cosmopolitan, too, working, mixing and having flings with migrants from the other side of the world whom they meet in some big city in Europe or the United States. And many of them have far more liberal attitudes to freedom of movement than the westerners who campaign on their behalf. She quotes a Kurdish migrant to the Netherlands who thinks borders should be abolished: "I don't come from the sun or moon. I'm from earth just like everybody else and the earth belongs to all of us." Now that's an argument I can get behind.

Brendan O'Neill is the editor of "spiked" (www.spiked-online.com)