On Aug. 11, Amnesty International decided to support full decriminalization of the sex trade, which includes all aspects of selling and buying sex, for “the protection of the human rights of sex workers.” In a statement Salil Shetty, Amnesty’s secretary general, said, “Sex workers are one of the most marginalized groups in the world.”

I do not deny that people who choose to enter the sex industry are marginalized and should never be arrested for selling sex. Nor do I reject the notion that their human rights are regularly violated and that we need to do much more to improve their safety and health conditions. But I find it odd that Amnesty, which has a history of standing up for the most vulnerable, does not pay more attention to another marginalized group of individuals: the victims of sex trafficking.

Specifically, I and other critics of Amnesty’s proposal reject decriminalizing the purchase of sexual services because it risks increasing the prevalence of human trafficking. Amnesty and decriminalization activists have responded to our concerns primarily by making two dubious claims: that trafficking is not that common and that making it easier to buy sex does not make it easier to traffic human beings.

To make the first claim, they often refer to a report commissioned by New Zealand’s government, which concluded that only a “small number of sex workers reported being made to work by someone else.” But this is a poor interpretation of the data. The report was based on an original study, “The Impact of the Prostitution Reform Act on the Health and Safety Practices of Sex Workers.” Any study that focuses on sex workers is likely to underestimate the prevalence of trafficking in the way that counting the number of households is likely to underestimate the prevalence of homelessness.

It has also been suggested that the link between buying sex and trafficking is weak. “There is no evidence to suggest that somehow trafficking goes up when sex trade is decriminalized,” Gauri van Gulick, Amnesty’s deputy Europe director, said in a video statement. But this is patently false. In fact, several studies have found that countries where sex purchases are decriminalized tend to have higher rates of sex trafficking. Decriminalization advocates seem to think that making it easier to buy sex would not make it easier to exploit victims, as long as abuse and trafficking are still illegal. But this belief suffers from the same faulty logic as the argument that making it substantially easier to buy guns would not make it easier to shoot others, as long as homicide and aggravated assault are still illegal.