Indonesian commercial female sex workers cover their face behind the glass inside a brothel last year in Surabaya, Indonesia. (Ulet Ifansasti/Getty Images)

AMNESTY INTERNATIONAL adopted a resolution last week urging the worldwide decriminalization of prostitution, and D.C. Council member David Grosso (I-At Large) was among many to applaud the vote: He announced that he is considering introducing matching legislation this fall. Amnesty International and Mr. Grosso are well intentioned but wrong: The policy would do more to hurt victims of sex trafficking than it would to help them.

Amnesty International’s vote authorizes its international board to adopt an official policy asking countries to decriminalize what it refers to as “sex work” — the exchange of sex for money. The human rights organization makes its recommendation on the grounds that it would allow women to report abuse, gain access to health care and leave the business if they want without fear of legal consequences. Those are noble ends. Yet these means won’t achieve them.

Supporters of the resolution assume that sex work can be a profession like any other and that sex transactions can be consensual. This is probably true for some prostitutes. It is not true for the vast majority, who resort to selling their bodies because they feel they have no other option. Decriminalizing prostitution entirely might give some of these women a way out. More often, it would allow pimps to operate with impunity, using the money and status that comes with their newfound legitimacy to scale up trafficking operations that hurt the most vulnerable — the young, the very poor and especially the undocumented. The evidence seems to bear that out in Germany and the Netherlands, where trafficking has increased dramatically since the decriminalization of the sex industry in the early 2000s.

Though no policy on prostitution is perfect, some have yielded better results. The so-called “Nordic model” — a set of laws first passed in Sweden — decriminalizes the sale of sex but keeps the purchase illegal. It has its flaws, too. Clients increasingly afraid of punishment may arrange for transactions in underground and unsafe locations, and police might manipulate prostitutes seeking protection into helping track down their pimps, instead of looking first to their safety. But overall, the Nordic model appears to lead to fewer women being in danger. In Sweden, for example, street prostitution has gone down by half. Men report soliciting sex less, and some traffickers who find the country inhospitable to business have moved away.

Prostitution and human trafficking will never be stamped out, and no legal approach to reducing the harm they cause will be perfect. But wholesale decriminalization is surely wrong. The way to solve a problem is not to protect the very people who cause it — not in the District and not anywhere else in the world.