In the United States, prostitution is thought to be worth at least $14 billion a year. Most of that money doesn’t go to girls like my teenage self. Worldwide, human trafficking is the second largest enterprise of organized crime, behind drug cartels but on a par with gunrunning.

In countries that have decriminalized the sex trade, legal has attracted illegal. With popular support, the authorities in Amsterdam have closed down much of the city’s famous red light district — because it had become a magnet for criminal activity.

In Germany, where prostitution was legalized in 2002, the industry has exploded. It is estimated that one million men pay to use 450,000 girls and women every day. Sex tourists are pouring in, supporting “mega-brothels” up to 12 stories high.

In New Zealand, where prostitution was decriminalized in 2003, young women in brothels have told me that men now demand more than ever for less than ever. And because the trade is socially sanctioned, there is no incentive for the government to provide exit strategies for those who want to get out of it. These women are trapped.

There is an alternative: an approach, which originated in Sweden, that has now been adopted by other countries such as Norway, Iceland and Canada and is sometimes called the “Nordic model.”

The concept is simple: Make selling sex legal but buying it illegal — so that women can get help without being arrested, harassed or worse, and the criminal law is used to deter the buyers, because they fuel the market. There are numerous techniques, including hotel sting operations, placing fake ads to inhibit johns, and mailing court summonses to home addresses, where accused men’s spouses can see them.

Since Sweden passed its law, the number of men who say they have bought sex has plummeted. (At 7.5 percent, it’s roughly half the rate reported by American men.) In contrast, after neighboring Denmark decriminalized prostitution outright, the trade increased by 40 percent within a seven-year period.