San Diego has pinpointed a reason for the recent growth of sex trafficking: gangs.

Not only is it a more profitable business than arms dealing, but it is also safer than drug trafficking, as seen in the Urban Institute’s latest study.

Traveling to different jurisdictions is more difficult for law enforcement to track, and also for lawyers to prosecute. Thus gangs have been working together to create circuits – series of cities that they transport people across to sell for sex or forced labor.