Victor Clark-Alfaro, a lecturer at San Diego State and director of Tijuana’s Binational Center for Human Rights, began doing field work with human smugglers along the U.S-Mexico border in the late 1980s. In a Q-and-A, he discusses the business of smuggling people across the border, including the role of referrals and pricing and how it’s all changed in recent years.

The chief of San Diego’s Border Patrol section said he can’t disclose what agents look for before stopping someone, but that race and ethnicity don’t come into play. A review of enforcement actions recorded between 2011 and 2014, though, shows agents stopped people for sometimes ambiguous reasons like sitting up straight or driving slowly.