Human societies run on trust. Every day, we all trust millions of people, organizations, and systems -- and we do it so easily that we barely notice. But in any system of trust, there is an alternative, parasitic, strategy that involves abusing that trust. Making sure those defectors don't destroy the very cooperative systems they're abusing is an age-old problem, and we've developed a variety of societal pressures to induce cooperation: moral systems, reputational systems, institutional systems, and security systems. Understanding how these different societal pressures work -- and fail -- is essential to understanding the problems we face in today's increasingly technological and interconnected world.