Why are the Taliban such a threat in Afghanistan? And why are violent radical Islamists such resilient terrorists? You won’t believe this but the answer is communism. I’m not kidding. Not capital C Communism, the discredited ideology of the Soviet Union, but mutual aid, as practiced in communes. This might sound like a joke from The Colbert Report—but stick with me.Religious extremists such as the Taliban are incredibly successful rebels. The Taliban controlled Afghanistan, and we now know how hard that is. Their success cannot be solely due to the Taliban’s theology, which is shared by many rebel groups, most of which are flimsy organizations that quickly crumble.So what is the Taliban’s secret? Well, the single biggest threats to terrorist organizations are leaks and defection. Radical religious organizations tend to succeed because they can select recruits carefully to create defection-resistant organizations. How do they resist defection? Communism. Religious radicals, such as the Amish, Hutterites, and Ultra-Orthodox Jews, are masters of mutual aid, creating tight-knit communities that are incredibly supportive of their members.Yet economists (like me) know that mutual aid shouldn’t work in large communities, since individuals pursuing their own self-interest will shirk their responsibilities to the commune, which will then unravel. In fact, mutual aid communes generally do unravel. But not those run by religious radicals—they carefully select new members and monitor the behavior or existing members through sacrifices and prohibitions. The latter are surprisingly similar among religious radicals of vastly different religious traditions. (Economists call that structure a “club,” and Larry Iannaccone of Chapman University is responsible for the insight.)One more logical step: Once the shirking problem is solved in the mutual aid community, controlling defection is much easier in the violent sub-group—since members have been screened for loyalty and are well-monitored.What’s the evidence for this communist conspiracy? Religious radicals with a social-service provision base, such as Hamas, Hezbollah, Sadr’s Mahdi Army, and the Taliban, are much more effective at violence than groups that share the same theology but lack a service provision base, such as the Palestinian Islamic Jihad.