Pain wires us with warning signs. When it’s big pain, we may build big warning circuits that get labeled phobias or posttraumatic stress. Smaller pain builds smaller warning circuits that we’re less aware of. We end up with alarmed feelings that don’t always make sense. It would be nice if we could just delete a circuit that made bad predictions. But there’s a good survival reason why we can’t. Imagine your ancestor watching someone die from eating a poison berry. His cortisol would surge and he would remember that berry forever. Years later, on a day when he was very hungry, he would be able to resist eating that berry. Your ancestor survived because his cortisol...