Second, while willpower may be fine under normal conditions, when you get stressed (saber-toothed tiger, email from the boss, fight with a spouse, exhaustion, hunger), your old brain takes control and overrides your new brain, basically shutting the latter down until the stress is gone. So exactly when you need your willpower—which resides, remember, in the prefrontal cortex/new brain—it’s not there, and your old brain eats cupcakes until you feel better and your new brain comes back online.
