The neurotransmitter dopamine is the source of desire (via the desire circuit) and tenacity (via the control circuit); the passion that points the way and the willpower that gets us there. Usually the two work together, but when desire fixates on things that will bring us harm in the long run—a third piece of cake, an extramarital affair, or an IV injection of heroin—dopaminergic willpower turns around, and does battle with its companion circuit.
