Only 0.0005 percent of brain cells produce dopamine—one in two million—yet these cells appeared to exert an outsized influence on behavior. Research participants experienced feelings of pleasure when they turned dopamine on, and went to great lengths to trigger the activation of these rare cells. In fact, under the right circumstances, pursuit of feel-good dopamine activation became impossible to resist. Some scientists christened dopamine the pleasure molecule, and the pathway that dopamine-producing cells take through the brain was named the reward circuit.
