Now—surprise!—the monkey presses the lever ten times and gets two raisins. Whoa: twenty units of dopamine are released. And as the monkey continues to get paychecks of two raisins, the size of the dopamine response returns to ten units. Now reward the monkey with only a single raisin, and dopamine levels decline. Why? This is our world of habituation, where nothing is ever as good as that first time.
