Brandy

“To his surprise, rather than reducing the odds that the rodents would press the lever, this variable reinforcement actually made them respond more. It also made them persist in trying for far longer periods of time after he’d stopped providing rewards. Later studies found that the most effective schedule of reinforcement to hook animals on a behavior pattern was the most unpredictable: if pigeons, for example, get rewarded only 50% of the time for doing a task (basically, completely randomly), they will perform the required behavior more frequently and will be more resistant to changing it than if they are always rewarded or are rewarded in a more predictable way. Humans behave quite similarly.”
― Maia Szalavitz, Unbroken Brain: A Revolutionary New Way of Understanding Addiction