Habits stick because they create craving.

Imagine this scenario: every afternoon for the past year, you’ve bought and eaten a delicious, sugar-laden chocolate-chip cookie from the cafeteria at your workplace. Call it a just reward for a hard day’s work.

Unfortunately, as a few friends have already pointed out, you’ve started putting on weight. So you decide to kick the habit. But how do you imagine you’ll feel that first afternoon, walking past the cafeteria without indulging? Odds are, you will either eat “just one more cookie” or you’ll go home in a distinctly grumpy mood.

Kicking a bad habit is hard because you develop a craving for the reward at the end of the habit loop. Research from the 1990s conducted by the neuroscientist Wolfram Schultz shows how this works at the level of the brain. Schultz was studying the brain activity of a macaque monkey named Julio, who was learning to perform various tasks. In one experiment, Julio was placed in a chair in front of a screen. Whenever some colored shapes were shown on the screen, Julio’s task was to pull a lever. When he did, a drop of blackberry juice (Julio loved blackberry juice) would drip down on his lips through a tube.

At first, Julio didn’t pay much attention to the screen. But when he happened to pull the lever at the right moment, thus triggering the blackberry-juice reward, his brain activity spiked, showing a strong pleasure response.

As Julio gradually grasped the connection between seeing the shapes on the screen, pulling the lever and getting the blackberry juice, he not only stared at the screen, but Schultz noticed that, as soon as the shapes appeared, there was a spike in Julio’s brain activity similar to when he actually received the reward. In other words, his brain had begun anticipating the reward. This anticipation is the neurological basis of craving and helps explain why habits are so powerful.

Schultz then altered the experiment. Now, as Julio pulled the lever, either no juice would come or it would come in a diluted form. In Julio’s brain, Schultz could now observe neurological patterns associated with desire and frustration. Julio got decidedly mopey when he didn’t get his reward, just as you might if you forewent your cherished end-of-the-day cookie.

The good news is that craving works for forming good habits as well. For instance, a 2002 study from New Mexico State University showed that people who manage to exercise habitually actually crave something from the exercise, be it an endorphin rush in the brain, a sense of accomplishment or the treat they allow themselves afterward. This craving is what solidifies the habit; cues and rewards alone are not enough.

Given the power of habits, it should come as no surprise that companies work hard to understand and create such cravings in consumers. A pioneer of this tactic is Claude Hopkins, the man who popularized Pepsodent toothpaste when countless other toothpaste brands had failed. He provided a reward that created craving: namely, the cool, tingling sensation that we’ve come to expect toothpaste to have. That sensation not only “proved” that the product worked in consumers’ minds; it also became a tangible reward that they began to crave.