Charles Duhigg

Habits, scientists say, emerge because the brain is constantly looking for ways to save effort. Left to its own devices, the brain will try to make almost any routine into a habit, because habits allow our minds to ramp down more often. This effort-saving instinct is a huge advantage. An efficient brain requires less room, which makes for a smaller head, which makes childbirth easier and therefore causes fewer infant and mother deaths. An efficient brain also allows us to stop thinking constantly about basic behaviors, such as walking and choosing what to eat, so we can devote mental energy to inventing spears, irrigation systems, and, eventually, airplanes and video games.

He created a craving. And that craving, it turns out, is what makes cues and rewards work. That craving is what powers the habit loop.

First, find a simple and obvious cue.

Second, clearly define the rewards.

If you get those elements right, Hopkins promised, it was like magic. Look at Pepsodent: He had identified a cue—tooth film—and a reward—beautiful teeth—that had persuaded millions to start a daily ritual. Even today, Hopkins’s rules are a staple of marketing textbooks and the foundation of millions of ad campaigns.

And those same principles have been used to create thousands of other habits—often without people realizing how closely they are hewing to Hopkins’s formula. Studies of people who have successfully started new exercise routines, for instance, show they are more likely to stick with a workout plan if they choose a specific cue, such as running as soon as they get home from work, and a clear reward, such as a beer or an evening of guilt-free television.2.13Research on dieting says creating new food habits requires a predetermined cue—such as planning menus in advance—and simple rewards for dieters when they stick to their intentions.2.14

“The time has come when advertising has in some hands reached the status of a science,” Hopkins wrote. “Advertising, once a gamble, has thus become, under able direction, one of the safest of business ventures.”

It’s quite a boast. However, it turns out that Hopkins’s two rules aren’t enough. There’s also a third rule that must be satisfied to create a habit—a rule so subtle that Hopkins himself relied on it without knowing it existed. It explains everything from why it’s so hard to ignore a box of doughnuts to how a morning jog can become a nearly effortless routine.