Willpower is the most important keystone habit.

In the 1960s, researchers at Stanford conducted what would become a very famous study. A large group of four-year-olds was brought, one by one, into a room. In the room, there was a table with a marshmallow on it.  A researcher gave each child a choice: either eat the marshmallow now or wait a few minutes and have two marshmallows instead. The researcher then left the room for 15 minutes. Only about 30 percent of the children managed not to devour the marshmallow in the researcher’s absence.

But here’s the interesting part. When, years later, the researchers tracked down the study’s participants, who were now adults, they found that those who had exhibited the greatest willpower and waited the full 15 minutes had ended up with the best grades in school, were more popular on average and were less likely to have drug addictions. Willpower, it seemed, was a keystone habit that could be applied to other parts of life, too.

More recent studies have shown similar results. For instance, a 2005 study on eighth-graders showed that students exhibiting high levels of willpower had better grades on average and were more likely to get into selective schools.

So willpower is a key habit in life. However, as you might have noticed if you’ve ever tried to start exercising more, willpower can be highly inconsistent. Some days, hitting the gym is a breeze; on others, leaving the sofa is nigh impossible. Why is that?

It turns out that willpower is actually like a muscle: it can tire. If you exhaust it by concentrating on, say, a tedious spreadsheet at work, you might have no willpower left when you get home. But the analogy goes even further: by engaging in habits that demand resolution – say, adhering to a strict diet – you can actually strengthen your willpower. Call it a willpower workout.

Other factors can also affect your willpower. For example, Starbucks found that, on most days, all of its employees had the willpower to smile and be cheerful, regardless of how they felt. But when things became stressful – for example, when a customer began screaming – they would soon lose their cool. Based on research, executives at the company determined that if baristas mentally prepared for unpleasant situations and planned out how to overcome them, they could muster enough willpower to follow the plan even when under pressure.

To help them, Starbucks developed the aptly named LATTE method, which outlines a series of steps to take in a stressful situation: Listening to the customer, Acknowledging their complaint, Taking action, Thanking the customer, and, lastly, Explaining why the issue occurred. By practicing this method over and over, Starbucks baristas learn exactly what to do should a stressful situation arise, and can stay cool.

Other studies have shown that a lack of autonomy also adversely affects willpower. If people do something because they are ordered to rather than by choice, their willpower muscle will get tired much quicker.