The Baumeister study hypothesized that Willpower or Self Control is exhaustible.

Self Control behaves less like a trait than it does a muscle. Muscles can be trained and strengthened. Muscles can be exhausted.

Some Self-Control Examples

Joe Saver is dieting. On a Monday he ate an egg white and strip of bacon for Breakfast. Later during the workday, he had a salad with light dressing and a piece of fruit. But the day has been hard on him. At the time he clocks out, he has been working for 10 hours. Joe gets in the car for his 45-minute commute home. He’s exhausted.

Joe has a plan to eat a lean steak for dinner with broccoli. Half-way through his commute his wife calls. She’s had a hard day as well. Joe’s wife suggests it might be easier to pick up a pizza for dinner. Joe caves. His Self Control is tapped out from the hard day at work. It’s easy to influence him to make poor decisions.

Or maybe this situation?

Joe Saver is dieting. It’s a Saturday. He’s eaten the same egg and bacon for breakfast. After breakfast, he drinks coffee on his couch and reads a book. Later that morning he goes for a run. For lunch has the same salad, dressing, and fruit. In the afternoon he does a couple chores and goes for a hike outside.

At dinner time Joe gets to make a decision about what he’ll eat. The steaks have been marinating but he could take the easy route and get the pizza.

Joe is more likely, in this situation, to cook the steak that fits his diet plan. He hasn’t been straining his willpower all day so the decision to stick to his diet is much easier than it is on Monday.