From VOA Learning English, this is the Health & Lifestyle report.

Willpower is a mysterious force that helps us to control our actions and achieve our goals. We also call willpower determination, drive and self-control.

The American Psychological Association (APA) defines willpower this way:

• Willpower is the ability to resist short-term temptations in order to meet long-term goals.
• Willpower allows us to ignore unwanted thoughts, feelings or desires.
• Willpower is a limited resource that you can use up.

That last part is important. The theory that our willpower is limited is where psychology experts are divided.

For many years in the world of psychology, the widely accepted theory has been that our willpower – our self-control – is limited. If we are not careful, we can use up our willpower before a task is completed or before a goal is reached.

Psychologists who believe that willpower is a limited resource say using up our willpower is the main reason some of us fail to achieve our goals.

However, in recent years, other psychologists have challenged this theory. A new theory says that the amount of our available willpower is affected by our attitudes about willpower.

But first, let's go back to the "limited supply" theory.

The Chocolate-and-Radish Experiment

An experiment back in the 1990s formed the popular theory in psychology that your willpower is limited. The experiment is known as the Chocolate-and-Radish Experiment.

Researchers put 67 study subjects in a room that smelled of freshly baked chocolate cookies.

Sounds good, right?

Wrong.

Instead of warm, rich chocolate cookies, researchers gave one group radishes to eat – cold, bitter radishes. The other group of subjects ate the chocolate cookies.

Then researchers asked both groups to solve a difficult puzzle.

The group that ate the radishes gave up on the puzzle after 9 minutes. The group that ate chocolate cookies worked twice as long on figuring out the puzzle.

The researchers concluded that the group that resisted the chocolate cookies had used up their willpower. Willpower is like a muscle that can lose strength and tire.

Other experiments that support this theory have found that willpower is like a hungry child in need of a "sugar fix." When we use our willpower, it robs our brain of energy, or glucose. So, feeding the brain a little sugar when doing a difficult task helps you to fill up your willpower resource.

The new theory on willpower

But a new theory contradicts the idea that willpower is limited and can be charged up with sugar.

Carol Dweck is a professor of psychology at Stanford University. In 2013 she and her team asked 87 college students in Germany, Switzerland and the United States to describe their beliefs on willpower.

Some said they believed willpower is a limited resource. Others said they believed willpower was plentiful and even increases the more it is used.

Dweck then asked the study subjects to complete a difficult mental task. This first task was followed by a second that required the subject to resist an impulse and use their willpower.

Dweck described the results in an interview with LiveScience. She said those who believed their willpower was limited grew tired after the first task. They performed poorly on the second task. But, if they received a sugary drink before the second task, they performed better.

Dweck also said that people who believed willpower is limited were always looking for signs that they were running out of willpower, or they thought they needed the sugar fix.

However, the study subjects who believed willpower was unlimited did not tire during the second task and they did not need a sugar boost.

Dweck said the results suggest that willpower is not in limited supply. Instead, it is people's beliefs about willpower that shape their behavior.

She and her team published their findings in August of 2013 in the journal The Proceedings of the National Academy of Sciences.

Tips how to use your willpower – limited or not

But what do all these studies mean to you?

Regardless of what you believe, the American Psychological Association website has advice for people trying to achieve their goals.

• Avoid Temptations. If you believe that your willpower is limited, remove outside temptations. This way you will not waste your willpower. As the expression goes, "out of sight, out of mind."
• Make a Plan. For example, if you have to study over the weekend, set aside a certain period of time to get your work done. But also make time for fun, too.
• Think about your motivations. Are your goals your own or someone else's? Studies and common sense suggest that people who pursue goals that are their own are more motivated to reach them.
• Focus on one goal at a time. Many push their willpower too much. They try to quit smoking, save money, lose weight and get a better job all at the same time. This may use up their willpower, if they believe in the "limited supply" theory. Instead, focus on one goal at a time.
• Think you can! There is evidence to support that if you believe you can do it, you can. A little positive thinking goes a long way!

I'm Anna Matteo.

What do you think? Is willpower a limited resource? Or do we have an unlimited supply to draw from? Let us know in the Comments section.

Anna Matteo wrote this for VOA Learning English. Ashley Thompson was the editor.

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determination – n. a quality that makes you continue trying to do or achieve something that is difficult

resist – v. to fight against (something) : to try to stop or prevent (something)

temptation – n. a strong urge or desire to have or do something

contradict – v. to say the opposite of (something that someone else has said) : to deny the truth of (something)

impulse – n. a sudden strong desire to do something

focus – n. a main purpose or interest

motivation – n. a force or influence that causes someone to do something