For a long time, the commonly held belief among psychologists was that willpower is like a muscle - it gets fatigued after use, but over time it can become stronger. Also, the general consensus is that we have one common pool of willpower - so after spending a day studying instead of watching reality tv reruns, you just might find yourself unable to resist the temptation of that delicious chocolate cake. Operating under this assumption, scores of research studies showed that people get depleted after engaging in a task that requires willpower.
A quick digression: one famous task used in psychological research that examines interference and self control is called the Stroop Task. This task requires you to read a list of words out loud. In the classic version, the words are a list of colors, such as 'yellow,' 'red,' and 'blue' and you have to read them as quickly as you can. Sounds easy right? Not so fast. The words are all colored. Give it a quick try below: