Forty years ago, Walter Mischel conducted a simple yet elegant experiment in which he asked four year old children to resist eating a marshmallow while he was out of the room. He promised that if they hadn't eaten it by the time he returned, they could have two marshmallows. Mischel observed that, while some children were able to resist the tantalizing treat, many others could not. But he didn't stop there—20 years later, he checked again and found that the children who were able to delay their gratification had grown into adults who exhibited greater levels of self control.

It has now been four decades since this experiment, and Mischel and his colleagues are still following some of the participants of the original marshmallow test. In this week’s PNAS, the team reports that the children's differences in self control are still evident after forty years, and there is new information about why some people's brains may be worse at controlling impulses.

In the first part of the new experiment, Mischel and his team were looking for more subtle behavioral differences between "high-delayers" (those who didn’t eat the marshmallow early) and "low-delayers" (those who could not resist the temptation). They wanted to test whether positive social cues would be more difficult for low-delayers to resist. Fifty-nine participants from the original marshmallow experiment were invited to take part in this new experiment; these people had either performed above average on the marshmallow task or had performed particularly poorly.

The participants were first told to press a button when they saw a face of a particular gender on the computer, and male and female faces were flashed on the screen in random order. In the first part of the study, these faces had neutral expressions. In the second part of the trial, these faces were either happy or fearful. The accuracy with which each participant pressed the button was then calculated.

All participants were equally accurate when the faces were neutral. However, low-delayers were much less accurate when the faces showed emotion. This effect was almost entirely due to false alarms when happy faces were shown—that is, low-delayers often pushed the button when the face was happy, no matter which gender it was. High-delayers, on the other hand, rarely made this particular mistake.

The researchers believe that while the high-delayers were able to separate the emotion from the gender, the happy faces were just too compelling for low-delayers to resist. The neutral faces didn’t cause this problem, indicating that some aspects of impulse control are directly related to the stimuli one has to resist. Additionally, individuals’ behavior in this experiment was directly tied to their behavior in the marshmallow test 40 years ago; self-control is clearly very stable over time.

In the second part of the experiment, the participants performed a similar test inside an fMRI machine, and scientists compared the brain activity of high-delayers and low-delayers. When the participants were suppressing the urge to press the button (when the face was of the wrong gender), an area of the brain called the right inferior frontal gyrus was very active. The researchers found that there was much less activity here in the low-delayers than the high-delayers.

Instead, the ventral striatum in low-delayers became particularly active at this point; the ventral striatum is part of the limbic system, and it deals with the processing of rewards. In those with low-self control, instead of the right inferior frontal gyrus being able to hold off temptation, the ventral striatum goes into overdrive and the person can’t ignore the attraction. Low-delayers, it seems, are much more sensitive to positive social cues and have trouble suppressing them to deal with other information.

Clearly, more research is needed, but Mischel's long-term study has significantly furthered our understanding of self-control—and resulted in much more nuanced information about why we can’t always resist that marshmallow.

PNAS, 2011. DOI: 10.1073/pnas.1108561108 (About DOIs).