Experiments like this demonstrate why Puritanism is so psychologically unrealistic:
A paper in The Journal of Consumer Research looks at the effects of self-restraint on subsequent impulse purchases.
In one experiment, college students spent a few minutes free-associating and writing down their thoughts, under instructions not to think of a white bear. Given $10 afterward to save or spend on a small assortment of products, they spent much more money than students who had free-associated without having to avoid thoughts of bears.
This isn’t the first time people have explored the impact of mental exertion on self-control. Stanford professor Baba Shiv invented an experiment where he manipulated the “cognitive load” of subjects. Shiv gave half of the subjects a two-digit number to memorize (low load), while the other half were given a seven-digit number (high load). Subjects were then instructed to walk to another room in the building. On the way they passed by a table at which they were presented with a choice between a caloric slice of chocolate cake or a bowl of fruit salad. Fifty-nine percent of the people trying to remember seven digits (high load) chose the cake, while sixty-three percent of the two-digit subjects (low load) chose the fruit salad. In other words, having people memorize an extra five digits made them exhibit significantly less self-control.
Why did the number of digits have such a strong effect? Shiv speculates that the effort required to memorize seven numbers drew cognitive resources away from our ability to control our urges. This makes anatomical sense, since working memory and self-control are both located in our pre-frontal cortex. Having to remember seven numbers occupied neurons that would otherwise help us decide what to eat, which causes us to become more reliant on our impulsive emotions. While we tend to think of self-control as being an innate trait, it is actually dependent on a range of extrinsic factors, all of which affect the way our brain responds to a given situation.
This model of limited “thinking resources” has now generated a large amount of supporting evidence. Our decisions really are swayed by the computational limits of our brain. For example, in 2003 neuroeconomists noticed that subjects on diets who resisted temptation in the morning (by foregoing the chance to grab snacks from a nearby basket) later ate significantly more ice cream in an ice-cream taste test than subjects who hadn’t exercised self-control. They also quit 40 percent earlier when confronted with a difficult math problem. By resisting the morning snacks, they had temporarily “used up” their ability to resist further temptation. (Other variables that seem to exhaust our self-control are alcohol, stress, and sleep deprivation.)
The moral of this data is that we have to pick our battles. Everybody occasionally splurges on the slice of cake, or quickly gives up on a difficult problem. Instead of trying to never be bad, we should focus on being good when it matters. If you are trying to diet, then let yourself think about white bears. Self-control is a mental muscle, and we should always be conscious of not tiring it out.