Check out this report from NPR's Radiolab.

In his book How We Decide, and in a recent Wall Street Journal article, Jonah writes about an experiment by Stanford University professor Baba Shiv, who collected several dozen undergraduates and divided them into two groups.

In the WSJ article, Jonah writes:

"One group was given a two-digit number to remember, while the second group was given a seven-digit number. Then they were told to walk down the hall, where they were presented with two different snack options: a slice of chocolate cake or a bowl of fruit salad."

And then he writes:

"Here's where the results get weird. The students with seven digits to remember were nearly twice as likely to choose the cake as students given two digits. The reason, according to Professor Shiv, is that those extra numbers took up valuable space in the brain — they were a "cognitive load" — making it that much harder to resist a decadent dessert. In other words, willpower is so weak, and the prefrontal cortex is so overtaxed, that all it takes is five extra bits of information before the brain starts to give in to temptation."

It turns out, Jonah explains, that the part of our brain that is most reasonable, rational and do-the-right-thing is easily toppled by the pull of raw sensual appetite, the lure of sweet. Knowing something is the right thing to do takes work — brain work — and our brains aren't always up to that. The experiment, after all, tells us brains can't even hold more than seven numbers at a time. Add five extra digits, and good sense tiptoes out of your head, and in comes the cake. "This helps explain why, after a long day at the office, we're more likely to indulge in a pint of ice cream, or eat one too many slices of leftover pizza," Lehrer writes.