We all like to think we’re in control of our decisions. But, even aside from the fact that we have no free will (thought I’d just slip that in there; UPDATE: here’s another great article about free will), other factors–things we wouldn’t want to admit are affecting our judgment–are influencing our decisions. There are many many experiments out there that show this but here’s one that’s been showing up a lot recently even though it’s over ten years old.

The idea was simple. Put students in a classroom and give them either a two or seven digit number to memorize. At that point, the students are asked to leave, one at a time, and go to a second room. On their way to the second room, they’re asked if, as a thank you for participating, they would like either a bowl of fruit salad or a piece of chocolate cake.

Seems simple enough. When I first heard the description of this experiment, I couldn’t figure out what the trick was. I’ll let Wired editor and author of How We Decide Jonah Lehrer explain:

“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.”

Although the students in the experiment were from Standford, they’re really no different than you or me. That could be you in the experiment getting your snack choice influenced by how many number you’ve been asked to remember. Part of me still thinks that my choice between cake and fruit salad wouldn’t be influenced by something as simple how many numbers I’m trying to remember but I know I’m probable just deluding myself.

Another consequence of this experiment is, couldn’t fast food restaurants and all sorts of junk food makers take advantage of this? They couldn’t really ask you to remember seven-digit numbers, but they could find other ways of increasing your cognitive load in order making it more difficult for you to resist that bacon double cheeseburger. After all this talk of psychology, a bacon double cheeseburger sounds pretty good right now.

You can hear an audio description of this experiment from Radiolab and for more examples of irrational decisions check out this Dan Ariely video.