You've just moved to town and need a place to live. You've narrowed your choices to three apartments that seem suitable. The first is spacious, 800 square feet, but it's a good 15 miles from your new job. That's a long daily commute. The second is much closer, only about seven miles away, but at 450 square feet the space is a bit cramped. The third is 350 square feet and 10 miles from work. You're running out of time and need to get yourself settled. Which do you choose?

Well, if you're like most people, you will choose the second apartment. That may be a perfectly fine choice, and chances are you'll be happy there. But it's not a rational choice, and here's why: Eliminating the third apartment is a no-brainer; it's both smaller and more remote than the second apartment. So that should leave you with a tossup between two decent places, and you should be just as likely to choose one as the other. But you're not. Instead you are irrationally swayed by the similarity between the second and the third apartments. You pick the second not because it is better than the spacious apartment, No. 1, but because you're still comparing it to the loser apartment, even though you ruled that one out.

Cognitive psychologists call that third apartment a mental "decoy." It is so clearly inferior to the other two, neither spacious nor well-located, it really shouldn't even be in the mix, but dinging it does not make it go away entirely. It lingers in your mind, tugging you toward apartment No. 2.

This is not a good thing. We make choices like this every day. We decide where to go to college, what to eat for dinner, who to date. And a lot of our choices are irrational, influenced by irrelevant information. We are of course capable of making deliberate, logical choices as well; recent science suggests that the brain is like a hybrid engine, constantly switching back and forth between reasoned calculation and rapid intuition. But what determines how we will handle a particular problem in life? How do we know what part of our cognitive repertoire will be in play today?

A couple of Florida State University psychologists may have part of the answer to that. If the brain truly is like a hybrid engine, E. J. Masicampo and Roy Baumeister reasoned, then why not look at the fuel system? All of that cognitive crunching doesn't come cheap, and effortful deliberation is especially greedy for energy. This is not just a metaphor: they wanted to see if the brain's supply of fuel--blood glucose--might determine whether we make logical choices or irrational ones. They decided to explore this in the laboratory.

The experiment was fairly simple. They started by having all the subjects do an exercise meant to deplete their power--both their willpower and the glucose that fuels self-control and decision making. Specifically, they had them watch a silent video of a woman talking. A series of words also flashed on the screen, but they told the subjects to ignore the words; if they did find themselves distracted by the words, they were to refocus their attention on the woman. This is actually very hard to do; it requires a lot of mental effort to not read the words right in front of you.