Shankar Vedantam

Across the United States, college administrators are poring over student essays, recommendation letters and SAT scores as they select a freshman class for the fall.

If this is like most years, administrators at top schools such as Harvard and Stanford will try hard to find talented high school students from poor families in a push to increase the socioeconomic diversity on campus and to counter the growing concern that highly selective colleges cater mainly to students from privileged backgrounds.

If a stranger attacks you inside your own home, the law has always permitted you to defend yourself. On the other hand, if an altercation breaks out in public, the law requires you to try to retreat. At least, that's what it used to do.

The millions of Americans who make New Year's resolutions to lose weight often have pictures in mind.

They're pictures that have been repeatedly supplied by the health and beauty magazines at supermarket checkout lines. They feature skinny models in bikinis, or toned guys with six-pack abs, and captions about how you could look like this by summer.

Some people go so far as to tape these pictures onto their refrigerators and cupboards. When they're tempted to reach for a cookie, they reason, the sight of that toned model might dissuade them from breaking their resolutions.

If you're a student at the halfway point of the academic year, and you've just taken stock of your performance, perhaps you have reason to feel proud of yourself.

But a recent study suggests some of the pride you feel at having done well — especially in science — may be unfounded. Or at least your sense of your performance may not be a very accurate picture of how good you actually are.

If I asked you to think of a murderer, what's the image that springs to mind?

If you're like most people, you'll probably think of an evil psychopath, or someone bent on revenge. Perhaps you'll see a criminal mastermind, who eliminates rivals on his way to riches. Or a strung-out drug addict, who kills because she needs money to get high.

All of these images have something in common: As a rule, we tend to associate murder with the behavior of individuals who behave in aberrational ways.