Carol Dweck turns nice kids into liars.

It's not quite as malevolent as it sounds. Indeed, the way the Stanford University psychologist brings out the worst in children is with kindness. She praises them lavishly, much like engaged and doting parents do with their own kids every day. She tells them how smart they are. And at the end of the day, she has them telling whoppers.

Here's the whole story. Dweck studies personality and achievement. She is one of a growing number of psychologists who believe that personality is forever malleable, which is a radical departure from the old thinking. The old thinking was that our personality—the sum total of our human qualities—was an inherited legacy, fixed at birth and unchanging through life. So we had adventurous people and timid people; competitive Type As and laid-back Type Bs; conscientious, truthful types and—well, scoundrels and liars.

The new thinking is that these traits are not fixed but in flux, and there are many ideas about why personality might change. Dweck's theory is that our beliefs about ourselves and the world—our "self theories," in the jargon—are a powerful influence on who we become in life. In other words, our own lay theories about personality and aptitude actually shape our character. Consider the lying experiment.

Dweck had hundreds of preteens take a test. The problems were from a standard IQ test, and most of the kids scored OK on the test. But when Dweck praised the kids' performance, she didn't praise them all the same way. She praised some for their natural talent (What a great score! You're so smart!), while others were praised for their effort (What a great score! You must have worked very hard!).

This may seem like a subtle difference, but to the developing mind the two messages are night and day. The former conveys the belief that people's abilities and traits are fixed, written in concrete, while the latter underscores the potential for growth and the value of old-fashioned effort. The results were immediate and unambiguous: the kids who were told they were smart immediately became cautious, shying away from any further testing that might expose weaknesses. The kids who were praised for their effort, by contrast, became hungry for new challenges. What's more, when the kids were subsequently required to solve very difficult problems, on which they all did poorly, the "smart kids" took the failure as a blow to their self-worth; where they had been smart, they were now dumb, irrevocably. The effortful kids just dug in more.