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Suppose you could raise your IQ by exercising your brain like a muscle. That's the analogy researchers are using to explain a new study that finds the IQ of teenagers can rise or fall significantly during adolescence. NPR's Allison Aubrey reports.

ALLISON AUBREY, BYLINE: If you know what the word obstreperous means, raise your hand. Obstreperous. Yup. I can't see you out there, but I would bet all of you straight A types are waving your hands. You were paying attention in school.

So what does this have to do with the story? Well, there was a time when a child's IQ was thought to be a fixed and stable measurement of intelligence, that captured a kid's intellectual potential, if you will. So a teen who could use fancy words like obstreperous would score well on intelligence tests. Stephen Ceci is a psychologist at Cornell University.

STEPHEN CECI: So obstreperous would be - a group of words like obstreperous would be highly predictive of how people would do on the vocabulary part of an IQ test.

AUBREY: Now, a strong vocabulary, Ceci adds, is the result of acquired knowledge, how much you've read or been taught at school and home. And in any given child these skills were thought to develop fairly consistently. How a kid performed at six would hold steady at age 12 or 18, even as tests get harder.

But in recent years, this theory has unraveled a bit. Ceci says a slew of studies show kids' performances on IQ tests can vary. Sometimes it's the academic environment that changes or sometimes kids are just late bloomers.

CECI: The timetable does get highly individualized for kids, and there's a lot of variability in neural developments during adolescence.

AUBREY: The most recent evidence is published this week in the journal Nature. Researchers at the University College London administered IQ tests to a group of healthy schoolchildren between the ages of 12 and 16. And then they repeated the tests four years later. Researcher Cathy Price says she expected to see IQ's inching up or down a few points, but what she documented were dramatic fluctuations.

CATHY PRICE: We had individuals that changed from being on the 50th percentile, you know, with an IQ of 100, right the way up to being on the third percentile with an IQ of 127.

AUBREY: Now, we're only talking about 33 kids here. So it's not clear that this could be true for all teens. But to confirm that these swings were not random or just a fluke of kids having a particularly good or bad day, the researchers did MRI scans of the teens' brains.

PRICE: We looked at the structure of their brain in early teenage years and in the late teenage years. And we were able to see that the degree to which their IQ had changed was proportional to the degree to which their different parts of their brain had changed.

AUBREY: This means that the brain really does change in line with teens' emotional and intellectual development. There are lots of factors that may explain changes, though this study did not attempt to nail them down. Cornell's Stephen Ceci says prior research has shown that rigorous academics can lead to improved IQ scores, similar to prepping for the SATs. Teen personalities, work ethic and the home environments are important too. Some teens are just defiant or obstreperous at age 13, but by 19 or 20 they've dialed in.

CECI: Studies like this one should definitely caution all of us against assuming that that one low IQ score at one point is really capturing all that that individual's capable of.

AUBREY: There's still plenty of evidence that over time IQ tests do measure something innate about a person's intellectual gifts. But there's emerging consensus that, particularly in the teen and young adult years, intellectual development is a bit like physical exercise. The harder you push your brain, the more you have to show for it.

Allison Aubrey, NPR News.

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