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Identical twins share the same genes, but they can have very different personalities. One twin may be outgoing while the other is shy. Scientists haven't figured out exactly how this happens.

But NPR's Jon Hamilton reports researchers are now a bit closer to understanding what causes these differences.

JON HAMILTON, BYLINE: Scientists used to think that identical twins turned out differently because they were treated differently by friends or teachers or their parents. So about 10 years ago, a psychologist named Eric Turkheimer started reviewing the evidence. He asked a simple question: Does different treatment really explain the development of different personalities?

ERIC TURKHEIMER: And the answer was, it didn't explain it at all. And that just sort of produced a mystery of, well, how do you explain it? How do they get different?

HAMILTON: Turkheimer, who is at the University of Virginia, had a hunch. It came from studies showing that differences tend to increase during childhood.

TURKHEIMER: Siblings often start out very, very similar to each other in something like personality or intelligence. But as time goes by they slowly drift apart from each other.

HAMILTON: Turkheimer suspected that was because tiny differences that appear early in life are somehow being amplified as children grow up. But there was no easy way to test this idea because researchers can't observe every moment of a child's life.

Mice, though, are a lot easier to study. And a group of scientists in Europe thought they might begin to solve the mystery of personality difference by studying 40 genetically identical mice as they grew up together. Gerd Kempermann, of the Center for Regenerative Therapy in Dresden, says the mice lived in a pretty elaborate home.

GERD KEMPERMANN: Yeah, it was a large cage of five square meters floor area, divided into five different levels.

HAMILTON: With plastic tubes leading from one level to another and food and water available in lots of different locations. Each mouse carried a special tracking chip that showed where the animal went. And Kempermann says over a three-month period, individual behavior changed dramatically even though the mice had the same genes and lived in the same place.

KEMPERMANN: Some mice explored a lot and continued to explore a lot over this long period, whereas others might start high but then would go down and become increasingly less interested...

(LAUGHTER)

KEMPERMANN: ...if you want, in the environment.

HAMILTON: And Kempermann reports in the journal Science that the behavior of these mice wasn't the only thing that changed. So did their brains. The scientists measured the exploratory behavior of each mouse. Then, Kempermann says they looked to see whether this measure was linked to changes in the hippocampus, a part of the brain that's highly involved in exploration.

KEMPERMANN: We found that this measure correlates with the production of new neurons in the hippocampus.

HAMILTON: The mice that explored a lot were generating new brain cells. So their brains were becoming increasingly different from those of less curious mice.

Eric Turkheimer says it's hard to know if something similar is happening with identical twins. But he says the results are consistent with his idea that even a tiny personality difference can be greatly amplified by experiences during childhood. He says it may work something like this. Say you have identical twin girls. One has a random experience that makes her just a tiny bit more extroverted than her sister.

TURKHEIMER: And because that twin one is a little bit more extroverted, she meets somewhat more extroverted friends, who then make the kid a little bit more extroverted.

HAMILTON: Creating a feedback loop that keeps amplifying the personality trait and may even cause the growth of new brain cells.

TURKHEIMER: Until finally, in the end, you have these two identical twins, one of whom is out at parties with all the extroverted friends and the other is sitting in the library by herself.

HAMILTON: Turkheimer says that's still just his hypothesis. But now it should be easier to test by doing more studies of genetically identical mice.

Jon Hamilton, NPR News.

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