Peer pressure can be a powerful force, and sometimes a positive one. For example, hanging out with active peers may lead kids to exercise more, making a child's social network a potential vehicle for promoting healthy habits and reducing obesity.

That's what researchers led by Sabina Gesell, a research assistant professor in pediatrics at Vanderbilt School of Medicine, and her colleagues are reporting in the journal Pediatrics. The scientists studied networks of friends in an after-school program involving students aged 5 to 12. Using a pedometer-like device that recorded minute muscle movements, the researchers tracked kids' physical activity levels over a period of 12 weeks.

At the start of the program, none of the children knew one another well, so the researchers were able to track how the youngsters made and dropped friends, and what effect these changing relationships had on their physical activity level.

"We see evidence that the children are mirroring, emulating or adjusting to be similar to their friends," says Gesell. "And that's exciting because we saw meaningful changes in activity levels in 12 weeks."

The results are encouraging also because they suggest a potentially inexpensive and effective way to change children's behavior.

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