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It's also likely that siblings directly influence one another with regard to diet and activities. "Every parent of more than one child has seen this," Katz said.

For the study, Harding's team collected data on over 10,000 American households and found that childhood obesity risk varies with the number of children and their sex.

Specifically, the researchers found that in a single-child home where a parent is obese, the child is 2.2 times more likely to be obese. In families with two children, however, they found an even stronger link between siblings.

The study also found a link between gender and obesity risk. In homes with one child, girls were less likely to be obese than boys.

In homes with two children, Harding's group found that younger kids are influenced by older siblings, especially if they're same gender.

The youngest boy in a two-child home is 11.4 times more likely to be obese if the older brother is, they noted. If the older child is a girl, the boy is 6.6 times more likely to be obese.

If the youngest child is a girl, she is 8.6 times more likely to be obese if she has an obese older sister. But if she has an obese older brother she is not significantly more likely to be obese, the researchers found.

Exercise and how much the kids eat both play a key role in the prevention of obesity. Harding's team found that an only child was less likely to be physically active and more likely to eat fast food than those who had brothers and sisters.

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