A ban on such commercials would reduce the number of obese young children by 18 percent, and the number of obese older kids by 14 percent, researchers found.

They also suggested that ending an advertising expense tax deduction for fast-food restaurants could mean a slight reduction in childhood obesity.

Some experts say it's the first national study to show fast-food TV commercials have such a large effect on childhood obesity. A 2006 Institute of Medicine report suggested a link, but concluded proof was lacking.

"Our study provides evidence of that link," said study co-author Michael Grossman, an economics professor at City University of New York.

The study has important implications for the effectiveness of regulating TV advertising, said Lisa Powell, a researcher at the University of Illinois at Chicago's Institute for Health Research and Policy. She was not involved in the research but was familiar with it.

The percentage of U.S. children who are overweight or obese rose steadily from the 1980s until recently, when it leveled off. About a third of American kids are overweight or obese, according to Centers for Disease Control and Prevention estimates.

The causes of childhood obesity are complicated, but for years researchers have been pondering the effects of TV advertising. Powell, for example, found fast-food commercials account for as much as 23 percent of the food-related ads kids see on TV. Others have estimated children see fast-food commercials tens of thousands of times a year.

The new study is based in part on several years of government survey data from the late 1990s that involved in-person interviews with thousands of U.S. families. The researchers also looked at information about local stations in the 75 largest TV markets, including locally seen fast-food commercials and the size of viewing audiences.

The researchers used a statistical test that presumes TV ads lead to obesity but made calculations to address other influences such as income and the number of nearby fast-food restaurants. They also took steps to account for the possibility that some children may already have been overweight and inactive regardless of their TV-watching habits.

The study is being published this month in the Journal of Law & Economics. The authors, funded by a federal grant, included Grossman and researchers from Lehigh University and Georgia State University.

The authors stopped short of advocating an advertising ban or eliminating the advertising tax deduction.

Grossman said it's possible that some families benefit from advertising by finding out what restaurants are nearby and what they're serving. "A lot of people consume fast food in moderate amounts and it doesn't harm their health," he said.

McDonald's Corp., the giant fast-food chain responsible for the widely seen "I'm Lovin' It" ad campaign, referred questions about the study to the National Council of Chain Restaurants. Officials with that organization could not be reached Wednesday evening.

The Journal of Law & Economics:http://www.journals.uchicago.edu/toc/jle/current