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Ever noticed your kids clicking on ads when they're playing mobile games? Turns out many kids' apps are full of ads, according to a new study, and some children might not be able to tell the ads apart from the games.

Researchers led by the University of Michigan Medical School studied 135 popular apps marketed to and used by kids aged five and under and found that 95 percent of them contained at least one type of advertising. Some of the ads are "manipulative" and "deceptive," according to the study, published in the Journal of Development and Behavioral Pediatrics.

Ads on kids apps come in many forms, says the study, including teaser videos that interrupt game play, in-app purchases, prompts to share on social media, and hidden ads with misleading symbols like "$." For young children, it can be difficult to distinguish ads from the game, according to the study.

Researchers also found that free apps have significantly more ads, which tend to be manipulative, distracting and not appropriate for young kids. Children's apps also often use game characters for ads to engage the young viewers.

"Children are known to develop trusting, emotional parasocial relationships with media characters and pay more attention to and learn better from familiar characters," wrote the researchers, led by pediatrician Dr. Jenny Radesky. Using the characters in ads "is a misuse of parasocial relationships," they said.

The researchers said some of the ad practices could break Federal Trade Commission rules around advertising to children and suggested that app stores need to be better gatekeepers when it comes to offering apps to kids.

"Children from all socioeconomic backgrounds deserve access to quality media that isn't saturated with marketing or persuasive design," Radesky said in an emailed statement. She added that ad content regulators need to consider limiting ads in apps for kids; app developers need to give ads less time in the apps; and app stores need to vet apps more before displaying them in family-oriented sections.

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