DVDs and videos that claim to help boost infants' ability to learn new words may actually hinder their language development, a new study says.

For every hour a day spent watching baby DVDs and videos, infants between 8 and 16 months old understood an average of six to eight fewer words than infants who didn't watch them. The baby DVDs/videos had no positive or negative effect on the vocabulary of toddlers ages 17 to 24 months.

The study, which included more than 1,000 families with infants or toddlers, was published Tuesday in the Journal of Pediatrics.

"The most important fact to come from this study is, there is no clear evidence of a benefit coming from baby DVDs and videos, and there is some suggestion of harm," lead author Frederick Zimmerman, an associate professor of health services at the University of Washington and Seattle Children's Hospital Research Institute, said in a prepared statement. "The bottom line is, the more a child watches baby DVDs and videos, the bigger the effect. The amount of viewing does matter."

"The results surprised us, but they make sense," added study co-author Andrew Meltzoff, co-director of the University of Washington's Institute for Learning and Brain Sciences, in a prepared statement.

"There are only a fixed number of hours that young babies are awake and alert. If the 'alert time' is spent in front of DVDs and TV instead of with people speaking in 'parentese' -- that melodic speech we use with little ones -- the babies are not getting the same linguistic experience," Meltzoff said.

"Parents and caretakers are the baby's first and best teachers," he noted. "They instinctively adjust their speech, eye gaze and social signals to support language acquisition. Watching attention-getting DVDs and TV may not be an even swap for warm social human interaction at a very young age."

While the study doesn't offer definitive proof that baby DVDs/videos are harmful, it's best to be safe and limit the amount of time infants watch them, Zimmerman said. He and his colleagues said more research is needed to examine the long-term effects of baby DVDs/videos on children's cognitive development.