Having the television on constantly in a household with infants and toddlers is bad for brain and language development because it reduces the number of words the kids hear and say, a study showed Monday.

"Audible television clearly reduces speech for both infants and their caregivers within the home and this is potentially harmful for babies' development," said University of Washington professor of pediatrics Dimitri Christakis, who led the study.

The study found that for every hour that the television is on with the volume turned up in a household with small children, the children heard between 500-1,000 fewer words from adults.

"Adults typically utter approximately 941 words per hour. Our study found that adult words are almost completely eliminated when television is audible to the child," said Christakis.

"These results may explain the association between infant television exposure and delayed language development," Christakis said.

Constant exposure to television could also explain attentional and cognitive delays in children, he said, since language development is believed to be key in brain development in early childhood.

For the study, 329 children ranging in age from two months to four years wore special vests with a chest pocket that held a small digital recorder.

The children wore the vests on random days for up to two years. The recorder captured everything the child said and heard during continuous 12 to 16 hour periods, and was only removed when the child napped or slept, was bathed or went for a ride in the car.

When the researchers counted the number of words uttered by adults and vocalizations by children, as well as vocal interactions between grown-ups and kids, they found that having the telly on resulted in significant reductions in all speech, regardless of whether it was being actively watched or just on in the background.

"Since 30 percent of American household report having the television always on, even when no one is watching, these findings have grave implications for language acquisition and perhaps even early brain development," Christakis said, counseling grown-ups against using the telly as a babysitter.

"Television is not only a poor caregiver substitute, but it actually reduces the number of language sounds and words babies hear, vocalize and therefore learn," he said.

"We are increasingly technologizing infancy, which may prove harmful to the next generation of adults."

The study was published in the June issue of the Archives of Pediatrics and Adolescent Medicine.