Lead researcher Dimitri A. Christakis of the University of Washington School of Medicine looked at 329 infants age 2 months to age 4. The children wore a small, business card-sized, 2-ounce digital recorder on random days monthly for up to two years.
A specially designed vest with a chest pocket held the recorders at a specific distance from the mouth, and captured everything the child said and heard during continuous 12-16 hour periods.
The recorders were removed only for naps, baths, nighttime sleep and car rides. A speech identification software program processed the recorded files to analyze sounds children were exposed to in their environment, as well as the sounds and utterances they made.
"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," Christakis said in a statement. "These results may explain the association between infant television exposure and delayed language development."
The findings are published in the Archives of Pediatrics & Adolescent Medicine.