Nancy Jensen, who took part with her now 6-year-old daughter, said the study was a wake-up call.
"I didn't realize how much Elizabeth was watching and how much she was watching on her own," she said.
Jensen said her daughter's behavior improved after making changes, and she continues to control what Elizabeth and her 2-year-old brother, Joe, watch. She also decided to replace most of Elizabeth's TV time with games, art and outdoor fun.
During a recent visit to their Seattle home, the children seemed more interested in playing with blocks and running around outside than watching TV.