An extra hour of television watching beyond recommendations set by the American Academy of Pediatrics (AAP) diminishes a preschooler's vocabulary and math skills as well as classroom attention, a new study suggests.

Every hourly increase in daily television watching from two and a half years old is also associated with bullying by classmates, and physical prowess at kindergarten, said Professor Linda Pagani of the University of Montreal and the CHU Sainte-Justine children's hospital.

The study's authors said it was because at that early age, children's brains are wired for play. "These kids are watching too much television at a time when they should be out there in the environment exploring and interacting, especially with other humans," she said.

Bullying is another consequence because excessive TV watching doesn't teach a child social or motor skills, Pagani added. "If you don't learn those skills, you get to kindergarten and cry every time someone takes your toy."

The AAP currently discourages watching television during infancy and recommends not more than two hours per day beyond age two.

The study involved 991 girls and 1006 boys in Quebec whose parents reported their television viewing behaviour as part of the Quebec Longitudinal Study of Child Development.