The study of 872 children found no correlation between the amount of time they had spent viewing television before they reached 2 years of age and their progress at age 3.

On average, the children had watched nearly one hour of television per day at the age of 6 months and 1.4 hours a day by age 2, according to the report published in Paediatrics.

Surveys show 68 per cent of babies under 2 view some screen media such as television or a computer on a typical day. The American Academy of Paediatrics recommends children younger than 2 never watch screen media, while some video makers promote their wares as educational.

In the study, children living in homes with lower household income or whose mothers had less education were more likely to watch more TV. Non-white children also watched more.

The children were given vocabulary, drawing, object matching and pegboard tests to evaluate their verbal and motor skills at age 6 months and again at 3.

Those who had watched more television as infants performed less well on tests at age 3, but the effect disappeared after adjusting for the mother's education, vocabulary, household income and other socioeconomic factors.

"Contrary to parents' perceptions that TV viewing is beneficial to their children's brain development, we found no evidence of cognitive benefit from watching TV during the first 2 years of life," wrote Marie Schmidt of Children's Hospital Boston with colleagues from Harvard Medical School and Harvard Pilgrim Health Care.

The findings paralleled those of some other studies that found television viewing had no impact on academic achievement among older children and adolescents - as long as socio-economic factors were taken into account.

But another study cited by the researchers did find more TV viewing at age 3 correlated to less verbal ability at age 6. They suggested the detrimental effect may not show up until children are older than 3 and more verbal. Or there may be more harm done by TV viewing between the ages of 2 and 3 years.