For small children learning language skills, watching TV and using computers are no substitute for a parent's love and attention, according to a group of leading psychologists.
The five childcare experts hammered home their message to parents at the world's biggest science conference in Vancouver, Canada.
All agreed that sitting toddlers in front of a TV or computer screen was unhelpful and may even obstruct their progress.
The group was discussing the problem of "late talkers" - children who are slow to learn to talk and often held back in their educational development.
Studies suggest that between 6% to 8% of all children fall into this category. Incidence rates are the same in the developed and undeveloped world.
By the age of two, a child should have a vocabulary of at least 50 words and be attempting to put together a couple of sentences.
However, some are still struggling to master words at three.
Although they often appear to recover, research has shown that impairments can show up in the later teenage years.
The experts all made the point that passive listening does not provide the same stimulation as the to-and-fro interaction of personal contact.
Professor Nan Bernstein Ratner of the University of Maryland said: "There's been recent work distinguishing between overheard speech as opposed to speech directed to the child, and overheard speech is much less effective as a predictor of what children learn than time spent actually interacting with the child.
"We have a very durable body literature that suggests that you can't substitute the kind of input that you give a child by plopping them in front of a television. That the child still needs to be engaged back and forth with another person in order to untangle language."
Colleague Professor Leslie Rescorla from Bryn Mawr College, Pennsylvania, said: "Children really need to have people talk to them in order for them to acquire language. Not that they don't learn anything from television and videos but really they need language partners."
Speech expert Professor Philip Dale from the University of New Mexico said: "I think the limitations of overheard or video observed speech are particularly strong at the early stages of language. It's a totally inadequate input for breaking into the code. It is the case that once you know a language you can perhaps learn more of than what you parents might wish you to learn from video, but that's only once you've essentially broken into the code. Its not a good way to start."
Professor Erika Hoff from Florida Atlantic University added that "brain training" computer programmes designed to stimulate children actually held them back.
"There was actually a study that found that the more parents buy these things and the more hours their children stay in front of them as one year olds, the lower the children's use of language," she said.
"Nobody thinks that these things actually do damage but.. they are bad for very young children."
Rescorla described a word "checklist" she developed that could help parents and child psychologists spot late talkers.
The checklist consists of around 310 words including some of those used most often by young children.
"Parents are asked to check off every word their child says spontaneously," said Rescorla.
Average-performing children can access around 250 to 300 words, she said, while late talkers only manage 25.
Late talkers tended to use the most common toddlers' words, such as "mummy", "daddy", "shoe", "dog" and "bye-bye".
The system was tested with a study in which children were monitored for a period of 15 years.
"What we found was that the children recovered from their language delay at varying rates," said Prof Rescorla. "By four, more of them were performing in the average range, and by the time they went to school most of them were performing in the average range.
"The not so good news was that after age 17 children performed significantly less well on all the language measures we gave. So it looked as if what we were picking up at age two was a sub-clinical but enduring relative weakness in the area of verbal language development."
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