Professor Craig Ramey, of the University of Alabama, told last week’s American Association for the Advancement of Science conference in Baltimore that children from the poorest backgrounds benefited most from such intervention. But programmes had to start very early - “intervening at kindergarten age produces only a minuscule benefit compared with intervening at the age of one,” he said.

There was now a substantial amount of evidence that intellectual development could be influenced by programmes in which the child was given the proper stimulation in a pleasant environment, he said. This might include simply talking to the child, something which most parents would take for granted.

Nevertheless, the type of programme advocated by Professor Ramey is not cheap. While Head Start, the US pre-school programme which has been criticised for failing to make much difference, costs Pounds 3,300 a year for each child enrolled, Professor Ramey’s approach is at least twice as expensive.

In some trials he has taken children as young as six weeks, rather than the four-year-olds who enter Head Start. His programmes are intensive, but do not attempt to “force-feed” the children. The main cost is paying the highly-trained teachers, who each look after a maximum of three children five days a week, 50 weeks a year.

Years of follow-up have demonstrated clear and lasting benefits, he said, with improved performances in maths and reading, and a reduced need for special education.

Experience with children from Romanian orphanages suggested that there was a critical period during which a child’s brain was still developing when it could be helped. “The longer a child has been in an unstimulating environment, the less responsive he or she becomes,” Professor Ramey said. “The old idea that there are critical periods for intellectual development appears to have had some truth in it.”

Nigel Hawkes is science editor of The Times