Putting a child into nursery or day care could put their future health at risk, a leading psychologist has claimed.

Writing in The Biologist, Dr Aric Sigman - a fellow of the Royal Society of Medicine, has said that being cared for by strangers could send the levels of stress hormones soaring in youngsters.

The increase in stress levels only appear up until the age of three, he said, although increases in the hormone cortisol has been linked to lower resistance to infection and, in the long term, an increased risk of heart disease.

Writing on his website (www.aricsigman.com), Dr Sigman said: "Few stop to consider that day care is an evolutionary novelty, which has grown suddenly and rapidly.

"In other areas of child health and development, when considering the potential effects of profound new developments, our society instinctively adopts a principle of precaution. This has not been the case with day care. The accepted position at the moment considers day care attendance as an accepted healthy practice which both scientists and society must consider equivalent in terms of child wellbeing and later development.

"While of course the long-term effects of day care and cortisol release are not fully understood, emphasising the possible negative implications in keeping with our tradition of a principle of precaution is justified and prudent. In the case of early child care we should remind ourselves that when it comes to an issue of such fundamental importance, we must invoke the Hippocratean medical principle of 'first do no harm', and subscribe to an a priori assumption that generally maternal care during early child development is better than day care for child wellbeing and later development.

"Moreover, it should be incumbent on those with an open mind on this matter to provide overwhelming evidence that paid day care workers can elicit the same intimate and often unique interactions that occur between mothers and babies."

However, Stuart Derbyshire, a University of Birmingham psychologist told the Daily Mail that children in day care may have higher cortisol levels because they run around more - not because they are stressed.

Dorothy Bishop, professor of developmental neuropsychology at Oxford University, added: "There is broad consensus that day-care influences cortisol levels in the short term, but there is no evidence that this has long-term detrimental consequences."

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