There is no evidence to date that another year in nursery under deferred entry rules makes any difference in the long term, according to two Glasgow psychologists.
Beth Hannah and Maureen Myant, senior psychologists in the south-west of the city, say there is little research to allow parents and nursery heads to make decisions.
Under the Education 2000 Act, parents were given the right to delay their child's entry to primary 1 if they felt they were too young at four and a half. Many argue that their child is not ready or too immature to cope because others can be a full year older.
Mrs Myant said: "Twenty years ago there was a surge of parents desperate to get their children into school at four and a half or even earlier. Now it seems to have gone the other way."
The two psychologists have conducted two surveys, one in 1999 and the other this year, covering 27 nurseries, but they say the results produce more questions than answers.
Research in the United States and Britain showed that teachers expect more from older children and tend to gear classes to their needs. Mrs Myant said that with older pupils being held back in nursery, the pre-school curriculum could begin to change.
Evidence from the south side of Glasgow showed that boys were far more likely to be held back.
There was "a gut feeling" among other psychologists that pupils referred to them had been left behind in school simply by being born in January or February.
The researchers say it is important to provide an early years curriculum that meets the needs of an increasingly diverse pupil population to bring about a better transition between nursery and primary.
Each of the city's 29 school clusters now has a nurture group for young primary pupils who are falling behind.