A study of 4,000 six- and seven- year-olds published today shows that children born in the summer do least well in national tests for seven-year-olds in English, maths and science, probably because they are the youngest in the class.
It also reveals that children who start school when they are either just four or already five are at a disadvantage.
The findings from the National Foundation for Educational Research come as the Government is considering changes in education for under-fives, including a statutory starting date for school of four rather than five.
Children born between May and August do significantly less well in the seven-year-old tests whenever they start school, the researchers say. Those born between September and December do best, followed by those born between January and April.
The report published in the journal Educational Research says: 'The season of birth should be taken into consideration when looking at national curriculum outcomes.'
Children who start school too young, when they are between four and four years and four months, are also at a disadvantage. The researchers reject the theory that some summer-born children suffer because they spend less time in the classroom than their classmates born at different times of year.
Caroline Sharp, one of the researchers, said: 'Summer-born children seem to suffer because the school year begins in the autumn and they are always the youngest in the class. Teachers may have lower expectations of them than they do of older children.'
Dates of admission to school vary in different local authorities. Because some schools admit children only in the September of the school year after they are five, pupils with summer birthdays may have to wait a year longer than those with September birthdays before they start school.
The researchers, Caroline Sharp, Dougal Hutchison and Chris Whetton, found that summer-born children who spent nine terms in infants school had similar results to those who had only six terms in school.
Autumn and spring-born children who spent longer in school did achieve better test results.
Boys born in the summer can be at a double disadvantage since girls do better in the tests in both English and maths.
The research also examines the theory that summer-born children admitted in the September before their fifth birthday are at a disadvantage because they are the youngest in the class.
Researchers in the United States have persuaded some parents to keep their children out of school for a year so that they become the oldest in the class.
The foundation's study suggests that the best age to start school is between 4 years 6 months and 4 years 11 months, though children who start at 4 years 4 months do only slightly less well than this group.
Researchers say teachers need to take a child's season of birth into account when using seven-year- old test results. They also need to be aware that childen may be making less progress than classmates because they are younger.
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