'Birthdate Effects: A Review of the Literature from 1990-on', published by the Cambridge Assessment, part of Cambridge University, says that developmental psychology suggests that children between the ages of four and five may not be ready for formal schooling. 
It says factors such as leaving familiar surroundings, facing separation from their parents and adapting to new routines could help to explain why children born in the summer perform less well overall in exams than those born in autumn or winter.

Data from 13 local education authorities (LEAs) providing GCSE results undertaken in 1990 to 1994 shows that summer-borns achieved the lowest results in ten LEAs and autumn-born children were the highest achievers in nine LEAs, the study found.

The review concludes that the birthdate effect is much weaker in countries where formal schooling begins at a later age.
In a letter to Sir Jim Rose, who is conducting a Government-backed review into primary education, Tim Oates, group director of assessment research and development at Cambridge Assessment, called for urgent research into how best to remedy the birthdate effect.
An interim report published in December by Sir Jim Rose said that summer-born children should start reception the September after they turn four but that some children should attend part-time (News, 8 December 2008).
Mr Oates said, 'For years, evidence of a birthdate effect has stared out of qualifications data; summer-born children appear to have been strongly disadvantaged. While those responsible for working on these data have, through mounting concern, periodically tried to bring public attention to this very serious issue, it has been neglected by agencies central to education and training policy.'
The study adds that teachers could be contributing to the birthdate effect by not taking into account children's relative levels of maturity when assessing their ability.