Two studies concluded that putting children into formal education too early was not just a waste of resources, which should rather be invested in primary and secondary schools, but is actually damaging to the children's later academic and social prospects.

A comparison of schooling methods and long term outcomes in Europe, the US, Japan and New Zealand conducted by the independent think tank Politeia concluded that the UK practice of putting children into pre-school at four, and even sometimes at three, yielded no additional academic benefit over starting children in primary school at six (as in most other countries). Furthermore, research showed that children who were allowed a less structured education through singing, playing, hands-on creative work and interacting with other children were more likely to marry, vote and stay out of prison.

Another, non-related study from the University of Wales found that children attending nursery school did no better academically than those who stayed at home.

Ed- There is also evidence that letting children read books close up before age 5-6 can lead to shortsightedness.