It is also a clear contradiction of the principle of equality of opportunity to which all OECD countries aspire. Allowing children to fall unnecessarily far behind brings in its wake a long list of practical costs and consequences. Causality is always difficult to establish, but many hundreds of studies in many different OECD countries have shown what the costs of falling too far behind may be. But this cannot change the fact that children who fall behind early in their lives, or who spend a significant part of their early years in poverty, are likely to find themselves at a marked and measurable disadvantage.