Socioeconomic background is a key factor in academic success in the OECD, with students coming from families in the top 10% of the income distribution being three years ahead in reading skills compared to those from the bottom 10%. However, the link between background and performance was weakest in Australia, Canada, Denmark, Estonia, Finland, Japan, Norway, South Korea, and the United Kingdom, meaning these countries have the most equitable education systems. A proposed method of assessing the equality of educational opportunities in a given society is to measure the heritability of academic ability as empirical evidence does support the hypothesis that the heritability of test results is higher in a country with a national curriculum compared to one with a decentralized system; having a national curriculum aimed at equality reduces environmental influences.