In most PISA-participating countries and economies, the average socio-economic background of students who attend privately managed schools is more advantaged than that of those who attend public schools. Yet in some countries, there is little difference in the socio-economic profiles between public and private schools. Why? An analysis of PISA results finds that while the prevalence of privately managed schools in a country is not related to socio-economic stratification within a school system, the level of public funding to privately managed schools is: the higher the proportion of public funding allocated to privately managed schools, the smaller the socio-economic divide between publicly and privately managed schools. This report also shows that those countries with narrow socio-economic stratification in their education systems not only maximise equity and social cohesion, but also perform well in the PISA survey. Annexed are: (1) Technical Background; and (2) Data tables.