Worldwide Shadow Education and Social Inequality: When Do Shadow Systems Become a Thread to Equality of Educational Opportunities? a Comparison of 50 Countries

International research highlighted the expansion of supplementary education markets worldwide and often emphasized that such a development would inevitably result in growing educational and social inequality in numerous countries. However, research often falls short of providing hard empirical evidence based on directly comparable, international data on the subject, enabling us to scrutinize at which developmental stage shadow education systems become a thread to equality in educational opportunities. Based on theoretical considerations using new institutionalist theory, differences in the effects of social origin on shadow education participation according to developmental stage of shadow education systems, which are classified as being either advanced, diversified, or traditional, are hypothesized. To test whether the impact of shadow education varies according to a country’s developmental stage of its shadow system, data of the 2012 Programme for International Student Assessment (PISA) are used to examine prevalence (country-specific enrolment rates), intensity (country-specific duration of enrolment per week), institutionalization level (country-specific proportion of students enrolled in supplementary schools), and the primary functions of shadow education (country-specific strategies with which shadow education is pursued) to classify shadow education systems in 50 countries. Finally, the impact of students’ family background on their probability to participate in shadow lessons according to shadow system is calculated (social inequality increase or reduction). Findings show that shadow education considerably affects social inequality in advanced (e.g. Korea, Greece, or Russia), and increasingly, diversified systems (e.g. Poland, Australia, or Germany). A reduction of social inequality is highly unlikely, even in traditional shadow systems (e.g. Finland, Belgium, or the United States). These findings imply that with the international expansion of shadow education, inequality of educational opportunities will only grow further, wherefore the development of these shadow systems and how to adequately respond to their expansion calls for more recognition of and more research in the field.