In analyses of labour market returns to education three competing theoretical approaches dominate the empirical research literature: education as productive skills, education as positional good, and social closure. We argue that, while these approaches might be suitable to explain the mechanisms behind labour market returns to vertical differences in educational levels, they are not appropriate to account for variation in labour market returns if field of study as a horizontal dimension of education is considered. We present a theoretical approach that addresses this horizontal dimension of education by disentangling the various mechanisms behind the field of study differences in graduates’ labour market returns.