This paper examines between-country differences in why education promotes trust using data from 29 countries (and 146 regions) participating in the OECD's Survey of Adult Skills (PIAAC). Results indicate that education is strongly associated with trust and that individuals' literacy, income, and occupational prestige are important mediators of this association. Contrary to previous studies we do not find that country level or regional level birthplace diversity is associated with average levels of trust. However, education gradients in trust and the extent to which these are due to social stratification or cognitive mechanisms vary both at the country and regional level depending on birthplace diversity. Multilevel mediation-moderation analyses reveal that in countries and regions with greater birthplace diversity there is a greater polarization in levels of trust between individuals with different educational qualifications. This polarization is primarily due to cognitive mechanisms.