In many European countries public opinion is polarized on issues such as immigration, inequality, populism, and trust in institutions. Although for each issue there is an extended literature, there is a pressing need for integration. Are opinions on these issues related and, if so, what is the glue that binds them? Do different groups of people polarize on different issues and/or for different reasons? Our first objective is to determine how identities and threat combine to generate multiple polarized attitudes. First, we use the novel technique of correlational class analysis to identify subpopulations with unique belief systems, consisting of threats, identities, and polarized attitudes. These analyses are followed by experiments that test causal effects of identities and threats, and how these may differ between subpopulations with different belief systems. Our second objective is to compare subpopulations of belief systems across countries and over time. Therefore, cross-country differences in belief systems will be related to variation in the political landscape (e.g., political polarisation), and differences in social structural country characteristics (e.g., inequality and meritocratic beliefs). Longitudinally we will examine the impact of the financial crisis on belief systems. Crucially, identifying subpopulations with different belief systems will help not only in understanding polarisation, but also in identifying solutions, which are expected to differ depending on the belief system. Democratic innovations such as citizen fora have been developed to overcome polarisation. We will test whether using our insights on threats and identity can make such fora more effective.