Around 60% of people report some form of non-monogamy – current, past or fantasized – yet they believe only 35% of others do the same. And while few would criticize someone in an open relationship, most assume others would. So is monogamy held in place by real agreement, or by misperception? In this talk, sociologists Antonia Velicu and Heiko Rauhut of the University of Zurich present first findings and a new research agenda exploring how relationship norms are changing in Switzerland, Austria and Germany – and what helps people choose non-monogamy despite social pressure.