Which psychological and linguistic factors allow a person to understand related, but different, languages? In 2012, the European Commission called for research to investigate the mutual intelligibility among closely related European languages to improve communication within the EU. Existing correlational evidence suggests that mutual intelligibility may depend not just on linguistic determinants (e.g., lexical, syntactic, pronunciation similarities) but also on extra-linguistic determinants (e.g., exposure, cognitive skills, personality). I devised an innovative questionnaire which includes not only tasks to measure the level of mutual intelligibility, but also cognitive games and questions to measure personal attributes, as well as proxies for respondents' effort. The survey design allows me to identify the causal impact of language itself, of cultural exposure, and of personal attributes on an individual's ability to understand related, but different, languages. I distributed the survey to 200 students in Slovakia divided into three groups: native speakers, non-native speakers, and heritage speakers (individuals who have grown up in a home where a language other than Slovak is spoken). I tested their ability to understand listening and reading tasks in Slavic languages of the European Union (Polish, Czech, Slovak, Slovene, Croatian, Bulgarian). Looking across these groups allows me to tease out the role of language itself in mutual intelligibility (which is equal for all respondents) and the impact of extra-linguistic determinants such as exposure, which is not present for heritage or non-native speakers. Controlling for a person's score on tasks in their native language, as well as for response times, and audio replays - proxies for exerted effort - allows me to obtain unbiased estimates of the determinants of mutual intelligibility. I provide recommendations that lay the foundation for an improved pedagogical approach to the teaching of languages which emphasizes the factors, skills, and personal attributes that facilitate mutual intelligibility.