People differ in how they use language, just as they differ in many other abilities. These differences can be seen in language skills such as vocabulary and grammar, as well as in general-cognitive abilities like processing speed and working memory. While people easily handle these differences in everyday communication, research in the field of psycholinguistics has traditionally treated such individual differences as random or unimportant. This thesis moves beyond the average and focusses on these individual differences by introducing the Individual Differences in Language Skills test battery for German speakers (IDLaS-DE) – a new tool to measure different language skills. This test battery is based on a similar tool developed for Dutch (IDLaS-NL), and both are designed to be as comparable as possible. The aim of this thesis is to encourage researchers to study individual differences more systematically and to support cross-linguistic work in general. In doing so, this thesis provides valuable insights into the overall architecture of the language system, helping us to better understand how language works, both within and across different languages.
Sandra Bethke was born in Haren (Ems), Germany, in 1993. She obtained her Bachelor’s degree in Applied Linguistics from Technical University Dortmund in 2016, with minors in Philosophy and Political Science. She then obtained her Master’s degree in Linguistics from Heinrich Heine University Düsseldorf in 2021, with a specialization in Neuro- and Psycholinguistics. While obtaining her Master’s degree, Sandra was working as an HR Manager in Cologne, Germany. In 2021, she started her PhD in the Psychology of Language department at the Max Planck Institute for Psycholinguistics, where she was also actively engaged in the Diversity, Equity and Inclusion Committee and the Sustainability Working Group, among others. Sandra completed her PhD in November 2025 and is currently still working at the Max Planck Institute for Psycholinguistics as an HR Advisor.