Studies on academic inbreeding have mostly focused on institutional inbreeding and its negative effects, whereas little research has explored its causes. We identify current explanations of the macro-, meso- and micro-level factors that sustain academic inbreeding as well as research gaps. We address a main research gap regarding what macro-level factors contribute to academic inbreeding, by analysing systems' norms and rules regulating access to senior academic positions and teaching language requirements in France, Germany, Italy and Spain, the largest public university systems of the European Union. The analysis reveals that career rules designed to guarantee quality may have unintended effects in terms of academic inbreeding. Most importantly, the habilitation procedures pose greater challenges to international candidates and often increase barriers between disciplines as well. In some disciplines and regions, language requirements contribute substantially to academic inbreeding.