It is conventional political science wisdom that electoral systems have political consequences. In order to systematically examine these consequences, we focus on the effects of electoral reforms in Central and Eastern European democracies. By analysing the consequences of electoral system changes for party systems and disproportionality, we make use of a quasi-experimental approach by isolating a single treatment – the electoral law change – and controlling for all other variables. Along the lines of classical electoral research, we argue that the introduction of more permissive rules will result in an increase of electoral and legislative fragmentation and a reduction of disproportionality while more restrictive rules will have the opposite effects. The article’s genuine contribution to the debate in the field of electoral research is the introduction of a dynamic perspective. Once we clearly distinguish between mechanical and psychological consequences of electoral systems and analyse them simultaneously, we can develop different hypotheses on the temporal patterning of these effects and hypothesise on the variations of electoral rule change effects over time. While mechanical effects set in immediately and quickly wear off, psychological effects, which are based on learning processes and strategic coordination, develop over time.