(Ir)rational choices : the impact of learning on party policy moves Die Arbeit untersucht den Einfluss von Lernen auf die ideologische Veränderung von 137 Parteien in 22 OECD Staaten von 1950 bis 2013. Parteien lernen über die Effektivität von policy moves mit Blick auf Stimmengewinne bzw. verluste von sich selbst ihren Konkurrenten und anderen Parteien im Ausland und treffen anschließend eine (begrenzt) rationale Entscheidung für die Richtung einer Veränderung. Die interne Machtbalance zwischen Parteeliten und Aktivisten bedingt dabei welche Informationsquellen die finale Entscheidung beeinflussen. The thesis is about ideological change of political parties and the way parties gather information learn by updating their beliefs and ultimately make rational choices . Analyzing 1451 policy moves of 137 parties in 22 OECD countries from 1950 to 2013 it is a story about rational learning about emulating other parties abroad and chasing public opinion. Yet the internal life of a party conditions the effects when activists have some influence over the formation of party policy. As volunteers facing a scarcity of time and resources members of the party on the ground have a different information horizon and may arrive at the opposite decision where to move than party elites which (can) rest their decision on a broader set of information resources. In some parties the party on the ground thus constitutes an internal wall of resistance to the strategy party elites would choose if they were free from constraints.