Does the probability to join a political party, to become a party functionary, and to leave a party depend on individuals' socioeconomic status? Political parties are central mediating actors between the population and the state; thus, it is reasonable to assume that unequal participation within parties fosters unequal political representation. However, due to data limitations no study has hitherto examined the social selectivity of the whole party membership cycle. We shed light on these issues by analyzing original data from the German Party Membership Study 2017. We find that socially disadvantaged individuals are less likely to become and to stay party members and have a lower proclivity for holding political offices. These effects persist even after controlling for socialpsychological variables and the general incentives for party membership. However, in line with recent findings on voter turnout we show that social selectivity is partly mediated by political efficacy.

Die regelmäßige Berichterstattung über die Mitgliedschaften der deutschen Parteien hat in der Zeitschrift für Parlamentsfragen eine lange Tradition. Seit 2001 legt Oskar Niedermayer in jedem Jahrgang differenziert die Entwicklung der Mitgliederzahlen dar. 1 Die Datengrundlage dieser Publikationen bilden die von den Parteien geführten Mitgliederverzeichnisse. Niedermayer informiert auch über die Zusammensetzung der Parteimitgliedschaften im Hinblick auf die beiden Merkmale Geschlecht und Alter. Dies sind die einzigen soziodemographischen Variablen, die in den Mitgliederverzeichnissen aller Parteien einigermaßen verlässlich erfasst sind. Im Falle der beiden Unionsparteien kommt noch die Konfessionszugehörigkeit zum Zeitpunkt des Parteibeitritts hinzu. So wichtig die von Niedermayer regelmäßig publizierten Zahlen sind, so notwendig ist ihre Ergänzung um detaillierte Informationen über die Zusammensetzung der deutschen Parteimitgliedschaften hinsichtlich weiterer sozialstruktureller Merkmale. Solche Informationen können allerdings nur im Rahmen groß angelegter Befragungen generiert werden, die in den wichtigsten deutschen Parteien gleichzeitig mit einem weitgehend identischen Fragebogen auf der Grundlage bundesweit repräsentativer Stichproben durchgeführt werden. Solche umfassenden nationalen Parteimitgliederstudien hat es in Deutschland bislang dreimal gegeben.

In this paper, the thesis is put forward that selective outcome incentives for party membership gain relevance over time. Two possible mechanisms are identified as the cause of this increase in importance: a supply-side mechanism based on processes of societal change that took place through generational succession and a demand-side mechanism focusing on shifts in the self-image and organizational structure of political parties. The supply-side mechanism should lead to changes in the motives of potential members, whereas the demand-side mechanism alters the incentives potential and current members are exposed to.The empirical analyses are based on the German Party Membership Studies of 1998, 2009 and 2017. These three studies are nationwide representative surveys of the members of the following six parties: Christian Democratic Union (CDU), Christian Social Union (CSU), Social Democratic Party of Germany (SPD), Free Democratic Party (FDP), Alliance90/The Greens (Bündnis90/Die Grünen), and The Left (Die Linke). Within the German Party Membership Studies, both the motives for joining the party and the current membership motives are surveyed.Empirically, it is shown that there is indeed an increase in the importance of selective outcome membership motives over the period under study. The mechanisms behind this increase in importance are investigated using multivariate Age-Period-Cohort (APC) models based on the cumulated data of the German Party Membership Studies. These analyses are based on a total of nearly 30,000 cases. The results of the APC analyses are largely consistent with the supply-side explanation of the increased importance of selective outcome motives for party entry and membership. The demand-side explanation is only partially confirmed.