This article is a response to calls in prior research that we need more longitudinal analyses to better understand the foundations of PSM and related prosocial values. There is wide agreement that it is crucial for theory building but also for tailoring hiring practices and human resource development programs to sort out whether PSM-related values are stable or developable. The article summarizes existent theoretical expectations, which turn out to be partially conflicting, and tests them against multiple waves of data from the German Socio-Economic Panel Study which covers a time period of 16 years. It finds that PSM-related values of public employees are stable rather than dynamic but tend to increase with age and decrease with organizational membership. The article also examines cohort effects, which have been neglected in prior work, and finds moderate evidence that there are differences between those born during the Second World War and later generations.