Previous research has described, theoretically and empirically, the negative consequences of a politicized civil service, recruited on political instead of professional merits. However, we know very little about what explains politicization, especially outside the context of the USA. This study reviews literature in the field and identifies three commonly used explanations that are then tested in a study of recruitment of agency heads in Sweden between 1960 and 2010. For this purpose, we collected new data that includes information about 1,812 agency heads' political and/or professional backgrounds and 1,608 appointments to agency head positions. Our study demonstrates that none of the previously used explanations – ideological bias of the government, the number of years that the government has been in power, and the parliamentarian support for the government – holds. Our general conclusion is therefore negative and emphasizes the need for more systematic research on this question.