The high rate of increase of ruling politicians' wealth has been empirically proven many times. However, in the literature it is almost always assumed that politicians grew rich faster due to political rent-seeking or corruption. The aim of this article is to discuss the assumption whether corruption and rent-seeking is indeed the only possible cause, and to present empirical findings undermining the assumption. The results of the analysis of levels and rate of growth of Polish politicians' wealth clearly show that the other explanation is the selection of people exercising authority. Based on statistical analysis of 2024 asset declarations of 689 councillors from Polish voivodeship assemblies from two terms in the period of 2010–2018, the paper demonstrates that the different rates of changes of the value of assets of coalition and opposition councillors are at least partly the effect of the selection bias.