This article seeks to identify the determinants of the existence and powers of upper legislative chambers. By analyzing their constitutionally established powers, including all of the minimally democratic bicameral countries, it was possible to reach conclusions not previously suggested in the literature. Using a two-step model whose calculation includes the possibility of a selection bias (Heckman model), the study evaluated both the factors leading to the existence of an upper chamber and those defining its powers. Federalism proved to be an important variable for explaining the difference between bicameral and unicameral countries, but not for explaining the political strength of upper chambers. The most relevant explanatory variable in this sense was the system of government: in Parliamentary systems, upper chambers have limited powers; in Presidential systems they exercise broad powers