Contrary to the long held view that political institutions are persistent
and help structure the political process in such a way that policy
equilibriums emerge, recent research has found an astonishing number of
institutional changes in established democracies. Why is that? Theory
building lags behind our empirical knowledge. I propose a model that starts
from the time horizons of political actors with regard to government
participation. It seeks to explain why political actors opt for
institutional reform and when they take the initiative. Political actors
advocate reform to the extent that they are disadvantaged with regard to
time in government. Once they get into government they use this window of
opportunity and establish institutions that help them increase their future
share of government time. I derive a number of empirical implications from
the model. These are tested both quantitatively and qualitatively.