Much of the empirical work and the conceptual discussion of the impact of institutions on economic development either implicitly or explicitly assumes that institutions persist. Although Acemoglu et al. (2001) provide evidence that constraints on the executive persist, many aspects of institutions change frequently. Less-developed countries cycle between democracy and dictatorship and often change their constitutions. Relatedly, while the current economic problems in Latin America are often traced back to colonial times (Stanley L. Engerman and Kenneth L. Sokoloff, 1997; Acemoglu et al., 2002), the specific institutions that once underpinned the colonial economy, such as the encomienda, the mita, or slavery, vanished long ago. These observations suggest that we need to develop a framework in which changes in certain dimensions of institutions are consistent with overall institutional persistence. In this paper, we make an attempt to highlight some important mechanisms for understanding simultaneous change and persistence in institutions. Institutional persistence, in this context, refers to the persistence of a cluster of economic institutions, such as the extent of enforcement of property rights for a broad cross section of society (Acemoglu et al., 2001). Such lack of property rights enforcement may be driven by quite different specific institutions, e.g., risk of expropriation, entry barriers, or economic systems such as serfdom or slavery. In turn, these different specific economic institutions may exist under different political institutions, including dictatorships, absolutist monarchies, oligarchies, and corrupt or even populist democracies. Given this rich array of possibilities, a useful framework must specify which aspects of institutions can change, which others have a tendency to persist in equilibrium, and how the persistence of certain types of institutions could have lasting effects on economic outcomes. In this paper, we provide a simple model of the coexistence of change and persistence in institutions. First we describe some existing approaches to the persistence of social arrangements, as well as the essence of the mechanism we propose. Next, we illustrate the key issues using the experience of the southern United States, and last we provide a simple model that formalizes the main mechanism in the paper. We conclude by discussing a complementary mechanism to the one presented in this paper.

The introduction of Medicare in 1965 was the single largest change in health insurance coverage in U.S. history.Many economists and commentators have conjectured that the introduction of Medicare may have also been an important impetus for the development of new drugs that are now commonly used by the elderly and have substantially extended their life expectancy.In this paper, we investigate whether Medicare induced pharmaceutical innovations directed towards the elderly.Medicare could have played such a role only if two conditions were met.First, Medicare would have to increase drug spending by the elderly.Second, the pharmaceutical companies would have to respond to the change in market size for drugs caused by Medicare by changing the direction of their research.Our empirical work finds no evidence of a "first-stage" effect of Medicare on prescription drug expenditure by the elderly.Correspondingly, we also find no evidence of a shift in pharmaceutical innovation towards therapeutic categories most used by the elderly.On the whole, therefore, our evidence does not provide support for the hypothesis that Medicare had a major effect on the direction of pharmaceutical innovation.