An Evolutionary Theory of Democracy: Dynamic Evolutionary Models of American Party Competition with an Empirical Application to the Case of Abortion Policy from 1972-2010

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Montgomery, Jacob Michael


Aldrich, John H

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In this dissertation, I challenge the unitary-actor assumption of contemporary theoretical models of American politics and re-conceptualize party competition as an evolutionary process. I begin by discussing the assumptions of Darwin's theory and their applicability to American party competition. Building on these assumptions, I then develop a formal evolutionary model of party competition that I test against empirical data regarding the two parties' shifting stances on abortion policy from 1972-2010.

Chapter 2 presents several single-party models that focus on explaining the conditions that must hold for parties to emerge as populations in an evolutionary sense. I show that only when candidates experience common selection pressures will population dynamics arise.

Chapter 3 extends this model to two-party competition wherein population dynamics are sustained by inter-party competition for votes and intra-party competition for activist resources. Two-party competition provides the necessary selection pressures needed to foster the emergence of coherent and distinct party populations. However, this will only be the case when: (1) party resources are valuable for winning elections, (2) the distribution of party resources are biased towards ideologically extreme candidates, and (3) parties have sufficient resources.

In Chapter 4, I extend the model to a multi-dimensional setting. Previous theoretical work on multi-dimensional party dynamics has been divided between (1) analytical models that provide stable equilibria results and (2) qualitative theories that seek to explain the dynamic process of party change. In this chapter, I present a formalized model that makes precise predictions regarding both the environmental conditions that lead to locally stable policy positions and the dynamic process that occurs as the parties drift from one stable configuration to another in response to changing environmental conditions.

Finally, in Chapter 5, I apply my model to the case of abortion policy in the United States from 1972-2010. Using data from public opinion surveys and Congressional roll-call votes, I show that party polarization on abortion was driven by changing activist preferences and that this shift occurred almost entirely as the result of incumbent replacement. These results support my ecological party model and demonstrate its ability to account for the kinds of gradual party movements -- driven by incumbent replacement -- that characterize many important historical shifts in party platforms.





Montgomery, Jacob Michael (2011). An Evolutionary Theory of Democracy: Dynamic Evolutionary Models of American Party Competition with an Empirical Application to the Case of Abortion Policy from 1972-2010. Dissertation, Duke University. Retrieved from


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