Why did the incumbency advantage in US House elections grow?

Theory: A simple rational entry argument suggests that the value of incumbency consists not just of a direct effect, reflecting the value of resources (such as staff) attached to legislative office, but also of an indirect effect, reflecting the fact that stronger challengers are less likely to contest incumbent-held seats. The indirect effect is the product of a scare-off effect-the ability of incumbents to scare off high-quality challengers-and a quality effect-reflecting how much electoral advantage a party accrues when it has an experienced rather than an inexperienced candidate. Hypothesis: The growth of the overall incumbency advantage was driven principally by increases in the quality effect. Methods: We use a simple two-equation model, estimated by ordinary least-squares regression, to analyze U.S. House election data from 1948 to 1990. Results: Most of the increase in the incumbency advantage, at least down to 1980, came through increases in the quality effect (i.e., the advantage to the incumbent party of having a low-quality challenger). This suggests that the task for those wishing to explain the growth in the vote-denominated incumbency advantage is to explain why the quality effect grew. It also suggests that resource-based explanations of the growth in the incumbency advantage cannot provide a full explanation.