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Cox and Katz: Why did the incumbency advantage in U.S. House elections grow

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Cox and Katz. 1996. Why did the incumbency advantage in U.S. House elections grow?. AJPS 40 (May): 478-497.

In Brief

The authors seek to explain the incumbency advantage (Y). By "incumbency advantage," they refer to the fact that incumbents tend to win a larger vote share (even though, as Jacobson 1987 found, this increased vote share doesn't translate into better survival odds for incumbents). They argue that three variables (X) explain the increase in incumbents' vote shares (what they term the "vote-denominated incumbency advantage").

  • First, there are the direct resources that candidates gain by holding office: Franking, staff, constituent service, and so on.
  • Second and third, there are the indirect effects conferred by incumbency. This has two multiplicative (i.e. interactive) elements:
    • The "scare-off" effect: High-quality candidates (usually people who currently hold some other office) face greater risk if they challenge an incumbent (i.e. they lose their current office), so having an incumbent in the race lowers the quality of challengers.
    • The "quality" effect: Clearly, the scare-off effect matters only to the extent that the "quality differential" (i.e. the difference in quality between the two candidates) matters. If the quality differential matters more for electoral outcomes today than it did fifty years ago (i.e.the "quality" effect has increased over time), then this will interact with the "scare-off" effect to strengthen the incumbency advantage.

Place in Literature

The authors identify two main strands of thinking about the incumbency advantage. The first stresses resources, and it is largely subsumed into the authors' "direct" effects. This approach has considerable evidence backing it from both the state and federal levels. The second approach (mainly Ferejohn's) argues that dealignment has caused voters to rely less on partisanship as a cue and more on other cues, like incumbency. This approach has very little empirical support.

The authors' main contribution is to identify a "quality" effect. (The "scare-off" effect is not new; see, for example, Jacobson and Kernell 1983.)

The Argument and Empirics

As stated above, the three primary variables (to explain the rise in incumbents' vote shares) are resources ("direct effects"), the scare-off effect, and the quality effect. The latter two interact to create the "indirect effects" of incumbency.


  • Direct effects refer to the money, staff, franking, casework, etc that directly helps incumbents win campaigns. Unclear how they measure this.
  • Indirect effects refers to the product of the scare-off effect and the quality effect.
    • Scare-off effect: Begin by measuring the Democratic candidate's quality advantage. Candidates are coded 1 if they have held elective office before and 0 otherwise. Subtract the Democrat's score from the Republican's score for the Democrat's quality advantage. Next, equation 2 (p 485) gives a regression to predict the Democrat quality advantage. Sigma (the coefficient on the incumbency variable) picks up the amount by which having an incumbent in the race changes the quality advantage. This is the scare-off effect.
    • Quality effect: Measured as the Democratic candidate's quality advantage (see above). The more this matters, the more quality matters.


Figure 1 shows how the incumbency advantage (in terms of vote shares) has increased over time. This is the outcome to be explained. Table 2 (Panel A) shows that the indirect effects have a significant relationship with this growth in the incumbency advantage. Of the two components that make up these indirect effects, the quality effect is the only significant one (see Table 2, Panel B). Figure 2 shows this relationship graphically: The quality effects have risen dramatically with time, raising the indirect effects with them.

Why Has the Quality Effect Increased?

The authors' primary purpose is to demonstrate empirically that the "quality" effect has become more important over time. They do speculate, however, on what might be driving this. With minimal evidence, they present the following basic speculative logic: The rapid series of court-ordered realignments that began in the 1960s (to correct malapportionment) undermined the ability of the old (local) party organizations to help candidates win campaigns; the party organizations found themselves less well-adapted to their local niches. Thus, candidates could rely less on local parties to help them win. As such, candidates with better campaign skills (i.e. high-quality candidates) had a considerable advantage.

Thus, redistricting drove a shift from party-centered to candidate-centered campaigns. "Rather abruptly, the value to a party of having an experienced candidate grew." As such, the importance of the quality differential grew.


Jacobson (1987) showed that incumbents still lose just as much as they ever did even though they tend to win by larger margins now. Thus, a theory of the "vote-denominated incumbency advantage" (like this one) must explain why incumbents win by larger margins without predicting that these larger margins will actual lower the probability of defeat. It's not clear that Cox and Katz's theory meets this standard. Their logic would (as I read it) predict that incumbents would actual win more frequently, not just win by larger margins. And if this is true, then their logic predicts something that (empirically) does not occur: incumbents win more frequently now than they did in the 1950s.