Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I even say who wrote them. If you have more recent summaries to add to this collection, send them my way I guess. Sorry for the ads; they cover the costs of keeping this online.
Mayhew. 1974. Congressional elections: The case of the vanishing marginals. Polity 6:295-317.
Mayhew observes that a rapidly decreasing number of incumbents win by a small margin. The margins have been growing larger. Thus, the incumbency advantage has grown rapidly as well. Note, in Figure 10.1, how the graphs in the left column change from being normal to bimodal. Thus, whereas most elections were once won with the Democratic candidate getting around 50% of the vote, there are now two modes: The Democrat either wins around 35-40 or around 60-65. At the same time, the same districts have remained competitive in presidential and open-seat elections--so the districts themselves aren't changing, incumbency is.
Mayhew's goal is to bring this puzzle to our attention, not to solve it. Nonetheless, he proffers five potential explanations and discusses each briefly.
SEE ALSO notes under Weisberg (1999), part III.
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