As most political scientists know, the outcome of the American presidential election can be predicted within a few percentage points (in the popular vote), based on information available months before the election. Thus, the general campaign for president seems irrelevant to the outcome (except in very close elections), despite all the media coverage of campaign strategy. However, it is also well known that the pre-election opinion polls can vary wildly over the campaign, and this variation is generally attributed to events in the campaign. How can campaign events affect people's opinions on whom they plan to vote for, and yet not affect the outcome of the election? For that matter, why do voters consistently increase their support for a candidate during his nominating convention, even though the conventions are almost entirely predictable events whose effects can be rationally forecast?
In this exploratory study, we consider several intuitively appealing, but ultimately wrong, resolutions to this puzzle and discuss our current understanding of what causes opinion polls to fluctuate while reaching a predictable outcome. Our evidence is based on graphical presentation and analysis of over 67,000 individual-level responses from forty-nine commercial polls during the 1988 campaign and many other aggregate poll results from the 1952–92 campaigns.
We show that responses to pollsters during the campaign are not generally informed or even, in a sense we describe, ‘rational’. In contrast, voters decide, based on their enlightened preferences, as formed by the information they have learned during the campaign, as well as basic political cues such as ideology and party identification, which candidate to support eventually. We cannot prove this conclusion, but we do show that it is consistent with the aggregate forecasts and individual-level opinion poll responses. Based on the enlightened preferences hypothesis, we conclude that the news media have an important effect on the outcome of presidential elections – not through misleading advertisements, sound bites, or spin doctors, but rather by conveying candidates' positions on important issues.
* Gelman, Department of Statistics, University of California, Berkeley; King, Department of Government, Harvard University. We thank Eric Oliver and Maggie Trevor for research assistance, and Larry Bartels, Neal Beck, Tom Belin, Mo Fiorina, John Kessel, Mik Laver, Eileen McDonaugh, Phil Paolino, Keith Poole, Doug Price, Phillip Price, Sid Verba and D. Stephen Voss for helpful comments, and the National Science Foundation for a research grant. All graphs were made using the S system. This is a revised version of a paper which received the Pi Sigma Alpha award for the best paper at the annual meeting of the Midwest Political Science Association, Chicago, 1992.