Why are American Presidential Election Campaign Polls so Variable When Votes are so Predictable?

British Journal of Political Science, Vol. 23, No. 1, pp. 409-451, October 1993

43 Pages Posted: 17 Jan 2008

See all articles by Andrew Gelman

Andrew Gelman

Columbia University - Department of Statistics and Department of Political Science

Gary King

Harvard University


As most political scientists know, the outcome of the U.S. 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 election 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 and yet reach 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 1952-1992 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 which candidate to eventually support 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. 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 due to misleading advertisements, sound bites, or spin doctors, but rather by conveying candidates' positions on important issues.

Winner of the Pi Sigma Alpha Award for the best paper at the annual meetings of the Midwest Political Science Association.

Suggested Citation

Gelman, Andrew and King, Gary, Why are American Presidential Election Campaign Polls so Variable When Votes are so Predictable?. Available at SSRN: https://ssrn.com/abstract=1084120

Andrew Gelman (Contact Author)

Columbia University - Department of Statistics and Department of Political Science ( email )

New York, NY 10027
United States
212-854-4883 (Phone)
212-663-2454 (Fax)

Gary King

Harvard University ( email )

1737 Cambridge St.
Institute for Quantitative Social Science
Cambridge, MA 02138
United States
617-500-7570 (Phone)

HOME PAGE: http://gking.harvard.edu

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