Scholars do not usually test for the duration of the effects of mass communication, but when they do, they typically find rapid decay. Persuasive impact may end almost as soon as communication ends. Why so much decay? Does mass communication produce any long-term effects? How should this decay color our understanding of the effects of mass communication? We examine these questions with data from the effects of advertising in the 2000 presidential election and 2006 sub-national elections, but argue that our model and results are broadly applicable within the field of political communication. We find that the bulk of the persuasive impact of advertising decays quickly, but that some effect in the presidential campaign endures for at least six weeks. These results, which are similar in rolling cross-section survey data and county-level data on actual presidential vote, appear to reflect a mix of memory-based processing (whose effects last only as long as short-term memory lasts) and on-line processing (whose effects are more durable). Finally, we find that immediate effects of advertising are larger in sub-national than presidential elections, but decay more quickly and more completely.2 Joseph Klapper's classic argument that exposure to mass communication rarely causes significant opinion change is no longer persuasive to many scholars (Klapper, 1960). In the laboratory and in the field, multiple studies have demonstrated fairly large and seemingly consequential effects of exposure to mass communication. 1 A handful of recent studies, however, has found that persuasion effects can be quite shortlived, decaying in a few weeks or even a few days. Best known is the Texas advertising study, which found no persistence of persuasion effects in the week following exposure to the ads (Gerber, Gimpel, Green, and Shaw, 2011). But other recent studies show similar results. For example, Mutz and Reeves (2005) found that exposure to "uncivil behavior" on TV talk shows reduced the public's level of political trust, but that trust bounced back to baseline levels in a follow-up survey.Social psychologists have grappled for decades with findings that persuasive communication often produces only short-lived effects (Cook and Flay, 1978;Petty and Wegener, 1998 raised, much less investigated, are the focus of this paper. We answer them with evidence on the effects of political advertising, but our analysis is framed in terms generally applicable within the field of mass political communication.The paper begins with the theoretical observation that, in politics as in the rest of life, citizens 1 For reviews, see Iyengar and Simon, 2000; Kinder 2003. 3 form opinions by one of two routes (Hastie and Park, 1986). In the first, memory-based evaluation, people express opinions on the basis of information available in memory when asked. In this model, the effect of persuasive messages is to create opinions that may persist only as long as the messages underlying them remain accessible in memory, which may not be long. In the...