The holy trinity of media effects research is "agenda setting - priming - framing." We've used all of these terms at some point in this blog. Since they are central to all kinds of communication work - and policy work, to quite some extent - we'll introduce all three a little more thoroughly, starting with agenda setting.
Agenda setting means the ability of the mass media to bring issues to the attention of the public and, related, of politicians. The basic claim is that as the media devote more attention to an issue, the public perceives the issue as important. When the media take up a specific topic - such as climate change, or manager bonuses - they make us think about it. The theory was introduced in 1972 by Maxwell McCombs  and Donald Shaw in their seminal study of the role of the media in the 1968 Presidential campaign in the US ("The Agenda-Setting Function of Mass Media ").
The authors claim that since the mass media are a major source of political information for voters, the media can shape public opinion by bringing particular issues to the forefront of their reporting. "The mass media set the agenda for each political campaign, influencing the salience of attitudes toward the political issues.” They content analyzed newspapers, magazines, and television news with regard to the issues the media reported on. At the same time they conducted a survey among undecided voters, asking them what they thought the key questions were in the campaign. McCombs and Shaw found a robust correlation between public opinion and newspaper reporting. Later studies established the causal direction of the effect through analyzing time series data, which showed that public opinion lagged behind media coverage, and followed the issue importance that the media prescribed. The agenda setting function of the media has been supported by a large number of studies.
Agenda setting effects are relevant with regard to policy making. In this blog we have discussed  John Kingdon's study  of who sets the agenda for policy making - he does not talk about mass media agenda setting, but the principle is the same. We can make a two-pronged argument about whether the media may be able to affect policy making: If we assume that it has been established that the media influence public opinion, we need to find studies that show whether public opinion influences policy making. The classic study in this regard comes from Page and Shapiro ("Effects of Opinion on Policy , 1983). Their conclusion: "Examining public opinion and policy data for the United States from 1935 to 1979, we find considerable congruence between changes in preferences and in policies, especially for large, stable opinion changes on salient issues." And they do find indeed that public opinion is more often a proximate cause of policy than the other way round. Burstein , in a somewhat recent (2003) and very helpful review of research on the impact of public opinion on policy, summarizes the existing research: "Public opinion affects policy three-quarters of the times its impact is gauged; its effect is of substantial policy importance at least a third of the time, and probably a fair amount more." The more salient an issue is in the public, the stronger its influence on policy. And the "impact of opinion on policy remains substantial when the activities of interest organizations, political parties, and elites are taken into account."
The available data on the relationship media reporting - public opinion - policy is not great, but there is good reason to assume that media coverage has an indirect, but nontrivial effect on policy making. Therefore the media should be considered a relevant player for policy and reform work - through the channel of public opinion the media can help shaping policies.
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