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Empirical evidence about the agenda-setting role of the communication media now confirms and elaborates Lippmann’s broad-brush observations. When agenda setting was first proposed, it ran counter to the prevailing paradigm among communication scholars that the mass media had limited effects in changing people’s perceptions and attitudes. Agenda setting, on the contrary, showed that the news media can have strong, direct effects in the short term by influencing not what people think, but what they think about.

However, the empirical currency of agenda setting as a theory about the formation of public opinion came much later than Lippmann’s essay. When Public Opinion was published in 1922, the first scientific investigations of the influence of mass communication on public opinion were still more than a decade in the future. Publication of the first explicit investigation of the agenda-setting role of mass communication was exactly fifty years away.

Systematic analysis of mass communication’s effects on public opinion, empirical research grounded in the precepts of scientific investigation, dates from the 1940 US presidential election, when sociologist Paul Lazarsfeld and his colleagues at Columbia University, in collaboration with pollster Elmo Roper, conducted seven rounds of interviews with voters in Erie County, Ohio.⁷ Contrary to both popular and scholarly expectations, these surveys and many subsequent investigations in other settings over the next twenty years found little evidence of mass communication effects on attitudes and opinions. Two decades after Erie County, Joseph Klapper’s The Effects of Mass Communication declared that the so-called Law of Minimal Consequences prevailed: ‘Mass communication ordinarily does not serve as a necessary and sufficient cause of audience effects, but rather functions among and through a nexus of mediating functions and influences.’⁸

However, the early social science investigations during the 1940s and 1950s did find considerable evidence that people acquired information from the news media even if they did not change their opinions. Voters did learn from the news. And from a journalistic perspective, questions about learning are more central than questions about persuasion. Most journalists are concerned with informing. Persuasion is relegated to the editorial page and, even there, informing remains central. Furthermore, even after the Law of Minimal Consequences became the accepted conventional wisdom, there was a lingering suspicion among many social scientists that there were major media effects not yet explored or measured. The time was ripe for a paradigm shift in the examination of media effects, a shift from persuasion to an earlier point in the communication process, informing.

After Lippmann, other authors in the social sciences alluded to the idea that the news media influence what people deem to be the relevant issues of the day.⁹ However, it was only when two young professors at the University of North Carolina’s School of Journalism launched a small investigation in Chapel Hill, North Carolina, during the 1968 US presidential campaign, that the notion was put to proper empirical testing. Their central hypothesis was that the ‘mass media’ set the agenda of issues for a political campaign by influencing the salience of issues among voters. These two professors, Donald Shaw and Maxwell McCombs, also coined a name for this hypothesized influence of mass communication, ‘agenda setting’.¹⁰

Testing this agenda-setting hypothesis required the comparison of two sets of evidence: a description of the public agenda, the set of issues that were of the greatest concern to Chapel Hill voters; and a description of the issue agenda in the news media used by those voters. Illustrated in Box 1.1, a central assertion of agenda-setting theory is that those aspects emphasized in the news come to be regarded by the public over time as being important. In other words, the media agenda sets the public agenda. Contrary to the Law of Minimal Consequences, this is a statement about a strong causal media effect on the public – the transfer of salience from the media agenda to the public agenda.

To determine the public agenda in Chapel Hill during the 1968 presidential election, a survey was conducted among a sample of randomly selected undecided voters. Only undecided voters were interviewed because this new agenda-setting hypothesis went against the prevailing view of media effects. If this test in Chapel Hill failed to find agenda-setting effects under rather optimum conditions, voters who had not yet decided how to cast their presidential vote, there would be little reason to pursue the matter among the general public, where long-standing psychological identification with a political party and the process of selective perception often blunted the effects of mass communication during election campaigns.

In the survey, these undecided voters were asked to name the key issues of the day as they saw matters, regardless of what the candidates might be saying. The issues named in the survey were ranked according to the percentage of voters naming each one to yield a description of the public agenda. Note that this rank ordering of the issues is considerably more precise than simply grouping sets of issues into those receiving high, moderate, or low attention among the public.

The nine major news sources used by these voters were also content analysed. This included five local and national newspapers, two television networks and two news magazines. The rank order of issues on the media agenda was determined by the number of news stories devoted to each issue in recent weeks. Although this was not the very first time that survey research had been combined with content analysis to assess the effects of specific media content, their tandem use to measure the effects of mass communication was rare at that time.

Five issues dominated the media and public agendas during the 1968 US presidential campaign – foreign policy, law and order, economics, public welfare, and civil rights. There was a near-perfect correspondence between the rankings of these issues by the Chapel Hill voters, and their rankings based on their play in the news media during the previous twenty-five days. The salience of five key campaign issues among these undecided voters was virtually identical to the salience of these issues in the news coverage of recent weeks.

Moreover, the idea of powerful media effects expressed in the concept of agenda setting was a better explanation for the salience of issues on the public agenda than was the concept of selectivity, which is a keystone in the idea of limited media effects. To be clear, agenda setting is not a return to a ‘magic bullet’ or ‘hypodermic needle’ theory of all-powerful media effects. Nor are members of the public regarded as automatons waiting to be programmed by the news media. But agenda setting does assign a central role to the news media in initiating items for the public agenda. Or, to paraphrase Lippmann, the information provided by the news media plays a key role in the construction of our pictures of reality. And, moreover, it is the total set of information provided by the news media that influences these pictures.

In contrast, the concept of selectivity locates the central influence within the individual and stratifies media content according to its compatibility with an individual’s pre-existing attitudes and opinions. From this perspective, it is often assumed that the news media do little to alter the issue priorities of individuals because individuals maximize their exposure to supportive information and seek out news about issues that they already deem important. For instance, during an election, voters are expected to pay the most attention to those issues emphasized by their preferred political party.

Which does the public agenda more closely reflect? The total agenda of issues in the news, which is the outcome hypothesized by agenda-setting theory? Or the agenda of issues advanced by a voter’s preferred party, which is the outcome hypothesized by the theory of selective perception?

To answer these questions, those undecided Chapel Hill voters who had a preference (albeit not yet a firm commitment to vote for a candidate) were separated into three groups – Democrats, Republicans, and supporters of George Wallace, a third-party candidate in that election. For each of these three groups of voters, a pair of comparisons was made with the news coverage on the CBS television network: the issue agenda of that voter group compared with all the news coverage on CBS, and the issue agenda of the group compared with only the news on CBS originating with the group’s preferred party and candidate. These pairs of comparisons for CBS were repeated for NBC, the New York Times, and a local daily newspaper. In sum, there were a dozen pairs of correlations to compare: three groups of voters times four news media.

Which was the stronger correlation in each pair? The agenda-setting correlation comparing voters with all the news coverage, or the selective perception correlation comparing voters with only the news of their preferred party and candidate? Eight of the twelve comparisons favoured the agenda-setting hypothesis. There was no difference in one case, and only three comparisons favoured the selective perception hypothesis. A new perspective on powerful media effects had established a foothold.