Do mass media determine or codetermine the political agenda? Available answers on this question are mixed and contradictory. Results vary in terms of the type of political agenda under scrutiny, the kind of media taken into account, and the type of issues covered. This article enhances knowledge of the media's political agenda-setting power by addressing each of these topics, drawing on extensive longitudinal measures of issue attentiveness in media, Parliament, and government in Belgium in the 1990s. Relying on time-series, cross-section analyses, the authors ascertain that although Belgium is characterized by a closed political system, the media do to some extent determine the agenda of Parliament and government. There is systematic variation in media effects, however. Newspapers exert more influence than does television, Parliament is somewhat more likely to follow media than government, and media effects are larger for certain issues (law and order, environment) than for others (foreign policy, economic issues).
Some fundamental concepts in agenda-setting are related to a simple cognitive memory decay process. Accounting for issue obtrusiveness and amounts of prior coverage, predictions for the size of the relationship between declining accumulated television coverage and issue salience are derived. Levels of declining accumulated coverage are estimated by applying an exponential decay function to the prominence of daily television coverage. This function presumably models simple forgetting of coverage that occurs within individual audience members. Three issues (inflation, Iran, and the Soviet Union) were investigated over an 1,826-day period, using the daily prominence of television coverage obtained from television news archives and daily salience of the issues interpolated from monthly archived poll data. The size of the relationship between accumulated coverage and issue salience was found to decrease with the amount of coverage of an issue prior to the beginning of the study period. A new unobtrusive issue (Iran) was found to have the strongest agenda-setting effects and more rapidly declining coverage effects than other issues.