How does the media help set the public agenda?

There is a theory from political scientists Maxwell McCombs and Donald Shaw that refers to the "Agenda-Setting Function of the Mass Media." This theory says that while the media don't tell us what to think, they do tell us what to think about. That is, the stories the media focus on may not be the most important stories in the world, but because we see/hear/read about them so often, we may begin to believe they must be really important after all; conversely, what the media ignore or fail to cover (or cover in a one-sided manner) will leave the public misinformed, believing there is only one side to a story, or that what is not reported about must be unimportant.

Also, the media set the agenda by picking winners and losers. Some media outlets and publications offer relentless coverage of celebrities (or politicians) behaving badly, leaving the impression that this is how they all act; the same is true with crime stories and stories about the public schools: the media may focus on certain poor and minority neighborhoods, leaving the impression there no crime and no social problems exist in upper-class neighborhoods. Many critics have noted that the media rarely cover anti-war stories during a time when the country is fighting a war (or a political leader is trying to drum up support for going to war). And in a country like the USA, where the media are commercial, it is also rare to see much focus on worker's rights issues (low pay, lack of health care, etc), use of child labor in third-world countries, or corporate greed, since advertisers take a dim view of being the subject of negative reports.

And critics have also noted that the media may be promoting an "argument culture"-- because many in the mainstream media prefer to focus on conflict between two sides of an issue. These stories are often reported as if both sides are equally credible, when in fact, one side's views may be considered a fringe viewpoint. Additionally, the media often prefer a simple story-line that can easily be told during a newscast, giving the news consumer the idea that there is a simple answer to even the most complicated question. Further, the media may give the impression that a certain view is held by almost everyone, leading the public to believe certain views are more popular (or more disliked) than they actually are.