Where is the plumb line when you really need one? A plumb line defines straight up and down, deviating neither to the left nor to the right. Everybody agrees on the absolute verticality of the plumb line because it a useful assumption to make and nobody really cares.
For media, nearly everybody cares. And there isn’t a plumb line.
I have seen quite a few quantitative studies of “bias in the media.” Nearly always, they find that the “left-leaning media” charge is inaccurate. The goal of these scholars is to dummy up a plumb line. They want to show, on the basis of unchallengeable scientific methodology, that the line is straight up and down. Or that it “leans” a little to the left or to the right. They really don’t care.  What they care about is that some standard is established that everyone can agree on. The charge needs to be shown to be true or false. 
If that could be done, then everyone could agree that “left” means left of the line and “right” means to the right of the line in the same way that west Portland is west of the river and east Portland is east of the river. That’s what they want. That’s not going to happen and in this essay, I am going to feel around a little for why that is so.
I am the plumb line
It is very hard to get over the idea that your own views and your own values are the way things really are. All little kids are like that until the age of 5 (by some measures) or 10 (by other measures). Eventually, they learn to understand that the person standing outside the door can see the knocker that is not visible from the inside of the house. I think that must be a very difficult day. Or maybe it is exciting.
Before you learn this, what you see and what is true are the same thing. Afterwards, you have to admit that whatever the truth is, these people will see it one way and those people another. It is still possible to say that those people are wrong, of course, or willful or hardhearted or whatever. But all of those charges begin with the idea that different people see things differently.
And that is true even if we treat “things” just as physical objects, like inside the house or outside the house. What if we expand “things” to mean beliefs? When we get to beliefs, it gets difficult very quickly. There are three reasons for that.
Why it isn’t going to happen
The first is that beliefs are a team sport. “We,” we say, “believe this that THEY believe that.” The great power of this formulation is that is asks whether you are really one of us or one of THEM. If you are one of us, then a lot of good things follow. “We” help each other, we include each other, we take pride in each other, and should a dispute come we “have each other’s back.” For treasures like that, most people would swear that black is white and up is down. It would be hard not to.
So a liberal who holds in his hand a well-researched report showing that there is no genetic element at all in homosexuality and that, consequently, it is completely the choice of the individual—has a problem. A conservative holding in her hands a report showing that explicit instruction in sexuality dramatically reduces the rate of pregnancies among unmarried youth—has a problem. You would really like to follow the evidence, but you would also like to have friends and neighbors and the tribal nature of beliefs suggests that you may have to choose.
The second is that a belief system is indicated and supported by what topics are chosen for consideration. This is the issue of “salience.”  In politics, we talk about these things and THEY talk about those things. And that is true also of news sources. Entirely apart from any bias in evaluation—which is the next point—there is a bias in salience.
Liberal news sources blame President Trump for everything imaginable and credit him with no achievements at all. Conservative news sources exonerate President Trump from any blame whatever and call the slightest hint of hope, an “achievement.” Liberal news sources talk about civil rights and the equal protections clause. Conservative new sources talk about religious liberty and the inviolability of conscience.
Third, a belief system says that these things are good and those things are bad. Or, less combatively, that these things are better than those things. Or that these things are better than those things…right now. It is too late, for instance, to restore the integrity of the American family, now that it has been torn asunder by generations of liberal permissiveness. It is too late to stop the pollution which is rapidly heating the atmosphere, but it is too early to start planning expensive protective efforts for those communities about to be inundated by rising oceans.
If, as noted earlier, I am the plumb line, then all I have to do to form an opinion about the fairness of the media, is to see where they fall with reference to my own—that means to “our” own—views. To be “left-leaning,” a news source would have to do two things. They would have to focus on issues that we don’t like to talk about and they would have to say things that contradict our values and our loyalties.
A common conservative belief, for instance, is that the nations of the world are persistently taking advantage of America. We are the global policemen and the global lending agency and the global social workers and the global food bank and so on. And in return, we get hostility, refusal to cooperate, and binding agreements with our enemies. Now take a minute and think of what stories—we are considering only the salience question here—would violate that belief. Countries that actively refuse our “police services” are a story that would not be covered. Stories showing that what Americans call “gratitude” would require that a country do things directly opposed to their self-interest, would not be covered. Instances of opposition, which would be called “ingratitude” would be covered in detail.
A left-leaning news source would feature topics that we don’t really talk about in my (conservative) news “neighborhood.” And they would take positions on those topics that we conservatives find weak or relativistic or unpatriotic unrealistic on questions of the global environment.
“Left-leaning” has nothing at all to do with whether these sources are “fair and unbiased;” it has nothing to do with equal numbers of liberal and conservative commentators; it has nothing to do with a careful grounding of the conclusions in the facts of the matter. And how could anyone—political scientists among them—have thought it could?
It means leaning away from me. And the further to the right I am, the more powerfully true it is that “many sources” or “most sources” of news are “left-leaning.”
I’m saying it’s a conspiracy.
 The terms left and right could be said to be arbitrary. They derive from the 18th Century French politics that put the aristocrats on the right side of the assembly and the commoners on the left, but there was no rationale for the seating and it is just a taken for granted term now.
 It is easier, of course, to sell your results to your left-leaning student body or your left-leaning community of scholars, but lots of scholars would be really happy to defy their students and/or their colleagues if their data would hold up to unfriendly scrutiny.
 Or…you know…adequately or inadequately supported by the data so far.
 Literally, the issue of what “jumps out” for you.