The first accusation of media bias was probably hurled about 10 minutes after the first newspaper ran off the first printing press. Since then, anyone in the news business – whether in print, TV or on the Web – has heard the charge.

Amazingly, the Sun Journal often gets back-to-back complaints from readers arguing that our bias runs in opposite directions.

Two recent columns by Washington Post writer Shankar Vedantam says science can help explain why extremely partisan people perceive the world the way they do.

Vedantam cites one study in which 144 Israelis and Arabs were shown six television news segments about Israel’s 1982 war with Lebanon. Questioned separately and in depth, the Israelis felt the broadcasts were overwhelmingly hostile to their cause, while the Arabs felt the news was slanted against them.

This, according to social scientists, has become known as the “hostile media effect,” and it describes the “sincere belief among partisans that the news reports are painting them in the worst possible light.”

According to one psychologist quoted by the Post, “If I think the world is black, and you think the world is white, and someone comes along and says it is gray, we will both think that person is biased.”

Interestingly, extreme partisans always say they are worried about how perceived media bias affects neutral observers. “But,” according to Vedantam, “neutral observers are better than partisans at seeing flaws and virtues on both sides.” Partisans, meanwhile, generally overestimate how susceptible other people are to propaganda.

Other studies show that the people who see the world in black-and-white terms have brains that actually filter out or downplay information that contradicts or undermines their position.

In one experiment, 10 partisan Republicans and 10 Democrats were subject to scans that measured chemical changes in their brains.

One section of the human brain apparently modulates the strength of our emotional responses. If we see something we don’t like, this part of the brain tells us to calm down or get more angry.

So, the brains of most people would see a photo of a candidate they don’t like and this part of the brain would automatically dial down the negative response. The brains of extreme partisans, however, deliberately turn up the negative emotion.

“In other words, without knowing it themselves,” according to Vedantam, “the partisans were jealously guarding against anything that might lower their antagonism.” It is as if their brains are trying to ensure that their antagonism never fades.

“My feeling,” Jonas Kaplan, a psychologist at the University of California at Los Angeles, told the Post, “is in the political process, people come to decisions early on and then spend the rest of the time making themselves feel good about their decision.”

That certainly seems like the reality of the loudest political debate in the U.S. today. Regardless of evidence to the contrary, some will always stand by the president, and others will condemn him.

We can only hope that there is an even larger group of practical, open-minded people who can sift and weigh relevant information.