I almost put this one into Philosophy and Debate

Here's some good insights into why the polarization in politics/religion grows.
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Once they have formed an affiliation, people bend their philosophies
and their perceptions of reality so theybecome more and more aligned
with members of their political tribe.
Circling the Wagons

June 5, 2004
By DAVID BROOKS

Over the next few months, I hope to write a fair bit about
the dominant feature of our political life: polarization. I
hope to figure out how deeply split the nation is, and what
exactly it is we are fighting about - questions that leave
me, at present, confused.

Today's topic is what it means to be a partisan, because
partisanship is the building block of polarization.

In a perfectly rational world, citizens would figure out
which parties best represent their interests and their
values, and they would provisionally attach themselves to
those parties. If their situations changed or their
interests changed, then their party affiliations would
change.

But that is not how things work in real life. As Donald
Green, Bradley Palmquist and Eric Schickler argue in their
book, "Partisan Hearts and Minds," most people either
inherit their party affiliations from their parents, or
they form an attachment to one party or another early in
adulthood. Few people switch parties once they hit middle
age. Even major historic events like the world wars and the
Watergate scandal do not cause large numbers of people to
switch.

Moreover, Green, Palmquist and Schickler continue, people
do not choose parties by comparing platforms and then
figuring out where the nation's interests lie. Drawing on a
vast range of data, these political scientists argue that
party attachment is more like attachment to a religious
denomination or a social club. People have stereotypes in
their heads about what Democrats are like and what
Republicans are like, and they gravitate toward the party
made up of people like themselves.

Once they have formed an affiliation, people bend their
philosophies and their perceptions of reality so they
become more and more aligned with members of their
political tribe.

Paul Goren of Arizona State University has used survey data
to track the same voters over time. Under the classic
model, you'd expect to find that people who valued equal
opportunity would become Democrats and that people who
valued limited government would become Republicans.

In fact, you're more likely to find that people become
Democrats first, then place increasing value on equal
opportunity, or they become Republicans first, then place
increasing value on limited government. Party affiliation
often shapes values, not the other way around.

Party affiliation even shapes people's perceptions of
reality. In 1960, Angus Campbell and others published a
classic text, "The American Voter," in which they argued
that partisanship serves as a filter. A partisan filters
out facts that are inconsistent with the party's approved
worldview and exaggerates facts that confirm it.

That observation has been criticized by some political
scientists, who see voters as reasonably rational. But many
political scientists are coming back to Campbell's
conclusion: people's perceptions are blatantly biased by
partisanship.

For example, the Princeton political scientist Larry
Bartels has pointed to survey data collected after the
Reagan and Clinton presidencies. In 1988, voters were asked
if they thought the nation's inflation rate had fallen
during the Reagan presidency.

In fact, it did. The inflation rate fell from 13.5 percent
to 4.1 percent. But only 8 percent of strong Democrats said
the rate had fallen. Fifty percent of partisan Democrats
believed that inflation had risen under Reagan. Strong
Republicans had a much sunnier and more accurate impression
of economic trends. Forty-seven percent said inflation had
declined.

Then, at the end of the Clinton presidency, voters were
asked similar questions about how the country had fared in
the previous eight years. This time, it was Republicans who
were inaccurate and negative. Democrats were much more
positive. Bartels concludes that partisan loyalties have a
pervasive influence on how people see the world. They
reinforce and exaggerate differences of opinion between
Republicans and Democrats.

The overall impression one gets from these political
scientists is that politics is a tribal business. Americans
congregate into rival political communities, then embrace
one-sided attitudes and perceptions. That suggests that
political polarization is the result of deep and
self-reinforcing psychological and social forces.

This theory doesn't explain how the country moves through
cycles of greater and lesser polarization. Still, I have to
say, depressingly, this picture of tribal and subrational
partisanship does accord with the reality we see around us
every day.

http://www.nytimes.com/2004/06/05/opinion/05BROO.html?ex=1087438219&ei=1&en=e79d4b876da2f3d2

Copyright 2004 The New York Times Company