We organize the discussion into several questions.

1) Do Americans PERCEIVE that civility is declining?
Yes, there is a lot of evidence showing that most Americans perceive incivility to be a very serious and growing problem
–A 2010 report on Civility in America, by the P.R. firm Weber Shandwick, shows  that Americans find politics, and the roadways, to be the most uncivil domains of life. The 2011 update goes into much more detail, including many interesting facts, such as that Republicans are more likely than Democrats and Independents to believe that incivility is inherent in the political process

2) Is there objective evidence of a decline in civil BEHAVIOR in recent decades?

[we're looking for evidence on either side. If you know of any, please email haidt at virginia.edu]

3) Has political POLARIZATION increased in recent decades, among voters and citizens?
Yes. There is one (and only one) measure by which polarization has NOT increased: If you look at the attitudes of Americans on a range of specific issues (such as gay marriage, taxes, foreign policy…), you don't find that the distribution is widening. The voters are not moving to the extremes on the issues. See work by Fiorina, and a recent study by John Chambers [links to come]. This is important to note, and it is a hopeful sign.

But by most measures, things are getting more polarized.

• The number of "split results" in house districts has declined. This refers to districts that vote for a president from one party and a congressional representative from another. As Sabato's Crystal Ball puts it:  "In 1984, there were 196 split districts: the result of President Ronald Reagan’s romp combined with the reelection of a Democratic House. That number has declined since then to 83 split districts in the 2008 presidential race. As the conservative Democrat and (especially) the liberal Republican becomes extinct, one can imagine the number of split districts declining further." [Haidt, jan 20]
• Even if the voters views on issues have not spread out much, their ratings of their own political positions have gotten more extreme, according to both Gallup (which shows a decline in centrists and a rise in the percentage of self-identified liberals and conservatives since the early 1990s) and also according to ANES, as reported in these helpful graphs from Larry Sabato.
• The 2012 Pew Values Survey was publlished with this headline: "Partisan Polarization Surges in Bush, Obama Years." The graph below shows that differences on a set of value questions by race, education, and income have held steady for the last 25 years, but differences on values by party have nearly doubled, from around 9 or 10 points in the 1980s and 1990s to 18 points today

4) Has political POLARIZATION increased in recent decades, IN WASHINGTON and state capitals?
When we look at the attitudes and behavior of the parties and the political elites, the portrait of increasing polarization and demonization is quite compelling: 

• Testimonials from departing congressmen and senators are uniformly despairing. [E.g., Former Members of Congress for Common Ground; other links to come].  
• And here is the killer graph from political scientists Nolan McCarty, Keith Poole, and Howard Rosenthal, authors of Polarized America, who maintain a very useful site here.

The graph is based on quantifying party loyalty in roll call votes in congress. High numbers mean that votes tend to split neatly along party lines. Low numbers mean that there is a lot of cross-over. As you can see, the period from the 1930s to the 1970s was something of a historical anomaly. We may never be able to get back to such low levels of polarization. But as you can also see, polarization levels began skyrocketing in the 1980s, and have gone straight up. Based on everything we know about social and moral psychology, more polarized congresses are likely to have higher levels of incivility, but we are still looking for data to establish whether or not that is true.