I expect this book ("The Righeous Mind: Why Good People Are Divided by Politics and Religion") to be great and that seems to be the consensus opinion so far. It builds on the kind of work that Mercier and Sperber did and summarized in the excellent "Why Do Humans Reason", which is available (and summarized) here. Bottom line, most reasoning isn't clean syllogisms or dispassionately interested in truth: it's "I currently observe and believe this, I already believe (and especially want you to believe) that, so I'll build whatever rickety, incoherent verbal bridge between the current state of affairs and the beliefs I want to reinforce." That is, reasoning evolved not to figure out reality, but to win arguments.
If you think this sounds like a bad way to get at the truth, you would be right, but then again, it shouldn't be surprising that a bunch of apes that probably didn't even have real language until a couple hundred thousand years ago are not suddenly and magically perfect proposition-generating and -evaluating machines. The key is in recognizing this fact in humans, especially ourselves, and finding hacks and workarounds to increase the truth-finding, and more importantly, good decision-making, power of reasoning.
There is ample effusion on Haidt's book from economist Arnold Kling here, consisting mostly of money-quotes. My own favorite so far: "We do moral reasoning not to reconstruct that actual reasons why we ourselves came to a judgment; we reason to find the best possible reasons why somebody else ought to join us in our judgment." [One more I added later: "If you put individuals together in the right way, such that some individuals can use their reasoning powers to disconfirm the claims of others, and all individuals feel some common bond or shared fate that allows them to interact civilly, you can create a group that ends up producing good reasoning as an emergent property of the social system. This is why it's so important to have intellectual and ideological diversity within any group or institution whose goal is to find truth...or to produce good public policy."]
As you have no doubt noticed, I am at this very moment trying to get you to believe something that I already believe. The key is to try to make sure the things you say to alter others' belief-states and decisions is true, and this is an effort that we rationalists make and should always strive to improve.
Signaling Group Membership
54 minutes ago