I know I'm late to the party, but Jonathan Haidt's The Righteous Mind, published in 2012, is the most important books I've read in 2017.
Haidt's assertion that "anyone who values truth should stop worshipping reason" (page 104) neatly summarizes the book's first part. What he means by this is that we don't make judgements based on reason. Instead, we have a feeling about things, and we use reason to justify that feeling. Our reason is like a press secretary: Their job it is to come up with justifications for whichever statements and decisions the president has made, no matter how nonsensical they are. In this metaphor, the president is our subconscious gut feeling.
We're therefore unlikely to arrive at the truth by relying on reason alone, because reason will just confirm what we've felt all along. However, if we put individuals together the right way, they can use their reasoning power to disconfirm each other's claims. Finding the truth therefore must be a social enterprise, and one manifestation of such an enterprise is Science. Democracy would ideally be another one.
The second part of the book is about moral dimensions, or what Haidt calls Moral Foundations Theory. Many people, and especially those on the liberal end of the political spectrum, think there are only two moral rules: Don't harm others and don't cheat.
However, there are others too: Be loyal! Respect authority! Honor! Sanctity! Those additional dimensions have in common that their emphasis on self-control over self-expression and on duty over rights is likely to benefit families and groups rather than individuals. Ignoring those additional dimensions will results in an incomplete picture of morality and of how society works. Conservatives as well as many non-Western cultures value those moral dimensions more highly than do liberals, hence the conservative focus on "family values". Liberals put themselves at an electoral disadvantage by dismissing those dimensions.
I'm not convinced by Haidt's arguments regarding group selection, which is the theory that natural selection acts not only on individuals but also on groups. He tiptoes around the subject, but ultimately suggests that group selection is the most likely explanation for most aspects of human morality. This is problematic.
First of all, evolutionary theory indicates that group selection is only possible under implausibly narrow conditions that are unlikely to ever be met in reality. Haidt doesn't address this issue.
Secondly, his assertion that group selection is necessary to explain many aspects of morality is unconvincing. For example, that we feel moral outrage toward bullies can adequately be explained by individual selection. Allowing others to bully you isn't going to improve your fitness in the evolutionary sense. It's simply not necessary to resort to groups selection to explain this.
However, Haidt's views on group selection are a relatively minor issue as the rest of the book holds up no matter how morality arose in the first place. I recommend The Righteous Mind to anyone who wants to understand why people on the other end of the political spectrum, but also in other cultures, have such wildly divergent opinions on moral issues. I'm also looking forward to the forthcoming The Coddling of the American Mind, which will likely expand on his somewhat controversial essay from 2015. This time, I won't wait five years to read it.