The Righteous Mind explores the polarization of American politics with a focus on the different moral foundations of conservatives versus liberals. He explores the question of why both conservatives and liberals tend to think they are morally in the right and that the other side is morally wrong. Haidt spends the first two chapters providing experimental evidence of why Hume was right to say that reason is the slave of passion. He suggests that you can't make reasoned decisions without emotional backing. Haidt uses Damasio's findings, presented in his book Descartes' Error, to back this up. Damasio found that people who do not feel emotion due to brain damage are flummoxed by even small decisions like what brand of milk to buy or what order to perform a set of tasks. Haidt then references studies that suggest that people will use emotional intuition to come up with a point of view, and then look specifically at evidence that supports this intuition, ignoring evidence that contradicts their own views.
In his own studies (see figure below), Haidt breaks down people's morals into categories: care about others, fairness, loyalty, authority, and sanctity. According to Haidt's studies, liberals value caring about others and fairness much more than loyalty, authority, and sanctity. Conservatives value loyalty, authority, and sanctity more, and value care and fairness to a lesser degree. The interesting bit is that conservatives have a broader (more diverse) range of what they care about than liberals do. They care about their less important values more than liberals care about their less important values. Haidt suggests that this is why Republican candidates purposely apply to the emotional side of people more than Democrats do. And since people's rational decisions are based on emotion, the Republican candidates have an advantage with moderate voters.
Haidt goes on to answer the question: "Why do poor, rural, white folk vote against their economic best interests?" He suggests that they're voting for their moral interests - loyalty, authority, and sanctity.
The next part of the book is about evolutionary group selection. Many people believe that groupish behavior evolved in our species because a group of individuals was more successful at producing offspring than single individuals. But Haidt asks the question: how can such behavior evolve when selfish individuals within a group can take advantage of the group to be even more successful than the altruistic group members? Wouldn't evolution then favor the selfish ones? He answers this question describing a situation breeding chickens. If the chickens who produce the most eggs are inbred, then the offspring end up laying more eggs. But they also end up more aggressive, which is a trait linked to high production of eggs. Because the chickens are more aggressive, they attack and kill each other, leading to lower egg production per cage. If you, instead, breed the cage that produces the most eggs compared to the other cages, the cages of offspring end up with more eggs. Thus, evolutionary group selection is about selecting an entire group, and not about breeding individuals within a group. Haidt suggests that this is why humans tend to break into groups.
Haidt goes on to suggest that religious belief evolved as group selection (groups of religious people survive better than groups of nonreligious people) rather than as an evolutionary trait of individuals. So people not only have a tendency to break into groups, but to have a formal set of values within each group. In the end, we follow our values and tend not to understand that another group's values may be founded in truth as well.