It's good; you should read it. There are multiple good reviews elsewhere, but suffice it to say, he builds a case using empirical psychology data with some evolutionary psychology arguments that human morality is built on six different foundations, and the differences between individuals and groups are mostly differences between how much we emphasize each foundation (rather than completely ignoring them). There is a clear problem that arises in the context of a globalized world, one that he didn't address, perhaps deliberately.
That is: the only way to tolerate differences in your neighbor's values, is either 1) not be in a position to do anything about it anyway, or 2) not consider your values to apply to all humans. And the only two way to accomplish #2 are a) consider your neighbors to be less than human or to otherwise believe their moral character is irrelevant, or b) admit that your values are provincial.
Haidt draws a strong distinction between the WEIRD experimental subjects that have produced most moral psychology data so far (Western, educated, industrialized, rich, democratic) who are actually quite strange relative to most humans, and maybe even relative to most other people in their own countries. He points out that secular WEIRD individuals place most of their morality on the harm/care foundation, neglecting others like authority and sanctity. He doesn't make an explicit value judgment to either the more "complete" traditional moralities vs. the harm-based WEIRD morality, but he does recognize that oppression and unhappiness can arise more easily from strongly authority and sanctity-based morality. Our WEIRD morality is strange precisely because it's not a natural condition of human minds and in each of us who hold it, is a product of long education and conditioning. Have we made a mistake, or might this be progress? After all, individuals suffer, and groups don't, and if individuals suffer because of arbitrary commitments made in morality-space by their culture, it's hard to see why that's acceptable. Someone is depressed because they happen to be part of a culture that says homosexuality is impure and the prophet said so and can't be questioned?
I am obviously reacting to such moral systems with my WEIRD harm-based morality. And here is the problem: people with these very different moral systems meet each other, in trade, in tourism, on the internet, and increasingly, as next door neighbors. Haidt does point out a difference bewteen universals and social conventions, but both of these become problematic. For one thing, when there is a group with sacred practices mandatory for its members, but not for the greater population, it's unclear how it's not dehumanizing the outgroup by dismissing their ignorance of said practices. For example, Jews circumcise boys and do not allow pig products to be consumed, but it's fine if gentiles do it. It seems that either people notice the difference from what they're doing compared to what they're Christian neighbor is doing, and on some level don't take their own rituals too seriously; or they do take them seriously, and don't consider the outgroup worthy of the same level of consideration. (Well of course you say, it's an in-group ritual, that's the point; but my point is it's different when you're one tribe of people surrounded by the desert, versus interspersed with other people that behave very differently who you have to constantly acknowledge. It's probably no surprise that assimilation becomes an issue.)
On the other hand, when something is considered a universal, in a globalized world we have the opposite problem. Where next door neighbors are either forced to on some level consider their neighbors inferior or just not take the rituals seriously, a universalist can only work to spread their value to everyone. Again, when it's the neolithic and you're an isolated city-state this doesn't present such a burden. But if you have (for example) a major prohibition against creating images of your prophet (let's just call him, oh I don't know, Allah) and then you see one such image in a cartoon from another country, it doesn't matter where it is or whether it's a Muslim country; you feel obligated to act on it.
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