Haidt has actually been at this sort of this for a while. All the way back in 2007, he was building up research on moral foundations theory, which became the foundation of his recent book The Righteous Mind, which is highly recommended. As he explains,
In moral psychology and moral philosophy, morality is almost always about how people treat each other. Here's an influential definition from the Berkeley psychologist Elliot Turiel: morality refers to "prescriptive judgments of justice, rights, and welfare pertaining to how people ought to relate to each other."