Green first describes the biological foundations of our morality, with first the defense of the "I" within the group, with all mechanisms needed to survive within a social context. Then he moves on to discuss the importance of group morality, the biological reflexes to protect the own tribe against neighbouring and competing tribes.
This is the kind of default positions for humans, a kind of automatic moral response against the others, which is often rationalised afterwards (they are evil, they have no sense of justice, etc.), but which is in essence a deep emotional and pre-rational response.
Luckily, we humans can also revert to "manual" response, when our brain can take over and think about what will provide the most happiness for most of us. In that sense, Greene belongs to the school of "utilitariansim" of John Stuart Mill and Jeremy Bentham, which he would rather re-define as 'deep pragmatism'. The advantage of Greene's approach is that he has done extensive tests of people in a variety of moral dilemmas, looking at the statistics of behaviour and motivation of different test set-ups and thought experiments, such as the prisoners' dilemma or the trolley dilemma, the latter offering someone the choice wether they would push one person under a trolley to make it stop and hence avoid the death of five other people down the tracks. In fact, very few people will do that, but the majority would willingly turn a switch to re-route the trolley on a side-track, killing one person in the process to save five. Why would they do that? The fact that he looks at all these varieties and possibilities of choices and people's response to it, is not only an eye-opener, but framed and explained by Greene also illuminating to understand our most unconscious reactions and moral behaviours.
Greene stears us clearly outside of the traditional moral positions of collectivism versus individualism, and he manages to come with a quite compelling proposal consisting of six rules :
1. In the face of moral controversy, consult, but dot not trust, your moral instincts
2. Rights are not for making arguments, they're for ending arguments;
3. Focus on the facts, and make others do the same
4. Beware of biased fairness
5. Use common currency
Each of these rules get ample attention and explanation in the book.
"Moral Tribes" is a fascinating exploration of our morality, and quite a convincing and highly readable one. As said at the beginning, how do we get thoughts like these known to broader audiences? Greene's proposals are definitely worth hearing.