Who says so?
It would be self-contradicting in practice in any community. Self-serving applications of a moral code based in self-interest, in practice, is crime: theft, rape, even murder. A society filled with rampant theft, rape, & murder cannot even get started. It's not an evolutionarily stable strategy.No moral code can be self-serving.
Who says so?
I don't know if this is an analytic or a synthetic statement, but I can't see how such a society could ever survive, let alone thrive.
Can you?