Hi wavydog,
Unfortunately it is a fallacy to presume the value of integrity, goodness etc must derive from a God. They happily derive themselves just as well from the mathematics behind group dynamics.
Humans have social rules because social rules provide a public good to humans collectively. Behaving in favour of the group is an in-built form of social contract which we appear to be evolved to demand of one another. Like bees working together inside a hive.
A person who believes that their selfish interest is more important than the group's will usually be excluded and shamed by the group for behaving this way. The punishment is isolation. Isolation turns out to be quite an unpleasant experience.
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