If there is no god, why do people behave in a moral and ethical manner?

If there is no god, why do people behave in a moral and ethical manner?

If there is no god, why do people behave in a moral and ethical manner? One answer might be long-term self-interest: if you never tell a lie, for example, you will develop a favorable reputation among other people which will allow you to participate in all sorts of activities of which you would never be a part otherwise. Another answer might be "big picture" self-interest: people usually achieve more and have higher standards of living when they collaborate compared to when they compete: "competition" only works as a motivator when embedded in a broader collaborative structure first (i.e., if everyone plays by the rules, we aren't deliberately trying to injure a competitor because we don't want them trying to injure us and so we all place voluntary limits on our behaviors to promote a better outcome for all). While these answers are all well and good, there seems to be something missing: to be motivated SOLELY by self-interest, no matter how you dress it up, seems like a somewhat barren life. People also have passions, which one might consider to be supra-rational: while they may not be fully "rational" they are not "ir-"rational they transcend rationality. Passions, not logic, provide us the determination to persevere in the face of obstacles. People who believe that there is something greater than the self, of which we are a part, can draw upon this belief for a sense of connectedness with other people that provides a backdrop against which many great things can be accomplished. Much of what I just wrote, however, seems to me only to make sense in a spiritual tradition. If I understand correctly, atheists claim to reject these traditions. Other than "enlightened self-interest", is there anything else that would motivate an atheist to behave in a moral and ethical manner?

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