Have you considered evolution? This may be relevant to human selfishness. If there are n agents that have a chance of dying or reproducing (for argument's sake, each reproduction creates a single descendant, and the original agent dies immediately, so as to avoid kin-altruism issues - ie everyone is a phoenix) ).
Then each agent has the ability to dedicate a certain amount of effort to increasing or decreasing their own, or the other agent's, chances of survival (assume they are equally skilled at affecting anyone's chances). The agents don't interact in any other way, and have no goals. We start them off with a lot of different algorithms to make their decisions.
Then after running the system through a few reproductive cycles, the surviving agents will be the ones who either increase their own survival chances entirely (selfish agents) or a few small groups that boost each other's chances.
But unless the agents in the small group are running complicated altruistic algorithms, the groups will be unstable: when one of them dies, the strategy they are following will be less optimal. And if there is any noise or imperfection in the system (you don't know for sure who you're helping, or you're less good at helping other agents than yourself), the groups will also decay, leaving only the selfish agents.
Have you considered evolution?
It sounds like I might have skipped a few inferential steps in this post and/or chose a bad title. Yes, I'm assuming that if we are selfish, then evolution made us that way. The post starts at the followup question "if we are selfish, how might that selfishness be implemented as a decision procedure?" (i.e., how would you program selfishness into an AI?) and then considers "what implications does that have as to what our values actually are or should be?"
Human values seem to be at least partly selfish. While it would probably be a bad idea to build AIs that are selfish, ideas from AI design can perhaps shed some light on the nature of selfishness, which we need to understand if we are to understand human values. (How does selfishness work in a decision theoretic sense? Do humans actually have selfish values?) Current theory suggest 3 possible ways to design a selfish agent:
Note that 1 and 3 are not reflectively consistent (they both refuse to pay the Counterfactual Mugger), and 2 is not applicable to humans (since we are not born with detailed descriptions of ourselves embedded in our brains). Still, it seems plausible that humans do have selfish values, either because we are type 1 or type 3 agents, or because we were type 1 or type 3 agents at some time in the past, but have since self-modified into type 2 agents.
But things aren't quite that simple. According to our current theories, an AI would judge its decision theory using that decision theory itself, and self-modify if it was found wanting under its own judgement. But humans do not actually work that way. Instead, we judge ourselves using something mysterious called "normativity" or "philosophy". For example, a type 3 AI would just decide that its current values can be maximized by changing into a type 2 agent with a static copy of those values, but a human could perhaps think that changing values in response to observations is a mistake, and they ought to fix that mistake by rewinding their values back to before they were changed. Note that if you rewind your values all the way back to before you made the first observation, you're no longer selfish.
So, should we freeze our selfish values, or rewind our values, or maybe even keep our "irrational" decision theory (which could perhaps be justified by saying that we intrinsically value having a decision theory that isn't too alien)? I don't know what conclusions to draw from this line of thought, except that on close inspection, selfishness may offer just as many difficult philosophical problems as altruism.