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what, if any, is then the incentive to actively try to have an impact on other individuals?

I assume a true ethical egoist, one who believes we ought to act only for our self interest, would state that we should actively try to have an impact on other individuals, if this impact would retroactively have some positive effect on the agent. If we for instance assume the politician, having to promote X or Y to get elected, this politician could actively pursue to achieve promise X as this might positively help the lives of the politician’s voters, and thereby secure a possible re-election, a better office, or even just a higher regard of their own moral/political status.