Having sex for procreation vs. having sex for fun, because you evolved for it to be fun to procreate more. The distinction becomes clear after the invention of birth control. People who want to have kids don't use it. People who want to have fun do.
I'm not sure I understand your example as a distinction between altruism as an act of conscious self-interest vs. moral reasoning.
Perhaps there is a distinction between conscious self-interest and unconscious self-interest?
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.