A lower bound on your first claim: Most everyone accepts that there exist people with severe intellectual disability. For many causes, the degree of impairment may range smoothly from severe into the "normal" range, where the bright lines are imposed by functional requirements like living independently or managing health care, and not by any well-defined abstract mental capability.
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.