In the episode "Close encounters of the Rick Kind"

[Lasty]

Rick reveals that Morty functions as a brain wave camouflage.

I was just realizing that this was foreshadowed upon in the pilot when the effects of the mega seeds

affected Morty's intelligence, which was why Rick was acting so obtuse and weird as compared to

other scenes in that and other episodes where he seems more lucid. He had to adjust his thinking

to maintain the camouflage, which was probably a pain in the ass more than anything.  Also, so as not to

upset Jerry's insecurities about his intelligence.