i believe given the right circumstances a healthy adult could be made to believe complete lies
where have I seen this in history....
People, throughout history and the present, share some things in common: a strong affinity for contempt over other groups of people leading them to all manner of false belief and irrationality, and a certainty over their own correctness.
And in saying that... Here I am, expressing contempt over some poorly defined group of people with some hint of a suggestion that I am immune.
It's not all bad though, on a lot of the empirical economic work I've done a lot of people appear what I'd call "weakly locally rational": a rationality effect seems to effect people in aggregate and determines the direction they move, and tends to be the biggest effect, though the aggregate doesn't move perfectly in accordance with what rationality would expect. It also depends very much on framing: they are "locally majority rational" in the context of their frame of reference, but not in terms of the macro-world: which makes sense, because we're finite creatures who can't actually take in all the information, have limited processing ability, and each have different access to information and historical experiences. so most of us do the best with the limited experience of what we got and how we understand the world.