Hawke wrote:

I have to admit that I can't remember where I came across the correlation
between stubbornness and lack of intelligence. Maybe it came from the
comparisons of the stupidity of a mule which goes along with it's
stubbornness. On the other hand, it's more likely that I read that somewhere
for a psychology class I took.

Hawke

I think you are mistaken about the "stupidity" of the mule. From all
I've heard from "equine" folks (maybe not mule skinners) the mule is
a lot smarter than a horse. Now stubborn, maybe. :-)
...lew...