I'd go on to say, that when we are looking at a large group of people, "dumb" and "rich" not just usually coincide, but are the same thing. If someone is smart enough you won't make money off them, if someone is poor enough you won't, either. So if we look at people as sources of money, dumb and rich are interchangeable and in large groups, difficult to distinguish.
Also, because different groups of people are different by net worth and IQ, we will start (unknowingly) selecting them for either dumb, rich, or both.
Hence the myth that "Americans are dumb" - they of course aren't, it's just that in almost every other country people rich enough to make enough money off them are the elite and thus too smart to profit off them, and only in America, because the country is rich, even dumb people frequently have cash worth bothering. Germans may not be any smarter than Americans, but poorer - thus appear smarter as a group.
