If you're like most, your opinion of someone's intelligence will be formed partly by his accent -- no matter how unfair that snap judgment may be.
British accents, at least upper class ones, tend to be associated with superior intelligence. "I do believe the sun will rise in the West tomorrow" just doesn't sound as dumb as it should in a plummy accent.
William F. Buckley was brilliant, but the mid-Atlantic accent he affected usually does not connote intelligence. Americans who try to sound vaguely British are merely pretentious, which is almost always the opposite of intelligent.
Cockney accents often exude resentment. Perhaps in the rigidly stratified class system of England working class people have reason to be, but I associate the accent with spiteful comments. Not dumb comments, just spiteful ones.
Australian accents sound a bit like Cockney accents, neither smart nor dumb. But they are far more appealing, because they exude good fellowship in a way Cockney accents do not. Perhaps it's because there's no hereditary upper class in Australia.
I grew up in Massachusetts, so Boston accents sound dumb to me. Occasionally someone will inform me that I have a slight one; I'm never happy to hear that.
New York, New Jersey, and Long Island accents are all similar. Many smart people live in those areas, but none speak with those accents.
Many Southerners seem to have a certain sly way about them, and most seem to know about all sorts of things I don't. (Things like hunting, fishing, and good manners.) Nonetheless, Southern accents generally don't project high IQ scores.
Black accents don't sound intelligent either -- though I'd never dare say that. The smart black people I've known only spoke jive in jest.
Young whites who speak in a black accent -- wiggers -- are merely advertising that have an average IQ close to those they imitate.
Asian accents -- Chinese, Japanese, and Korean -- make the speaker sound as if he'd be good at math and science, and nothing but. I don't think I've ever once heard a truly funny joke delivered in any of those accents.
Midwestern accents, if you can call them that, project nothing in terms of intelligence either way. Canadian accents likewise provide no read. But when you hear one, make tracks as quickly as possible. Otherwise you are about to be bored out of your mind. Eh?
Spanish accents don't reverberate with book smarts. But they usually denote nice, friendly, down to earth people with common sense. Unless that accent is coming from young men with tattoos, in which case, run for your life.
German accents have long been associated, at least in Hollywood, with evil and brutality. And the harsh sound of the language is reflected in the accent. I tend to associate them with a disapproving view of our liberal country. But that is probably just my imagination: these days Germany is more liberal than we are.
Russian accents sound downright stupid, mostly because they are usually so thick. It's hard to sound intelligent when native English speakers can barely understand what you're saying. I usually have to ask Russians to repeat what they've said about three-quarters of the time. Of course, this makes me feel stupid too.
French accents give the opposite impression: French people often express impatience that I don't immediately grasp whatever concept they are trying to communicate. This makes me feel both stupid and resentful. However, their low opinion of my intelligence doesn't make them seem any smarter to me.
Despite all my ugly biases, after about five minutes I think I can judge someone based on the content of their speech rather than its accent.
It is only the accumulation of what I've heard in those accents that has led to these impressions.