Like most people on HN, I like to think I'm at least somewhat "smart" at least by some metrics, so what I've found works is actually "dumbing myself down" by not showing others my intelligence. I've made a lot of "normal" friends that way actually.

Here's why (written from others' perspective in the second person):

For whatever reason, most people are really put off by you "smart people" -- you're "different," and us humans are evolutionarily wired to be suspicious of things that are "different."

You say things we don't understand, or you may challenge our core beliefs, and we humans do not like people like you who say or do things like that -- it immediately incites distrust in you and makes it VERY hard to relate with you. We think you are trying to trick us.

You're deviating from the "herd" in society through thought, and we're a social species, so such deviations come with consequences in the form of ostracization.

Your interests are things we don't understand (and so we don't care about) like "reading," "coding," and "mathematics," and we can't get a good read on what you are like as a person, so we (unfortunately) resort to thinking you are arrogant -- that you think you are too good to show us what you're truly thinking about.

You might have used your intelligence to make money, now you're just another rich asshole, etc...

For all of these reasons, we feel "inferior" to you, because like most people, we feel very insecure about ourselves and where we fit in society. Thus, we are overly sensitive to anything you say that might even remotely imply that you are smarter than us. "That guy just used a 'big word,' he's full of himself." And we're jealous that you have something that we don't have because you are smart. We think you are happier than we are and have a better life than us (even though you actually aren't) because "the grass is greener."

-- and then there's the philistine aspect of American culture --

It's not "cool" to study and be smart 'round these parts. You're a "nerd." We don't associate with "nerds" because they will lower our social status.

You have trouble relating with us because of your "intelligence," but we see that as you being aloof or "stuck up." You think you're too good to be our friend (even though it's actually an oil and vinegar type mixture -- an incompatibility, if you will).

And just throwing it out there, I think a lot of smart people are misdiagnosed as "autism spectrum" because the diagnosing physician/psychologist falls more moderately on the "intelligence spectrum" and so is biased by their same visceral "weirded out" reaction to their diagnosee. Either that, or (less likely) autism and intelligence are two sides of the same coin. Or the "disorder" (or should I say, label?) was fabricated because such "intelligence people" -- THOSE people -- MUST have something wrong with them because of all of the above reasons.

It's also my hypothesis that at least some of the "techie backlash" in San Francisco is the result of the above -- unfortunately, the in-group effects caused by grouping together a lot of "intelligent people" together who suffer from the above ostracization is a perfect storm for dividing a population into "us" and "them" factions, even though, ironically, San Francisco's "nativist" population was really formed under similar ideals (i.e. "we're the weird ones, come join us").