From IEEE Software "Best Practices" Column by Steve McConnell:
Developers tend to be introverts. About three-quarters of developers are introverts compared to about one-third of the general population. Most developers get along with other people fine, but the realm of challenging social interactions is just not their strong suit.

That was from May 1996.  Wonder if it's still true.  The strange thing is the way programmers tend not to "perform introversion" so much around other programmers.  Then it's often your normal display of wolves playfully (and not so playfully) jockeying for alpha.  And if reddit and imgur don't show you that geeks have a very advanced, um, style of humor, you're not trying hard enough.  This manifests away from the keyboard too.  ;^)

Not sure how the author got those numbers, but I do think communication with folks that can't think as logically as programmers can (and unfortunately often programmers return the favor, and can only communicate in a sort of hyper-logical zero-sum fashion, approaching Sheldon or Spock, depending on your generation) be troubling.  I'd argue programmers, as a whole, aren't as empathetic as the general population.  You must see it not precisely their way, but what appears to them to be the [logical] way.

The inability to have others catch their meaning as easily as, well, other programmers often can, probably reduces confidence and has programmers perform introvertedly.  But that's like saying that you're introverted if you were dropped into a jail cell full of, well, sketchy folk and didn't talk to them like they're buddies. 

Avoiding painful situations isn't precisely introversion.  The key is finding a translator.  And hiring programmers who can blog, natch.  (See what I did there?)