I decided I should elaborate on the whole topic:
Here are some speculations about why the demograph might breaks down like it does.
INtuiters are more interested in MBTI than Sensers.
Possible reasons:
The highly abstract, theoretical nature of the MBTI seems more interesting or useful to an N than it would to an S.
INtuiters are somewhere between a fourth and a fifth of the American population, creating a personality minority status. People in this position are often more interested in psychological works for understanding the self and others.
Authors of MBTI material have an iNtuition favoring bias, and so the material they write is accordingly biased in who it attracts.
The forum is skewed by an iNtuition bias.
Possibly by coincidence, some other factors, or the factors already listed above, these forums my have started off on an iNtuitive limb. INtuiters, consolidating with other iNtuiters, may start to frame themseves against the predominatey Sensing world, creating a forum atmosphere that has the following effects:
It attracts yet more iNtuiters, who see a like crowd, a place of belonging, somewhere to gloat or commiserate.
It repulses Sensers, who see the forum atmosphere as being exclusive and hostile to their kind.
It persuades undecideds that they are iNtuitive, because regardless of whether or not they really are N or are actually S, people are drawn in by communities of solidarity and God knows almost everyone thinks it's themselves against a different and unsupportive world. If you are uneducated about the MBTI and see that talk on the iNtuitive forum, you'll think you must be an N.
Likewise, after a mostly iNtuitive forum becomes especially conceited, it's hard to imagine any newbie with a bit of self-esteem would type themselves as a Senser, if the only thing they hear is that Sensers suck in regards to everything. So if you like yourself, you must be iNtuitive.
Crappy amateurish tests.
In essentially the direct reversal of Ezra's theory, maybe tests are written in a way that produces an inaccurately large number of iNtuiters.
The population figures about the USA were conducted in a respectably conducted serving using the official MBTI form M. As best as I can tell, almost no one here has taken such a test. Instead, most have taken online tests written by random amateurs, likely barely refined after their original creation, not created without sufficient knowledge of psychometics to know what sort of questions to use or not use, and simply way too few questions a anyhow. Making matters worse, there are tons of these tests, mistaken in different ways, that add up to give confusing counter-results.
Those who do not take any tests are instead left to self-typing from their own knowledge, or letting themselves be type by others, which makes them seriously vulnerable to the other cause I've listed.
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I'm increasingly thinking the N bias is the reason for our demographics. Like I said, perhaps an increased interest in the MBTI among Ns is why the forum would start out N heavy in the first place, but given just how different this forum population is from the national population, I have to wonder if the types really differ that much in their interest. On the other hand, the anti-Sensing sentiment is really palpable on this forum (though I acknowledge it's gotten better over time.. probably had to shake off some of its initial INTPc influence). The Sensors I have spoken to it seem to be almost in consensus about the feeling that they are unwanted, disliked, or singled-out here, and their view is the most important factor in this theory.
I would guess the whole crappy test theory is in second place.
I'm really losing my belief in the theory that differing interests are the source of all of this. And when it comes to differing interests, I think the Ns looking for mutual validation is a bigger factor than them having a more inherent interest in the kind of material that the MBTI is.
EDIT: And I just realize, since this forum is in many ways a branch from INTPcentral, that alone could be most of the reason this place started-off N heavy.