The Rarest Type?
One of the explanations that has been put forth for why INFJs are so rare is that they are misidentified more than other types. Is this the case? If you go here, there's a chart on page 14 that shows the difference between the "reported type" and the "best-fit type." The reported type is what you get when you take the MBTI and it gives you your result. The best-fit type is when a type practitioner checks the result with you to make sure that it's accurate. As the table shows, the MBTI test by itself found 285 INFJs in the sample. But after the type practitioners double checked the results, it turned out there were actually 294 INFJs in the sample. I.e. the percentage of INFJs increased from 3.2% of the sample to 3.3% after the results were verified. That's not very much change compared to (say) the INTJs, who appear to be the most misidentified type (at least for the current version of the MBTI, Form M). But it has also been found that when Intuitive type practitioners do the verifying, they tend to "find" more Ns than Sensor type practitioners. (Helllooooo type bias.) And of course, most type practitioners are NFs.
However--another study found that NJs were the letter combination most likely to be misidentified. Interestingly, while NTs tended to mistype as NFs, NFs tended to mistype along the J-P axis. Do INFJs tend to mistype as INFPs?
One final thing of note is that there is actually a correlation between Intuition and Perceiving. So if you are an Intuitive, it is somewhat more likely that you will be a Perceiver (or exhibit more Perceiving traits) than a Judger. This would of course tend to rarify the NJ population.
Finally, when the MBTI is translated into various languages (even from British English to American English to Australian English), it is not quite possible to do an exact translation. Therefore results will fluctuate between countries not only because of genuine differences in the percentages of each type, but also because of the language barrier. From this it is evident that the national percentage of each type is dependent upon how the test questions are phrased. Have INFJs been "defined away" by the very phraseology of the test?
At this point, it's hard to say. We can't rule out the possibility that INFJs are misidentified, but neither can we rule out the possibility that they actually are the rarest type.