My own feelings on MBTI are similar to this SSC post—it’s unscientific but manages to kinda work as long as you don’t expect too much of it. I wouldn’t make any life decisions based on it!
For the third part of the question we don’t have to guess—the 2012 lesswrong survey included an MBTI question. Of the people who answered, 65% were INTP or INTJ, compared to 5-9% of Americans according to the MBTI website.
Wow! I did not this! Thank you for your answer Bucky! Has lesswrong done more surveys? Will they be doing more in the future?
The last one appears to be 2016 (this was a slightly wider survey which included other rationalist communities) which was before the lesswrong 2.0 relaunch. I haven’t heard of any plans for surveys—maybe a mod can fill us in.
Slatestarcodex does an annual survey of its readers. Scott pre-registers some investigations and then reports on results. This year, for example, he got a negative result on “Math preference vs Corn eating style” and more interesting results in the ongoing birth-order investigation.