I would not try to reinvent the wheel and choose a subset of the Big Five sun facets. https://en.m.wikipedia.org/wiki/Facet_(psychology)
One dimension that might be missing in Big Five but relevant for rationality and is present in HEXACO is honesty/humility with it's sub facets; https://en.m.wikipedia.org/wiki/Honesty-humility_factor_of_the_HEXACO_model_of_personality
Independently, it would be interesting to see how these correlate with
Iconoclastic, distrusting of mainstream institutions, seeing oneself as an outsider (vs. identifying more with the people inside institutions who are trying to make institutions work decently well)
Scrupulosity, especially about honesty/integrity/commitment/authenticity (e.g. when you say you'll do something that is an ironclad promise) (e.g. feeling uncomfortable w the job interview process where you know what you're supposed to say to improve your chances)
Demandingness of rigor vs. willingness to seek value from a broad range of lower quality sources (e.g. the tension in this post)
Being quick to dismiss of standard ways of doing things if other ways look better to you vs. treating the standard ways of doing things as a pretty good default (e.g., styles of social interaction) (e.g., lifestyle things such as eating soylent/huel)
Being in touch with your own emotions, living in your body
Avoidance of "woo" / things related to spirituality
Trying to form your own independent impressions, always trying to build your own gears-level models vs. trying to form all-things-considered beliefs, foxy aggregation of multiple viewpoints, inclined to favor variants of outside viewish reasoning
How much you identify with heroes vs. sidekicks vs. villains/antiheroes
How much you identify with various fictional characters (e.g. HJPEV, HPMOR!Hermione)
How easily/thoroughly you get nerd-sniped & delve deeply into random topics or puzzles
Head-in-the-clouds absentminded professorness vs. tracking the concrete practicalities around you
I remember having written some thoughts exactly on that but can't find it now, so some mostly obvious stuff from the top of my head:
Here are some attributes I've noticed among people who self-identify as rationalists. They are:
If you want to look into this in more detail the data from the censuses we have is good. It tells you how the intensity of LessWrong and other rationalist engagement correlates with all sorts of different factors.
I see the rationalist community as a center of a large onion. There are multiple filters one must pass in order to get there.
On the outside, there is the filter of intelligence and abstract thinking in general. It eliminates the people incapable or unwilling to think about complex matters.
Next is a filter of scientific/realistic orientation. It eliminates the people who decided to use their intelligence to study horoscopes or conspiracy theories.
Near the center is the filter of humility/nonattachment. It eliminates people who are unwilling to change their mind, or unable to stop fighting for their political tribe.
(This is a simplified model, and I forgot a few things, such as caring about the future and other people. If we add those missing things, the sequence of the filters will probably not remain linear.)
What I wanted to say here is that the answer to "who are rationalists" depends on the specific filter. Who are rationalists, as opposed to the general public? Who are rationalists, as opposed to university-educated people? Who are rationalists as opposed to skeptics or atheists in general? Depending on which question you choose, the answers will be quite different, and will sound wrong to a person focused on a different question.
I would love to see proper data on this. In particular, including the facets and not just broad buckets. Or if possible, even including findings for specific items.
The ones I've met at a meetup seemed (compared to the broader population):
-Very high in Interest in ideas, which was by far the most noticeable trend.
-Introverted
-Somewhat neurotic
Agreeableness was mixed. Some were unfailingly agreeable, and some were starkly low in agreeableness. Maybe data would show a clear trend on facets or items. For the more strongly utilitarian ones, as a group, I'd speculate they are lower in Honesty-Humility from HEXACO. Yet none ever seemed to make me "worry" in that way, as if they couldn't even manage to have Dark Triad traits without being helpful.
I have a severe psychometrics addiction, so I've regularly been tempted to create some sort of "rationalist type test" or similar, which rationalists could take to classify themselves compared to other rationalists. I particularly like to create such tests using data-driven methods, such as factor analysis. But factor analysis requires a bunch of primitive variables to summarize. And I thought, the most logical way to get such variables is to ask people what they've already come up with.
So, do you have any thoughts on ways that rationalists differ from each other, or ways that rationalists differ from nonrationalists, or perhaps even ways that nonrationalists differ from each other in ways that might be interesting to put into a test for rationalists?
Also, if you don't have a LessWrong account, or want to share this question with rationalists who don't hang out on LessWrong, I've created a Google Form that you can fill out instead. If you've got something spicier that you would prefer doesn't become public, then I have also created a private version of the form here.
I don't know how useful my analysis could become, but at least in theory it seems like it could become more generally useful. For instance, it might be useful for providing targeted advice to certain subgroups of rationalists.
Edited to add: A lot of the comments seem to be bringing up abstract, general traits that have been studied in differential psychology. Certainly those could also be interesting to investigate rationalist on, but I have numerous philosophical objections to many of the traits, and I would instead like to design a new test with traits specific to the rationalist community, in terms of more concrete things that rationalists do and experience.