Reformulating a bit to make the discussion not about the actual labels:
For example, the 'smartest' [read: some combination of high-IQ, high-rationality, knowledgeable] people I know tend to identify as X, rather than as Y. There are a few exceptions to this, but not many. From this, I could arguably assume that it would be correct to adopt a X, rather than a Y mindset without actually understanding why people identify as X.
That breaks down as soon as the knowledgeable people use the same algorithm for deciding what to believe! I assume that's why you put "groupthink" in your title.
The problem arises because we don't have a habit of distinguishing
A) "I believe X because I personally investigated the topic",
B) "I believe X because leading experts in the field, who investigated the topic closely, believe X", and
C) "I believe X because other smart-seeming people believe X".
(A gets the best knowledge but is not usually cost-effective, B is perfectly sensible, and C is the-blind-leading-the-blind, but is the cheapest)
(And that problem arises because everybody has an incentive to imply more knowledge than they actually have, while not being caught in outright lies, so being vague about where your knowledge comes from gets good enough results.)
The problem arises because we don't have a habit of distinguishing "I believe X because I personally investigated the topic", "I believe X because leading experts in the field, who investigated the topic closely, believe X", and "I believe X because other smart-seeming people believe X".
There are languages, real and artificial, in which every sentence grammatically must contain evidential markers, distinguishing such things as "I experienced this myself", "This is a conclusion I inferred from what I experienc...
There are some issues where a) I have no direct knowledge of the facts, b) there is some dispute over the facts, c) I hold an opinion on what the facts are. You probably do too.
For example, I believe that ~6mm Jews were killed in WWII, and that most people who deny this are anti-semites. If someone produced a theory which conflicted with my views, I would probably be very suspicious of the person's motives. I think this without ever having seen evidence that would satisfy me directly. [I've been to Auschwitz and a few other camps, and I've read a few books about WWII, but the only reason I have to cite the figure of 6mm rather than 3mm, or even 300,000, is that most knowledgeable people use that number]. I suspect most LWers are at a similar state of opinionated ignorance.
Now, I am in no way incentivised to investigate this: my opinions matter roughly zero to anyone, including me. So I don't have any reason to investigate the Holocaust. But there are other areas where the facts do matter to me.
I know very little about medicine. When I need medical assistance, I tend to do what a doctor tells me without criticising his diagnosis. For example, I suffered from eczema a few years ago. I was prescribed a medicine which contained hydrocortisone as the active ingredient. I Googled this ingredient and then took the medication until the affected area cleared up. I noted that there were other steroids available [Clobetasol propionate is one such] which are considered cures for eczema. I did not know why my doctor prescribed me one rather than the other, and the distinction between the two would likely affect me. But I took the 'expert advice' without a pinch of salt. Unlike the WWII question, here the facts are actually relevant to me, and a mis-diagnosis could have caused some complications. But just as I don't check the wiring of my house's electric systems, or check my walls' stability and capacity to support my ceiling, I take my doctor's advice.
There are some less clear-cut examples. For example, the 'smartest' [read: some combination of high-IQ, high-rationality, knowledgeable] people I know tend to identify as either libertarians or utilitarians, rather than as socialists. There are a few exceptions to this, but not many. From this, I could arguably assume that it would be correct to adopt a libertarian or utilitarian, rather than a socialist*, mindset to politics without actually understanding why libertarians are libertarian. Just as I don't need to know why my doctor prescribed hydrocortisone in order to take it, I don't need to know why most smart people I know favour relaxed drug laws in order to share their opinion.
*I'm aware that these are not the only options, but they're the most mainstream 'labels' which are clearly defined. ['Liberal' and 'conservative' mean different things in different countries, but I think those three are relatively constant, at least in the people who apply them to themselves]