In conversations on LessWrong you may be surprised (in fact, dismayed) to find an apparent majority of the community agreeing with each other, and disagreeing with some view you hold dear. You may be tempted to call "groupthink". Whenever that happens, please hold yourself to at least as high an epistemic standard as the people who are participating in the community, and substantiate your accusation of groupthink with actual evidence and analysis.
"Groupthink" can be an instance of applause lights, terms or explanations used not so much for their semantic content as for the warm fuzzies they are intended to trigger in your audience. Or... since "groupthink" isn't so much intended to generate applause for you, but to generate disapproval of those who disagree with you, we might coin the phrase "boo lights".
At any rate, you may be cheaply establishing (in your own eyes and the eyes of people "on your side") your status as a skeptic, without actually doing any critical thinking or even basic due diligence. Are you sure you that's what you want?
(N.B. links in this post either point to examples, or to more complete definitions of the concepts referenced; they are intended as supplementary material and this post stands on its own, you can ignore the links on a first read-through.)
Apparent consensus is not sufficient grounds for suspecting groupthink, because the "groupthink" explanatory scheme leads to further predictions than the mere appearance of consensus. For instance, groupthink results in "selection bias in collecting information" (from the Wikipedia entry). If the community has shown diligence in seeking contrary information, and yet has not rallied to your favored point of view, your accusations of groupthink are unjustified.
Disapproval of your contributions (in the form of downvoting) is not sufficient grounds for suspecting groupthink. Communities establish mechanisms of defence against disruption, in a legitimate response to a context of discourse where disruption is an ever present threat, the flip side of open participation. The voting/karma system is the current mechanism, probably flawed and probably better than nothing. Downvotes signal "we would like to see fewer comments like this one". The appropriate thing to do if you receive downvotes and you're neither a troll nor a crackpot is to simply seek feedback: ask what's wrong. Complaining only makes things worse. Complaining that the community is exhibiting censorship or groupthink makes things much worse.
Disapproval of your accusations of groupthink is still not sufficient grounds for suspecting groupthink. This community is aware of information cascades and other effects leading to groupthink, discusses them openly, and strives to adopt countervailing norms. (Note that this post generalizes to further concepts, such as censorship. Downvotes are not censorship; they are a collaborative filtering mechanism, whereby readers are encouraged to skip over some content; that content is nevertheless preserved, visible to anyone who chooses to read it; censorship, i.e. banning, does occur but much more seldom than downvoting.)
Here is a good example of someone
substantiating their accusations of groupthink by reference to the actual research on groupthink. Note how much more work this is.
If you're still thinking of calling "groupthink" without doing that work... or, perhaps, if you have already done so...
Please reconsider: your behaviour devalues the technical meaning of "groupthink", which this community does have a use for (as do other communities of sincere inquiry). We want the term groupthink to still be useful when we really need it - when we actually succumb to groupthink.