Going back to at least G.E. Moore, philosophers have steadily built a strong case that the social practice of assertion is constituted by a knowledge norm: you should assert Q only if you know Q is true. As best I can tell, this "knowledge account," as it's often called, is by now the best supported hypothesis in contemporary epistemology. It teaches us something deep and important about an absolutely central aspect of our lives as social beings. It's a hard-won discovery that illustrates philosophy's value, exemplifies genuine philosophical progress, and which we as a discipline can be proud of.
Whenever I explain the knowledge account as part of a presentation or in conversation, the primary -- indeed, pretty much the only -- objection I hear is that it's counterintuitive and, by way of illustration, purported counterexamples are adduced. The examples feature subjects who assert false but well justified beliefs. This is usually followed by the proposal that justified belief, not knowledge, is the norm of assertion. I'm also often told that this is the "ordinary" or "commonsense" view and that it's a foible of epistemologists to place knowledge, or anything other factive state, into that role.
In a new paper to appear in Cognition, I report a series of experimental studies which strongly suggest that these objections and examples do not reflect the commonsense view. Instead, just the opposite seems to be true: the ordinary view is that we should not assert false claims, even ones extremely well supported by the evidence. By contrast, asserting well justified true claims is perfectly fine.
Interestingly, it turns out that resistance to a factive account of assertion's norm is at least partly driven by a general psychological tendency that many people have to excuse blameless transgressions by (falsely) denying that any transgression has occurred.
(Unoriginal thought: norms of social practices seems to be an especially fruitful area of cross-fertilization among philosophy and the social and psychological sciences.)