Familiarity gets misread as truth, which is among the most reliably replicated findings in psychology. The more often a statement has been encountered, the more likely it is to be judged correct. The effect persists where the statement can be verified as false, and it persists where the reader knows about the effect. The mechanism is simple enough. Processing familiar information is easier than processing unfamiliar information, and that ease of processing gets mistaken for agreement with the content itself.
In a market information environment, the mechanism is unusually well fed. A single view surfaces in many forms. There is the original long piece, a condensed commentary, a quoted fragment, and a chart lifted out of context. Then come dozens of accounts that look independent while having all read the same material. From the receiving end this presents as several dozen separate confirmations arriving from unconnected directions. From the source it is one piece of material replayed several dozen times, which is a very different thing.
The composition of a following list makes matters worse, and it is rarely the product of deliberate choice. People continue following sources they find rewarding, and the judgement of what counts as rewarding depends heavily on whether the source articulates things the reader was already inclined to believe. The list gets slowly filtered into a homogeneous group over a period of years. Every step of that filtering feels like an improvement in quality at the time it is made, because unfollowing somebody unpersuasive genuinely does feel like raising a standard.
One feature of the resulting state deserves attention. Opposing arguments have not been defeated, they have merely failed to appear. Those two situations are hard to tell apart from the inside. Both are experienced as an absence of any compelling counterargument, and the experience carries no label indicating which one produced it. In the first case the counterarguments really are weaker. In the second they simply never entered the field of view, and the second case is by a considerable margin the more common.
Subjective confidence and evidential strength come apart as a result. Somebody can grow steadily more certain over time with no addition of evidence whatever, because certainty is being driven by exposure count rather than by anything the exposures contain. Confidence built this way tends to be unusually fragile when it finally meets a serious counterargument. It gives no visible warning before it collapses, which is why such collapses are usually described afterwards as sudden reversals of view. The suddenness belongs to the visibility, and the underlying weakness was there throughout.
The test is direct enough to run in an afternoon. Try writing out the strongest case against a currently held view, written well enough that somebody holding that opposing view would accept it as a fair statement of their position. Managing it indicates real exposure to the other side. Failing to manage it indicates that the certainty in hand was assembled out of repetition, whatever else it may have felt like at the time. The condition is common enough that failing the test says nothing unusual about the person taking it.
What makes this exercise hard is not the execution. It is that almost everybody overestimates their ability to do it before attempting it. Nearly all investors believe they know what the other side argues. What emerges on the page is frequently a weakened version of that position, stated in terms the opposing side would not recognise as their own. The size of the gap between the expectation and the result is itself a useful piece of information, and it is available at no cost.