Believing you’re good at something just because you do it—without any information that you’re doing it well—is indeed irrational. - Robyn Dawes

Prior to studies of unusually intelligent people that showed them to be generally much better adapted and happier than others, the popular belief in the United States was that exceptional intelligence was often associated with exceptional ability to “drive yourself nuts.” Hence, people believed that genius and lunacy were intimately connected. Perhaps, nearly all of us “drive ourselves a little nuts” by virtue of creating stories that lead us to the illusion that we understand history, other people, causality, and life—when we don’t.

The situation is very simple. Familiarity leads to availability and often to accuracy as well; hence availability is used as a cue to accuracy.
The problem is that mere assertion and repetition also leads to availability, whether or not this assertion and repetition involve reality, as familiarity generally does. Thus, the “big lie” of Nazi propaganda minister Joseph Goebbels was based on the idea that if something is repeated often enough, people will believe it—in large part simply because they have heard it before…
Goebbels apparently believed that providing a credible source, for him the German national government, was a critical component in having the repeated statements believed. Subsequent research has shown that the credibility of a source is not a necessary condition to develop beliefs…
Worse yet, mere repetition—which creates an availability bias due to familiarity, can also make people confident of their own decision making in the absence of any feedback that they have made good decisions.

Again, what cannot be is not, and what is can be regarded as an instance of what can be. Individuals who make pseudodiagnoses on the basis of “typical” characteristics—by attending only to the numerator of the likelihood ratio rather than to both numerator and denominator—will similarly be doomed to failure by making diagnoses that are not empirically supported. Because such a diagnostic procedure is based on irrationality, it cannot in general succeed. And similarly, people who argue that both the evidence and its negation support the same conclusion are arguing irrationally, and hence the conclusions will be empirically flawed. The principle is the same.