Typically a crank is impervious to contrary evidence. Those who have studied the phenomenon of crankery agree that this is the essential defining characteristic of the crank: being impervious to facts, evidence, and rational inference.
According to these authors, virtually universal characteristics of cranks include:
- Cranks overestimate their own knowledge and ability, and underestimate that of acknowledged experts.
- Cranks insist that their alleged discoveries are urgently important.
- Cranks rarely, if ever, acknowledge any error, no matter how trivial.
- Cranks love to talk about their own beliefs, often in inappropriate social situations, but they tend to be bad listeners, being uninterested in anyone else's experience or opinions.
In addition, many cranks:
- seriously misunderstand the mainstream opinion to which they believe that they are objecting,
- stress that they have been working out their ideas for many decades, and claim that this fact alone entails that their belief cannot be dismissed as resting upon some simple error,
- compare themselves with Galileo or Copernicus (or in a religious context, Noah), implying that the mere unpopularity of some belief is in itself evidence of plausibility,
- claim that their ideas are being suppressed, typically by secret intelligence organizations, mainstream science, powerful business interests, or other groups which, they allege, are terrified by the possibility of their revolutionary insights becoming widely known,
- appear to regard themselves as persons of unique historical importance.
Cranks who contradict some mainstream opinion in some highly technical field, such as mathematics or physics, frequently:
- exhibit a marked lack of technical ability,
- misunderstand or fail to use standard notation and terminology,
- ignore fine distinctions which are essential to correctly understand mainstream belief.
That is, cranks tend to ignore any previous insights which have been proven by experience to facilitate discussion and analysis of the topic of their cranky claims; indeed, they often assert that these innovations obscure rather than clarify the situation.
In addition, cranky scientific "theories" do not in fact qualify as theories as this term is commonly understood within science. For example, crank "theories" in physics typically fail to result in testable predictions, which makes them unfalsifiable and hence unscientific. Or the crank may present their ideas in such a confused, "not even wrong" manner that it is impossible to determine what they are actually claiming.