According to Wikipedia:
Some cranks claim vast knowledge of any relevant literature, while others claim that familiarity with previous work is entirely unnecessary.
In addition, the overwhelming majority of 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 shows that their belief cannot be dismissed as resting upon some simple error,
compare themselves with luminaries in their chosen field (often Galileo Galilei, Nicolaus Copernicus, Leonhard Euler, Isaac Newton, Albert Einstein or Georg Cantor),[citation needed] implying that the mere unpopularity of some belief is not good reason for it to be dismissed,
claim that their ideas are being suppressed, typically backed up by conspiracy theories invoking 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, (e.g. mathematics, cryptography, physics) may:
exhibit a marked lack of technical ability,
misunderstand or not 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.[6]