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The Dunning–Kruger effect is a cognitive bias manifesting in two principal ways: unskilled individuals tend to suffer from illusory superiority, mistakenly rating their ability much higher than is accurate, while highly skilled individuals tend to rate their ability lower than is accurate. In unskilled individuals, this bias is attributed to their metacognitive inability to recognize their ineptitude. Skilled individuals tend to underestimate their relative competence, erroneously assuming that tasks that are easy for them are also easy for others.[1]

David Dunning and Justin Kruger of Cornell University conclude, "the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others".[2]

Dunning and Kruger proposed that, for a given skill, incompetent people will:

fail to recognize their own lack of skill;
fail to recognize genuine skill in others;
fail to recognize the extremity of their inadequacy;
recognize and acknowledge their own previous lack of skill, if they are exposed to training for that skill.[5]

Dunning has since drawn an analogy ("the anosognosia of everyday life"[1][6] with a condition in which a person who suffers a physical disability because of brain injury seems unaware of or denies the existence of the disability, even for dramatic impairments such as blindness or paralysis.

If you’re incompetent, you can’t know you’re incompetent. […] the skills you need to produce a right answer are exactly the skills you need to recognize what a right answer is.