The Dunning–Kruger effect is a cognitive bias wherein unskilled individuals suffer from illusory superiority, mistakenly assessing their ability to be much higher than is accurate. This bias is attributed to a metacognitive inability of the unskilled to recognize their ineptitude. Conversely, highly skilled individuals tend to underestimate their relative competence, erroneously assuming that tasks which are easy for them are also easy for others.
As 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.” [Wikipedia]
Also read the Guardian’s article: “Democracy v Psychology: why people keep electing idiots“