Dunning-Kruger effect

"The Dunning-Kruger effect is an example of cognitive bias in which '...people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it'."

"They therefore suffer an illusory superiority, rating their own ability as above average. This leads to a perverse result where people with less competence will rate their ability more highly than people with relatively more competence."

"It also explains why competence may weaken the projection of confidence because competent individuals falsely assume others are of equivalent understanding 'Thus, 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.'"

see Wikipedia