The Dunning-Kruger effect probably is real

The Dunning-Kruger effect refers to the idea that people with low ability levels overestimate their ability by a lot, and people with high ability levels underestimate their ability by a little (Kruger & Dunning, 1999). This conclusion is largely based on quantile plots where objectively measured ability and subjectively estimated ability are plotted as a function of objective ability quartile.

Figure 1 from Kruger & Dunning (1999).