As you can see in the chart above, the most competent individuals tend to underestimate their relative ability a little, but for most people (the bottom 75%) they increasingly overestimate their ability, and everyone thinks they are above average. I sometimes hear the effect incorrectly described as, “the more incompetent you are, the more knowledgeable you think you are.” As you can see, self-estimates do decrease with decreasing knowledge, but the gap between performance and self-assessment increase as you decrease in performance.
The Dunning-Kruger effect has now been documented in many studies involving many areas. There are several possible causes of the effect. One is simple ego – no one wants to think of themselves as below average, so they inflate their self-assessment. People also have an easier time recognizing ignorance in others than in themselves, and this will create the illusion that they are above average, even when they are in the single digits of percentile.
More here – Neurologicablog
PS: If you are ever in the mood to look for not just an individual overwhelmed by this effect but an entire industry look no further than the local superannuation fund managers, who on average generate long term rates of return well below their industry benchmark but who think they are doing really well. And who also pay themselves really well for not doing very well.