The dunning cruger effect is a phenomenon named after two researchers who were at cornell university. It's basically an effect that, let me see if i do it justice explaining the gist of it: people who don't know very about any given domain tend to think they know a lot more than they do. So being ignorant about a domain leads to am self enhancement of the very biasd variety. At the same time, and will explain why, if you are at the top of your domain, if you're like an expert in your field, or in whatever performance you're good at, you tend to underestimate your performance relative to other people.