Thinking you’re a genius can be quite a handicap. People who believe they’re geniuses tend to be obnoxious to be around — especially if their belief is unfounded, which it often is. And having a negative impact on other people has a negative impact on oneself.
The relational drawbacks of thinking you’re a genius are the most common, egregious, and discussed. But there are several additional ways thinking you’re a genius is a handicap, for strictly selfish reasons:
It makes you a perfectionist. You expect that all your output should be genius-level, since you’re a genius. Since you are incapable of actually producing genius-level output, you produce nothing, and feel sad. Everything you make sits in your drafts folder, because if you published it people would immediately catch on to the fact that you’re not actually a genius.
It makes you think that everything you make is great, even though it’s not. You dash off some sloppy work, and think it’s the next Finnegan’s Wake, or On The Road. So what nobody can figure out what you’re talking about; that’s their problem. Pearls before swine! The real problem here, is that thinking you’re a genius prevents you from doing the work. The only way to produce anything approaching genius-level work, as far as I can tell, is to put in genius-level effort. But people who think they’re geniuses think they can skip the “doing the work” step because, duh, they’re a genius and everything they come up with is brilliant.
Eventually you will (probably) realize you’re not a genius, and will have a mid-life (or, if you’re lucky, quarter-life) crisis and be terribly disappointed.
The gradually- and painfully-dawning realization that I was largely devoid of special innate genius-hood has been, perhaps, the best thing to have happen to my creative efforts. (A lingering suspicion that one could potentially become a semi-genius isn’t necessarily a bad thing, however, since it helps with aiming high and not giving up.)
Realizing that most of what I create is downright mediocre means that I work harder to make it at least the best I can make it, to bring it from genuinely embarrassing first draft to faintly disappointing end-product. And the other good thing about mediocrity is that my expectations of what I should be able to do are less delusional — which means, once in a while I actually make things and put them out into the world.