The more confident a person is in his own wisdom or understanding, the more likely he is to be off base. That's the old Dunning-Kruger Effect.
More info on the topic: We Are All Confident Idiots. A quote:
Because it’s so easy to judge the idiocy of others, it may be sorely tempting to think this doesn’t apply to you. But the problem of unrecognized ignorance is one that visits us all. And over the years, I’ve become convinced of one key, overarching fact about the ignorant mind. One should not think of it as uninformed. Rather, one should think of it as misinformed.