the dunning-kruger effect

The more you know, the less confident you're likely to be.

Because experts know just how much they don't know, they tend to underestimate their ability; but it's easy to be over-confident when you have only a simple idea of how things are. Try not to mistake the cautiousness of experts as a lack of understanding, nor to give much credence to lay-people who appear confident but have only superficial knowledge.

“The whole problem with the world is that fools and fanatics are so certain of themselves, yet wiser people so full of doubts.”- Bertrand Russell

Further reading
Wikipedia | You Are Not So Smart (Please note this is an Amazon Associates link)