Apparently (via Tim Worstall) the least competent people are also the least likely to know they are incompetent - they lack self-monitoring skills:
"One reason that the ignorant also tend to be the blissfully self-assured, the researchers believe, is that the skills required for competence often are the same skills necessary to recognize competence.
The incompetent, therefore, suffer doubly, they suggested in a paper appearing in the December issue of the Journal of Personality and Social Psychology.
``Not only do they reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the ability to realize it,'' wrote Kruger, now an assistant professor at the University of Illinois, and Dunning.
This deficiency in ``self-monitoring skills,'' the researchers said, helps explain the tendency of the humor-impaired to persist in telling jokes that are not funny, of day traders to repeatedly jump into the market -- and repeatedly lose out -- and of the politically clueless to continue holding forth at dinner parties on the fine points of campaign strategy."
This result probably applies more broadly. Inexperienced people are not only going to be more likely to behave incompetently, they are less likely to expect that they will prove incompetent. Beyond that, they will find it harder to see when their colleagues are incompetent. That's why inexperienced politicians will struggle to make effective use of the advice of Civil Servants and other advisers - they don't have the experience necessary to spot the useless ones.