The Dunning-Kruger effect refers to the tendency for people to over-estimate their ability to perform a particular task, particular if they are low-skilled at that task.  Related research suggests that the converse is also true; high-skills performers tend to underestimate their ability.  The effect was validated through a number of studies covering a range of skills, though the cause of the effect is still a matter of debate.  Similarly, the practical implications of the effect is debatable, as over-confidence stemming from an inaccurate self-assessments can have positive and negative impacts.