Cognitive psychologists studied expertise across a variety of sectors--from silver-smithing to law, surgery to architecture. One trait differentiated experts from the rest. The experts were more likely to be self-critically focused on improving their skills.

Citation for this study:

Bereiter, Carl, and Scardamalia, Marlene. Surpassing Ourselves: An Inquiry into the Nature and Implications of Expertise.
New York: Open Court Publishing Co., 1992.