As a result of overdiversification, their (active managers) returns get watered down. Diversification covers up ignorance. Active managers haven't done enough research into any of their companies. If managers have 200 positions, do you think they know what's going on at any one of those companies at this moment?
Your ability to discipline yourself to set clear goals, and then to work toward them every day, will do more to guarantee your success than any other single factor.