Gene Fama and Ken French show that after you take account of fees, actively managed mutual funds underperform index funds. This is not new news really, but Fama and French present the evidence in an exhaustively robust way. The evidence against active management is very strong.
Felix Salmon summarizes the article nicely here.
Still, most investors will ignore the results of this and other similar studies and chase returns by trying to pick actively managed funds. In the long run they will fail.