As hard as investors try to wean themselves off stocks, the habit is proving impossible to kick. In a year when fund clients pulled about $60 billion from equities, the value of shares has climbed by $527 billion, pushing the total owned by households to $20 trillion, data compiled by Bloomberg and Ned Davis Research Inc. show. Today, Americans have 41 percent of their financial assets in stocks, matching the high in 2007 and trailing only the Internet bubble.