Marvell Technology (MRVL) jumped about 13% Monday after getting invited into the S&P 500 (^GSPC), the kind of move that can force index funds to buy and traders to pile in ahead of them. The catch is that the edge usually comes early. After analyzing 1,926 S&P 500 additions going back to 1957, Yahoo Finance found that stocks tend to outperform before joining the index, then fall behind the market after they get in. For funds that track the S&P 500, a new member has to be bought.