Despite attracting a massive amount of new cash, the nation’s largest funds continue to deliver strong returns, the Los Angeles Times reports. Companies that run the biggest funds include American Funds, Davis Funds, Dodge & Cox and Fidelity Investments.

The biggest stock fund, the Growth Fund of America, has more than quadrupled, from $36 billion in 2002 to $160 billion today. And last year it delivered 10.9%, four percentage points better than the average large-cap growth fund’s average rise. The track record is still impressive over the past five years, with the fun up an average of 8% a year, compared to 2.9% by its peers and the S&P 500’s average 6.2% rise.

One reason, of course, for the funds’ strong showing is that as new money pours into the fund, the portfolio manager frequently adds to current holdings, and the massive trading volume helps drive up the stocks’ value. But in American Funds case, because its funds are run by teams of portfolio managers, the firm doesn’t believe its funds can ever get too big.

But, certainly, as many funds grow, portfolio managers run out of good investment ideas. Also, too, the fund may begin to look like an index fund.

The mega funds are going through “a very interesting experiment right now,” said Roy Weitz, publisher of fundalarm.com.

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