PHOENIX - If there's a single, unqualified maxim in mutual fund industry, it's that performance, above all else, sells. But to what extent it keeps on selling has emerged as a topic of some debate, and that discussion, in turn, reveals some surprising factors that motivate investor behavior and inhibit economies of scale.

"When you have great performance, people open an account," said Erik Sirri, a professor of finance and the Walter H. Carpenter Chair at Babson College in Wellesley, Mass. "But if you continue to have good performance, you don't attract a whole lot of new money from existing investors. They tend to stay put."

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