Of all the crystal balls one could use to predict mutual fund performance — past returns, manager tenure, investing style, portfolio turnover — fees have historically held one of the tightest correlations to performance, according to research from Morningstar.

Yet even with fees steadily declining — down to zero for new index mutual funds from Fidelity — there may still be no better way to boost your chances of picking a winner than to go for a low-fee fund, according to a new analysis from Morningstar.