If you want customers to pay more for something, you need to convince them that it’s better than the cheaper options. Investment companies that offer actively managed mutual funds are finding it harder to close that deal. More investors are pulling their money out of actively managed funds (average expenses: about 0.8% of assets annually) in favor of ETFs and index funds (average expenses: about 0.2%) that keep costs down by simply mimicking the markets. Meanwhile, a growing body of research suggests that very few of the pricier active funds outperform the indexers over the long term.

Active fund managers have often argued that they’re particularly good at beating the broader market (or at least at losing less money than the broader market) when stocks are falling. And active funds did outperform index funds in 2016, losing 2.2% on average while the average indexer lost 2.7%, according to Morningstar morn . But fewer customers are sticking around to see if the managers can keep that streak alive in 2016.

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A version of this article appears in the February 1, 2016 issue of Fortune.