A small percentage of stocks account for most of the market’s gains, putting pressure on investment managers. This according to a recent article in The Wall Street Journal.

“Fear of being left behind is a powerful force among fund managers,” the article says, “even if it means taking big risks.” The article notes that in the first six months of this year, technology shares accounted for all of the S&P 500’s gains, adding that “managers not invested in tech stocks should be polishing their resumes.”

But other sectors are also attracting attention, the article reports, offering the example of Biogen–the biotech company saw a sharp rise in its shares last month after announcing (with its Japanese partner Eisai) “promising clinical data for patients with Alzheimer’s disease. All of a sudden, what had been a value stock became a compelling growth story, and Wall Street didn’t want to be left behind.”

The article cites FactSet data showing that growth stocks in the S&P 500 have outperformed value by 11 percentage points this year. “This reality,” it concludes, “makes the fear of missing out a powerful force—and means that the trend can feed on itself.”