The stock market is back. But not the shares of biotechs.

Biotech was long a favored sector for investors with a stomach for risky bets on companies developing breakthrough medicines. For three years running, biotech stocks massively outperformed the broader markets before peaking last summer and then sliding in earnest when the rest of the stock market dropped at the beginning of 2016 on global economic jitters.

But since then, the Standard & Poor’s 500 index, for example, has recouped its losses and was trading at an all-time high last month. By contrast, biotech stocks remain in a bear market — called the Bio-Bust, by some — and are still more than 25 percent below their peak.

“The bloom has come off the rose,’’ said Jim Weiss, senior portfolio manager at investment firm Segall Bryant & Hamill. “In the euphoria, the biotech valuations went to astronomical levels. But there’s a feeling that they may have gotten ahead of themselves. Biotech stocks are more risky and volatile, and there’s been a drift to less volatile, more established companies.’’

Mark Williams, finance and economics professor at the Boston University School of Management, said early-stage biotechs — including some that became public companies before even launching clinical trials of their experimental compounds — were “priced for perfection.’’ Subsequent setbacks sent their shares plunging 50 percent or more in some cases.

Investors have become more skeptical, Williams said. “If you look at many of these biotechs, there’s no there there. You’re buying a lottery ticket with a promise.’’

There doesn’t seem to be a consensus about why biotech stocks — buoyed by the rise of precision medicine and the revolution in genomics — have diverged in 2016 from the general market, which has recovered partly because low interest rates have left investors with few other options.

Beyond feeling they were overpriced, some market watchers think biotechs are suffering from a growing backlash against prescription drug prices, including critical statements by presidential candidates Bernie Sanders, Hillary Clinton, and Donald Trump.

While some analysts have discounted the likelihood the next president would make good on calls to rein in prices, industry executives are clearly worried the US government’s hands-off approach to drug pricing will change — possibly by empowering Medicare, the federal health insurance program for older Americans, to negotiate for lower prices.

Joe Jimenez, chief executive of Swiss drug maker Novartis AG, which bases its global research and development in Cambridge, last month told the Financial Times, “We believe that, no matter which candidate wins, we will see a more difficult pricing environment in the US.’’

That, in turn, could lower the ceiling on estimated returns from biotech shares.

“If you project that the pricing environment is going to become tougher going forward, that’s going to reduce the value of future earnings’’ that guide the calculations of investors, said John Dorfman, founder and chairman of Dorfman Value Investments in Boston.

Dorfman said the souring on biotech may be part of a broader shift in investor sentiment toward less risky stocks, including those paying dividends. “Biotechs always did well based on the promise of future earnings,’’ he said. “But investors have become more conservative. They want to see the performance now, not on the horizon, and they’re less willing to wait.’’

Market watchers can’t point to a single event triggering the biotech swoon. But several early warnings came last year. Biogen Inc., one of the largest US biotech companies, lost $20 billion on July 24, 2015, nearly a quarter of its market value, after posting disappointing earnings and lowering its sales forecast for its multiple sclerosis drugs.

In September, after Clinton on Twitter bashed drug “price gouging,’’ the biotech index tumbled nearly 5 percent.

The wary attitude of investors hasn’t deterred the parade of biotechs going public. Of the 62 initial public offerings completed at the end of July, no fewer than 17 have been biotechs, according to data tracked by the Nasdaq stock exchange. That would suggest the sophisticated investors who buy into IPOs continue to see value in the sector.

Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.