????股票研究和交易机构Summer Street Research Partners医药科技股分析师马克?兰迪研究了大量并购传闻。他指出：“传闻当然会影响市场。有人在市场上投资，有人在市场上投机——老实说，有时候投机的那部分人给投资的人带来了机会。”兰迪还提醒我们：“投资领域甚至有句老话，出现传闻时买入，传闻兑现时卖出。”
????Leigh Drogen remembers exactly how he first heard that Radware was going to be acquired.
????At the time, Drogen was running a hedge fund of momentum stocks, and he’d been watching Radware, an Israel-based Internet security firm listed in the U.S. “Everything lined up for me that I should be owning this company, but I hadn’t pulled the trigger yet,” he remembers. Then on Twitter, a respected trader Drogen follows tweeted that a slew of options trades on Radware stock—bets that its shares would rise—had just been made. The trader’s conclusion: The company was definitely a takeover target. “That was the thing that put me over the edge,” says Drogen, who quickly loaded up on Radware shares.
????Within a week, Wall Street was abuzz with acquisition rumors: Analysts had labeled Radware an attractive target, and an Israeli newspaper, citing anonymous sources, reported that the company was considering a nearly $1 billion offer from IBM or Hewlett-Packard . The story was picked up by American blogs and news outlets, sending Radware’s stock through the roof. “It seemed real,” Drogen recalls.
????The deal never materialized, but that didn’t matter to Drogen, who was already long gone by the time the dust settled—taking a handsome profit with him. “If one of your stocks goes up 40% in one day on a rumor, you get rid of it,” he says. “You don’t look a gift horse in the mouth.”
????It’s a classic example of the power that gossip wields in the market—and the potential rewards for those who play the right rumor before everybody else catches wind of it. The chance to capitalize on such speculation is hard to resist, even for professional investors who swear that they would never trade on anything so flimsy as a rumor (including every portfolio manager Fortune interviewed for this story). It’s their job, after all, to look out for anything that could impact a stock—and mergers and acquisitions, even rumored ones, can sway stock prices more dramatically than most other events.
????Sometimes, when M&A speculation catches on, it becomes so contagious it spreads to the market itself, becoming a self-fulfilling prophecy—moving prices as though a deal had just been announced.
????In a new Fortune series on Contagion, my colleagues and I set out to explore this phenomenon—from the classically (and fatally) contagious MERS-coV virus (here and here) to more figuratively viral manifestations like stock market selloffs, runaway bestsellers and even the advent of “the selfie.”
????With M&A rumors, the contagion effect is obvious, but the mechanics behind it are a complete mystery to many market experts. We know that stock-related rumors are an almost daily part of the trading life. Much less clear is why certain bits of gossip—including some that seem to have little substance at all—catch a wave of trader attention.
????“Certainly, rumors move the market,” says Mark Landy, a medical technology stock analyst with Summer Street Research Partners, who has vetted his fair share of M&A gossip. “You have people who invest in the market and people who gamble in the market—and honestly sometimes, the gambling portion of the investment community gives the investment part an opportunity.” There’s even an old investing saying, Landy reminds us: “You buy the rumor and you sell the news.”