Yahoo CEO Marissa Mayer delivers a keynote address at the 2014 International CES at The Las Vegas Hotel & Casino. (Photo by Ethan Miller/Getty Images)

When a woman is appointed to lead a major public company these days -- think GM's Mary Barra or Yahoo's Marissa Mayer -- it often becomes a major media event, met with wall-to-wall news coverage and an avalanche of headlines. That's partly because they remain such a rare breed: Just 21 companies in the S&P 500 index are led by women. And in 2015, just one woman was named CEO of a major North American company. (Yes, one.)

But when it comes to appointing female CEOs, a new study finds, there apparently is such a thing as getting too much press. Researchers have examined the link between the amount of media attention new CEO appointments receive and the performance of the stock immediately following the announcement. They looked at the news coverage on the day that more than 8,000 CEO appointments were made between 2000 and 2014 -- just 84 of them female.

They found that when a new female CEO is named, the stock tended to go down if the announcement generated lots of headlines. Yet "the opposite held true for men," said Ned Smith, a co-author of the paper and professor at Northwestern's Kellogg School of Management. "Firms that appointed men and got lots of attention in the media got a positive response in the market." 

The working paper has not been published yet but recently won an award from the Academy of Management. The researchers found that women do get a lot more press attention than men when they're named CEO -- 3.6 times as much, in fact. They also found that on the whole, the stock market reaction is not measurably different whether the newly picked chief executive is male or female.

But when the researchers also analyzed how many media outlets covered those appointments, they found the market appeared to respond differently to gender. Companies that named female CEOs who were showcased in the press found their stocks trading at a discount just after the announcement, while the stocks of companies that gave the top job to women quietly were more likely to receive a positive response. For men, the response was inverted: The announcement of a male CEO who got little attention in the press appeared to have no significant effect on the stock, while those that got showered with attention were linked with the stock going up.

Smith points to a few examples: When Lynn Elsenhans was named CEO of Sunoco in 2008, just one media outlet covered the news on the day the appointment was announced; the stock initially rose 5.5 percent, his data show. Meanwhile, he says, when Beth Mooney was named CEO of KeyCorp in 2011, 20 media outlets reported the announcement, and the stock fell 3.1 percent. (Of course, there are outliers. For instance, Marissa Mayer's appointment as CEO of Yahoo in 2012 -- which also included her sharing the news she was pregnant -- was met with a deluge of media coverage, but the stock fell less than 1 percent.)

Smith and his co-authors think what's going on isn't necessarily straight prejudice -- though he won't rule out that some investors may think that way. Yet if it was nothing but investors acting on their own gender biases, he says, then the results should be similar for men and women no matter how much media attention they receive.

Rather, he theorizes that what's going on is something a little more meta. There's a concept in sociology, variably known as "anticipatory bias," "preemptive discrimination" or "second-order sense-making" which basically says we behave in a way that may look like we're prejudiced. But what we're really doing instead is acting in response to how we think others will behave. Investors may be doing the same thing in these instances. "It's thinking about the way others are going to respond, and adjusting one's response accordingly," says Smith. "It's the nature of speculative trading in markets of all kinds."

But what if it's not bias at all? What if it's simply a realization that because female CEOs of publicly traded companies are so rare, and under so much scrutiny, the market won't cut them any slack, and will respond negatively with even the smallest hiccup? Smith argues that if that were true, the negative market response to female CEOs who get lots of press wouldn't go away over time, because the scrutiny they face wouldn't go away either. His results, meanwhile, show that after a couple of weeks, the stock effect seen right after the announcement goes away.

So what should companies do? Whisper the news and try not to call attention to the fact that they've named a woman CEO? Ignore the effect entirely? Smith says no.

"I really don’t think the right interpretation of this is to say companies should shut up when you're appointing women," he said. "I'm of a belief that we need more women in business. We need to break down the hurdles to get to a more even playing field. So there are plenty of reasons to highly publicize these things." Besides, when a company in the news like Yahoo names a woman to the CEO job, the press is going to cover it, whether the company touts it or not. 

Rather, he thinks firms need to simply be mindful and aware of the phenomenon his research shows, and evaluate male and female CEOs accordingly. While that immediate blip in the stock price may not seem like it should matter, Smith says past research has tied the way a market responds to a CEO appointment with their compensation and their likelihood for dismissal down the line. "I think the story there is the board really becomes imprinted with this idea that 'the market didn't like this woman when she was appointed,' and they start interpreting the woman's performance differently."

Smith admits his research, or at least his explanation for their results, is a little depressing. "If this market response was really just about investors being prejudiced against women -- which we're not completely ruling out -- then time alone should make that go away, education should make that go away, as more women are appointed CEO," he says. But if it's true that the bias seen is really investors anticipating the biases of other investors, then it could take longer for change to happen. "It's much easier to update ourselves than to update our beliefs about others. It becomes a much stickier problem."

Read also:

Only one woman was named CEO of a major company in North America in 2015

The median female CEO actually out-earned her male peer in 2015

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