This is easy to understand. Hillary Clinton is poised to become the first female major-party presidential nominee in U.S. history. Congress is more than 80 percent male. And women’s under-representation is similar at virtually every other level of office. So it’s reasonable to assume that it’s harder for women to get elected than it is for men.

Indeed, our own surveys find that nearly 60 percent of Americans thinks the media covers women in a sexist way. Roughly half believe that women have to be more qualified than men to win office. And nearly one-third say that women who run for office don’t win as often as men do, or raise as much money.

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But they’re wrong. There’s another factor that keeps women out of politics — and it’s far more overwhelming than gender bias. We’ll explain below.

Voters and the news media treat female and male candidates very similarly

In our new book “Women on the Run: Gender, Media, and Political Campaigns in a Polarized Era,” we show that the vast majority of women who run for office are treated — by the media and voters — no differently than men. Women are under-represented not because of what happens on the campaign trail, but because they are much less likely to run in the first place.

And misperceptions about bias against female candidates are one reason why.

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Our book focuses on U.S. House elections, campaigns that look similar in many ways to the kind of down-ballot elections where the vast majority of women (and men) run. During the 2010 and 2014 midterms, we collected data on candidate communications, media coverage, and voter attitudes in every congressional district in the country. Along with interviews with more than 70 journalists and campaign managers, we were able to paint a comprehensive picture of the landscape faced by female candidates when they run.

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Our findings are strikingly consistent.

Male and female candidates run virtually identical campaigns.

Candidate sex has almost no influence on the volume or content of campaign communications. Women and men run the same number of television ads, send out the same number of tweets, and emphasize the same issues and traits in their advertising and social media messages.

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Men and women receive the same kind of media coverage.

We collected, read and coded more than 10,000 local newspaper stories over two election cycles, and could find only the smallest differences in the way male and female candidates were covered.

Oh, we tried. We examined whether reporters were more likely to mention women’s appearance or gender roles than men’s. Nope. Were the media more likely to associate female candidates with “women’s” issues, such as child care or pay equity? Nope. We checked whether news stories were more likely to report on women’s “feminine” traits, such as empathy and integrity. No gender differences there. And the media were just as likely to describe women and men as strong, competent leaders, and to cover women’s positions on “masculine” issues, such as national security and crime.

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We found no evidence whatsoever that voters are biased against female candidates.

In our national surveys, voters evaluate the women and men running for Congress in their districts identically when it comes to issue-handling abilities and personal traits. Nor do they change who they vote for based on the candidate’s sex.

That’s consistent with what other researchers are finding: Voters’ attitudes toward female candidates aren’t primarily shaped by gender stereotypes. Some experimental studies find a bit of a subtle gender bias in voters’ attitudes toward candidates — but in the real world, powerful cues, such as party identification, may be more overwhelming.

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Why so few gender differences in how the media and voters treat candidates?

There are two reasons.

First is the declining novelty of female candidates. As women have become a prominent part of American politics, male and female candidates have fewer incentives to run substantively different campaigns. As a result, journalists are less likely to cover female candidates “as women.” And voters are less likely to evaluate them in gendered terms.

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Second is polarization. The growing ideological gap between the Republican and Democratic parties means that campaigns tend to divide along party, not gender, lines. Republicans, male and female, stake out divergent positions on a full range of issues, as do Democrats, men and women alike. As a result, news coverage is largely shaped by partisan conflict — as well as how competitive the race is (i.e., “horse race” coverage) and other factors, such as who’s the incumbent and who’s the challenger.

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And in a more polarized climate, voters assess candidates based on whether there’s an R or D next to their name, not on whether or not there’s a Y chromosome in their DNA. Candidate sex just doesn’t matter much.

Each race’s mileage will vary

Of course, sometimes a candidate’s sex can become central — usually when one candidate finds it strategically advantageous to highlight gender in some way. That’s happening now in the presidential campaign. Donald Trump appears to be appealing to men who feel threatened by women’s advancement. Clinton, meanwhile, is trying to capitalize on Trump’s derisive remark that she’s been playing the “woman’s card,” among other comments.

But our in-depth interviews with journalists and campaign managers suggest that such circumstances are rare. And when they do come up, it’s even more rare that they put women at a disadvantage.

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So why do so many people believe politics is overwhelmingly sexist?

Which brings us back to public perceptions. If female candidates don’t routinely face discrimination, sexism or other unique obstacles while campaigning, why do most Americans believe they do?

For some people, it’s as simple as looking around: if men hold most positions of political power, gender bias must be the reason. Others face sexism and gender bias in their personal and professional lives, so they naturally assume that female candidates do, too. Still others see high-profile examples of overt political sexism —cue Trump — and conclude that female candidates must experience sexism regularly.

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To be clear, we are not contending that there’s no sexism left in politics. Women undoubtedly encounter it on the campaign trail — whether in the form of pundits criticizing Clinton’s voice or Republican Joni Ernst having to endure comments about being “as good looking as Taylor Swift” during her 2014 U.S. Senate race in Iowa.

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But there is a distinction between occasional, albeit high-profile, examples of sexist behavior and systematic gender bias in campaigns. These two facts of modern political life — sexism sometimes happens, and women do not face a systematically biased campaign environment — can coexist.

Perceptions of bias keep women out of politics

Because of that perception, many women think that they’d need, as the saying goes, to be twice as good to get half as far as men. And researchers have shown that that discourages them from running for office.

If our new book can change that misperception, perhaps it can help close the political gender gap.

Perhaps just as important, we hope our findings can educate party leaders, donors and activists who recruit candidates and help them raise funds. Female candidates will face no more difficulties on the campaign trail than will the men these political networks have traditionally encouraged to run.