When a woman runs for President of the United States, it’s like she wraps herself in a giant roll of clear Scotch tape: everything sticks to her, and she can’t move. In American politics, a woman’s gender acts as both an invisible adherent and a tight constraint: it’s harder to shake off mistakes, harder to pivot, harder to throw punches and harder to avoid them. Sexism is transparent, easy to look through if you don’t want to see it, which makes it possible to pretend it isn’t there at all.

The 2020 campaign started with four accomplished women senators vying for the presidency. On Thursday, the last one dropped out. By the time Elizabeth Warren ended her campaign, it marked the third time in four presidential elections that a woman had a real shot at the American presidency. Two of those times it was Hillary Clinton, which had led many observers to conclude that the problem wasn’t women, but rather something specific to Clinton herself. The failures of Kirsten Gillibrand, Kamala Harris, Amy Klobuchar and finally Warren—a well-liked, well-prepared progressive who once seemed positioned to unite the Democratic Party’s moderates and left-wingers—suggests that all of them were wrapped in this invisible Scotch tape.

Warren made many mistakes running for President, of course. So did Klobuchar, Harris and Gillibrand. So did Clinton. So did Republicans Michele Bachmann in 2012 and Carly Fiorina in 2016, and Democrat Tulsi Gabbard in 2020. But so does every male presidential candidate. Sure, Warren’s health care plan was imperfect—but it had more detail than Sanders’s. Yes, Harris’s record on racial justice raised red flags—but Joe Biden’s is arguably worse. Klobuchar had some hits and misses in her long legislative career; but Pete Buttigieg got more money and attention for revamping downtown South Bend than she got for more than a decade of Senate accomplishments.

The hardest thing for women candidates is that by discussing the double standards, they inevitably end up feeding them. When asked about how gender affected this race, Warren let loose in a way that she never could have done just 48 hours earlier, when she was an active presidential candidate: “You know that is the trap question for every woman. If you say, ‘Yeah, there was sexism in this race,’ everyone says ‘whiner,’” she told reporters Thursday. “And if you say, ‘No, there was no sexism,’ about a bazillion women think, ‘What planet do you live on?'”

After she lost the presidency in 2016, Clinton voiced a similar feeling, one that was impossible for her to describe while she was in the running. “It should not be an impossible task for more women to achieve their own goals,” she told TIME for the inaugural FIRSTS issue. “But we face what is a pernicious double standard that is aided and abetted by the idea of perfectionism.”

Other countries don’t seem to have this problem. On TIME’s new 100 Women of the Year list, a tribute to 100 women who defined the last century, many of the influential women were heads of state of other countries.

Indira Gandhi became Indian Prime Minister in 1966. Golda Meir was elected Israel’s Prime Minister in 1969. Margaret Thatcher was Prime Minister of the United Kingdom throughout the 1980s. Corazon Aquino was sworn in as President of the Philippines in 1986. Benazir Bhutto took the top job in Pakistan in 1988. Ireland, Lithuana, France, Turkey, Poland and Canada all had women heads of state during the 1990s. Angela Merkel became Chancellor of Germany in 2005, and Ellen Johnson Sirleaf became president of Liberia in 2006. More recently, Finnish Prime Minister Sanna Marin briefly became the youngest head of state in the world; she is the third woman to lead Finland.

It’s not that these other countries are necessarily more egalitarian. India, for example, has long struggled to give women equal access to education and deal with epidemic-levels of sexual violence, and women’s rights in neighboring Pakistan are still far behind the U.S. Countries from Turkey to Indonesia to Thailand have all elected female heads of state despite lagging behind in gender equality in other areas.

So why has the U.S. not elected a female head of state?

It could be because the American political system is more of a popularity contest than other countries, which creates particularly thorny challenges for women leaders. It may be easier for women to achieve executive roles in a parliamentary system, like in Germany, the U.K. or Finland. Parliamentary systems might be more favorable to women because “you don’t vote for the Prime Minister, it happens within the institution,” explains Debbie Walsh, director of the Center for American Women and Politics at Rutgers University. “The party becomes the majority, and then the parties can do more structurally for women to attain leadership.”

The parliamentary system allows parties to be more intentional about advancing women leaders: some even have quotas about the number of seats that are reserved for women. Under a parliamentary system, the head of state is not decided by whether a particular group of voters—say, white working-class voters in Michigan—identify with a particular candidate. If the U.S. had a parliamentary system, then Nancy Pelosi might be President of the United States.

Other countries, particularly European states or colonized nations, may have more comfort with women heads of state because of the legacy of European monarchies. A woman prime minister of the U.K., for example, may seem less unusual because of the kingdom’s long history of being ruled by a Queen. Likewise, in India, where Queen Victoria once graced the colonial postage stamps, a woman prime minister might not seem so out of place.

“The challenge is in this country there is this underlying resistance and long held social construct of who can be head of state,” says Walsh. “So you tell people to close your eyes and imagine what a president looks like, and it still does not register as a woman, and barely as a person of color. It’s still an older white man.”

Other women leaders rose at times of rebellion, or in the early decades of young democracies, without centuries of political precedence working against them. Golda Meir was only the fourth prime minister of Israel, taking office just 20 years after the nation was founded.

Being part of a political family also helps. Corazon Aquino was the 11th prime minister of the Philippines, rising to power with the 1986 People Power Revolution, partly to avenge the assassination of her husband, a popular Filipino senator. Both Indira Gandhi and Benazir Bhutto were daughters of prime ministers.

But apart from these factors, there are aspects of the American political process that seem to uniquely disfavor women, especially now. Running an 18-month presidential campaign in a 24-hour news cycle creates infinite opportunities for intense scrutiny, which falls harder on women candidates. American politics has become infused with a culture of celebrity, which means voters often pick their candidates based on their personalities, making campaigns much more reliant on the dubious science of “likability” in a way that usually cuts against women. In other countries, where campaigns are shorter and the media scrutiny is less intense, elections are more often decided over policy distinctions rather than personality.

Donald Trump’s presidency has made the stakes of the 2020 election feel especially high to Democrats, in ways that may have made it even harder for voters to consider nominating a woman after Clinton’s surprise defeat in 2016. Many felt it was simply too big of a risk.

But more women will run, and more women will fail. As they do, that tape that binds them may become less sticky and less constraining, less transparent and more visible. One day, a woman will be able to shake it off altogether.

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Write to Charlotte Alter at charlotte.alter@time.com.