Tuesday's DNC session made it official: For the first time in the US, a major party has nominated a woman for president — Hillary Clinton. That's a real achievement for gender equality in the United States.

But looking to the rest of the world raises another question: What took America so damn long?

The United States, which positions itself as the world's foremost beacon of democracy and equality, is actually behind quite a few other countries when it comes to giving a woman the top job.

"There were countries like Sri Lanka, Pakistan, the Philippines, [or] the United Kingdom that were getting women into executive offices before the United States," Farida Jalalzai, a professor at Oklahoma State University who studies women in top national jobs, tells me.

The puzzle is why the United States has taken so long to even come close to catching up.

The answer, political scientists have figured out, is that there are a number of reasons why certain kinds of elections are harder for women to win — and a lot of them apply to the American presidency in a way they don't to, say, the German chancellery or the British premiership.

The stakes are also bigger in the US. Research from around the world suggests that when a woman takes office, it inspires other women to run for office more. The global nature of the US presidency, the most famous job in the world, means that a Clinton victory could increase political participation by women around the world.

It's harder for women to become presidents than prime ministers

Between 1969 and 2009, only 56 women were presidents or prime ministers of a democratic country. It's tough for any woman to surmount centuries of institutionalized sexism and win the top job. But the American context makes it especially hard.

First and foremost, the United States has a presidential system, not a parliamentary or mixed system (where a prime minister and a president share power).

Study after study has found that women are more likely to 1) become prime minister rather than president, and 2) more likely to gain either a presidency OR a premiership when it shares power with another office.

"I analyzed pretty much all the women who have ever run for president around the world," Jalalzai says, referring to a 2013 book. "One of the most striking findings was that almost never do women actually win their election contest when they're running for presidencies."

The problem here has to do with gender stereotypes. Prime ministers are only rarely elected by a direct popular vote; in parliamentary systems, people usually vote for parties rather than individuals.

That means a woman who wants to become prime minister takes power by cooperating with members of her party and convincing them to put her in the top job, rather than through grueling national elections. Presidential contests, as Ezra Klein writes, emphasize allegedly "masculine" virtues like oratorical skill and toughness over "feminine" ones like cooperation and consensus building.

Beyond the electoral system, voters see the very office of the presidency in gendered terms.

While prime ministers are seen as representatives of the party, governing by consensus and cooperation, presidents are seen as solitary leaders. Hence why women are more likely to win either presidencies or premierships when they share power in a mixed presidential/parliamentary system: Voters aren't electing a woman as their sole chief executive.

"Executive power is characterized by unity of command, hierarchical arrangements, and — with centralized control — a capacity to act quickly and decisively when circumstances dictate," Beloit College professor Georgia Duerst-Lahti writes. "These factors create circumstances in which women are understood as 'other' in contrast to a masculine norm, and they do so in a way that is predictable inside gender ideology."

Presidential elections, then, activate gendered stereotypes in the electorate. Consciously or subconsciously, voters tend to think that presidents should be men in a way that they don't when it comes to prime ministers.

This helps explain why women have led advanced democracies like the UK, Germany, Israel, and Canada, but not presidential countries like the US and France.

The US presidency is particularly tough for women

A handful of women have won presidential elections — Brazil's current president, Dilma Rousseff, is one (though she's currently facing impeachment). So it's not that gendered stereotypes make it impossible for a woman to win a presidential election; it's just a lot harder.

A number of factors in the US make it harder still.

For one thing, the US has an unusually low number of women in its federal legislature. Data from the Inter-Parliamentary Union finds that an average legislature is 22.7 percent female; the US Congress is 19.4 percent female, ranking a dismal 97th in the world when it comes to women's representation. A smaller female legislative bench means fewer women are likely to run for president.

Another important factor, somewhat strangely, is the stability of the American political system. Historically, women are more likely to take over executive positions in countries that have recently experienced or are currently undergoing fundamental crises.

"19 percent of women came to power after a period of political transition, 45 percent came to power in countries with a recent history of instability, and 33 percent after a military takeover," University of Texas Austin's Pamela Paxton and University of Pittsburgh's Melanie Hughes write in their book Women, Politics, and Power.

This appears linked to a well-documented effect in the business world called the "glass cliff": Corporations are more likely to appoint women executives when the company is failing or in trouble.

Experimental evidence finds that certain gendered stereotypes — like the idea that women "root for the underdog" out of compassion — lead people to conclude that women are better suited to leadership when things are bad.

The American political system is famously stable, with 227 years of democracy under the same Constitution. So here, would-be women presidents don't really benefit from crisis effects.

Finally, America's unique military might works against women candidates. The United States boasts the most fearsome military in human history. And voters think about electing a president in terms of electing a "commander in chief" with their "finger on the nuclear button."

These are, of course, highly gendered ideas: Leading troops into combat is stereotypically the most masculine of all masculine pursuits. People tend to envision a man in charge of the US military, creating another implicit barrier to a woman being elected.

There's some statistical evidence to back up this idea. According to Jalalzai's research, women are less likely to become executives in nuclear-armed countries: "Nuclear status statistically works against women when we're thinking about women breaking the glass ceiling," she says. This suggests that countries with greater military might are generally more hostile to female leadership, though more research needs to be done to confirm that.

No single one of these barriers makes it impossible for a woman to win the American presidency. But the unique combination of low numbers of women in Congress, political stability, and overwhelming military strength makes winning the office an astonishingly daunting task for any woman.

The Bill effect: why dynasties are good for women candidates

Hillary Clinton has done a lot to counteract these implicit biases, selling herself as a steely leader who isn't afraid to make the tough calls.

"Hillary Clinton has done a very good job, I think, of communicating her strength in leadership and adopting what many people stereotypically think are masculine traits," Jalalzai says. "She's done that purposefully, to overcome these obstacles that we know women have had to surmount."

But Clinton also has one major asset that almost no other American woman can claim: A husband who has been in the White House.

A lot of the time, this fact is used as a tool to bludgeon Clinton's candidacy. "Who wants another Clinton in the White House?" the line goes. "Dynasties are bad for democracy!"

But historically, dynasties have been a powerful tool for women seeking to attain political office.

Indian Prime Minister Indira Gandhi was the daughter of Jawaharlal Nehru, India's first prime minister. Former Argentine President Cristina Fernández de Kirchner took over after her husband, Néstor Carlos Kirchner. Guyanese leader Janet Jagan became president after the death of her husband, President Cheddi Bharat Jagan.

This effect is particularly strong in presidential systems, where women already face higher barriers to entry.

"Of 56 women leaders [between 1969 and 2009], only 9 have ever held fairly unrivaled authority in the position of president," Jalalzai writes. "All but one of these dominant female presidents ([Ellen] Johnson Sirleaf of Liberia) possess kinship ties to a former executive or opposition force."

Now, these women were all qualified in their own right; many were legislators or otherwise had real government experience. But without these family ties, it would have been even more difficult for these women to break the glass ceiling.

The issue is that when systems are overwhelmingly male-dominated, it's very hard for women to build up the kind of political support networks necessary to fuel a run for the presidency — owing to all the stereotype and networking effects we've already discussed.

Being connected by blood or marriage to an already-successful man gives a woman access to his support network. A woman can finally leverage the "old boys' club" in her favor.

You can see this not only in the international sphere but in congressional elections in the United States. The Huffington Post's Amanda Terkel explains:

Until the 1970s, one of the most common ways for a woman to enter politics was by following her husband. According to Pew Research Center, 90 women served in the House between 1916 and 1980; 34 of them were elected to fill their husband’s seat or replaced him on the ballot after he died. This practice became so common that it had its own term: the "widow’s mandate" or "widow’s succession." Male party leaders expected these women to quietly carry on their husband’s legacy until the party could find a permanent male successor. In total, 47 women have been elected or appointed to fill a congressional vacancy created by their husband’s death, according to the Center for Women and Politics.

Since the 1970s, Congress has opened up somewhat for women — but women with husbands in office helped break the congressional glass ceiling. We're now seeing the same pattern, just on a national level.

"It makes sense that Hillary would have had to wait, that Hillary would have had to come in second," Paxton, the UT Austin scholar, told Terkel. "She can run, but it’s after Bill. And that absolutely is a product of the fact that there were few women in Congress and there were few women in governorships."

It's hard to imagine a woman winning a presidential primary and election for the first time without some kind of family ties to the political establishment. Clinton's connection to Bill wasn't some kind of unfair advantage; it just puts her on a more level playing field with the men.

Breaking America's glass ceiling has global significance

Women winning elections creates a virtuous circle: Their win shows other women and girls that they, too, can win an election, leading to more women running and ultimately winning. As Matt Yglesias explains:

Amelia Showalter, a political consultant specializing in data and analytics, has studied the impact of electing women to statewide offices. Her research project initially focused on the impact of state-level efforts to recruit women to run for office — things like a state version of Emily's List — and she found that the presence of a recruiting campaign could increase the share of women in the state legislature by about a percentage point. That's not nothing. But she found that electing a woman to statewide office could have double or more the impact of a recruiting campaign.

International evidence points to a similar conclusion. Switzerland, for example, had all-male governments until it gave women the vote in 1971. When the University of Zurich's Fabrizio Gilardi looked at 168 municipalities between 1971 and 2010, he found that when a woman runs for office for the first time, it leads to significantly more women running for office in nearby municipalities. The "role model effect," while not the only reason for Switzerland's progress, appears to be quite powerful.

A Clinton victory could have significant inspiration effects in other countries, as well. The American presidency is by far the best-known political office in the world. A woman winning the office would send an unmatched signal to women everywhere that even the world's most powerful job could be held by someone like them.

"There is going to be, probably, this influence that we can detect, with her leading what is considered by most to be the most important country on the world stage," Jalalzai says (assuming Clinton wins).

Women's representation in politics isn't just a matter of domestic politics. A 2013 paper by scholars Suraj Jacob, John A. Scherpereel, and Melinda Adams found that countries tended to have more women in legislatures and cabinets when neighboring countries did, as well. Given the unique prominence of the American presidency, we could see a global version of this effect if Clinton wins.

"We would all be remiss to ignore the significance of Hillary Clinton’s nomination," Andra Gillespie, a political scientist at Emory University, writes in the Conversation. "Her nomination will no doubt serve as an example and inspiration to many girls and women who aspire to the pinnacles of political leadership in this country and around the world."

Whether or not you like Hillary Clinton, or agree with her politics, you can probably agree that's a good thing.