The United States could still earn this distinction faster than most of the world -- 79 countries accounted for by Pew hadn't had a female leader as of 2014 -- but that's in large part because there have been so few female leaders in Africa and the Middle East. Most of Europe and South America have already had one, as have India, China, Australia and Canada.

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But -- and there is a but -- Clinton's election would actually be more novel than it might seem. For a couple of reasons.

First, even though more than 60 countries had had a female leader as of 2014, the vast majority of them didn't have one for long stretches of time. According to Pew, "in nearly two-thirds of those nations a woman was in power for less than four of the 50 years – including 11 countries (17 percent) where a woman led for less than a year."

Clinton, of course, would serve a four-year term as president if elected -- longer than the vast majority of countries that have had a female leader actually saw that woman in power. And she could serve as many as eight years if reelected. Just 10 countries have had a female leader for a decade or more.

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Secondly, Clinton's triumph in the Democratic primary is more exceptional in one important way: because the United States has a presidential election system. These have proven more difficult for women to penetrate than parliamentary systems in which prime ministers aren't directly elected and there could be two leadership positions -- president and prime minister -- available to women.

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"I analyzed pretty much all the women who have ever run for president around the world," Farida Jalalzai, who has studied female world leaders, told Vox. "One of the most striking findings was that almost never do women actually win their election contest when they're running for presidencies."

Jalalazi explained further in a paper about German Chancellor Angela Merkel:

Restricting analysis to the 56 non-temporary leaders, 35 (63 per cent) are prime ministers while only 21 (37 per cent) are presidents. . . . Women therefore serve more often in systems where executive authority is relatively dispersed. Among those in unified systems, 11 are presidents and 7 prime ministers. Thus, several women have unrivalled authority as national leaders, though this is still the exception rather than the rule. However, recall that prime ministers are generally highly vulnerable in their positions. Even prime ministers that share power with a weaker president are still susceptible to being ousted and they cannot dismiss the president. However, if for a dominant president sharing power with a prime minister, the potential power is great. Only two women presidents have ever shared power with a weaker prime minister (Jagan, Guyana; Kumaratunga, Sri Lanka). Subsequently, of 56 women leaders only 9 have ever held fairly unrivalled authority in the position of president.

Because of this, European countries with parliamentary systems, for instance, have been much more likely to produce female leaders than countries who directly elect a president. For a variety of reasons, voting directly for a female leader has proven a more difficult task.