Japan now has its fifth prime minister in four years, after Yukio Hatoyama resigned this week and was replaced on Friday by former Finance Minister Naoto Kan. Hatoyama came to power just 10 months ago, but his popularity plummeted to below 20 percent over his inability to close a controversial U.S. military base. Still, Hatoyama is far from the only politician to have had such a short tenure in Japan's top job—the three leaders before him each resigned after less than a year in office. "It makes one wonder," says Kevin Voigt at CNN. "Is Japan’s political instability the result, or the cause, of Japan’s economic doldrums?" (Watch a Russia Today report about Japan's fast turnovers.) Here's a quick guide to Japan's "revolving door" prime ministers:

Why did Hatoyama resign?
One of Hatoyama's key pledges before taking office was to close the U.S. Marine Corps' Futenma Air Base in Okinawa. His failure to reach a deal with Washington is considered the main reason for his catastrophic collapse in the polls, although he is also facing damaging political funding scandals. But Japan's latest prime minister never warmed to the job, says Justin McCurry in Britain's Guardian, and could not wait to quit. It was evident in the way he "relinquished power with all the genuine grief, at least in public, of a student bidding farewell to his supermarket shelf-stacking duties."

Why have there been so many leaders in the past four years?
Although each left office for different reasons—Shinzo Abe (2006–07) was sick, Yasuo Fakuda (2007–08) resigned to protest legislative gridlock, Taro Aso (2008–09) lost an election, and Yukio Hatoyama (2009–10) quit for the reasons cited above—the four most recent leaders were all sons and grandsons of former prime ministers. Some have characterized them as "soiled 'silver spoon' weaklings," hereditary politicians with no experience of leadership and unsuited to the pressures of high office. 

But short-lived prime ministers are quite normal in Japan, aren't they?
Yes. Fifteen men have held the position in the past 21 years. Leaving Junichiro Koizumi out of the equation — he held the post for five years, from 2001 to 2006 — 14 prime ministers have lasted a total of 16 years. And Japan has had 32 prime ministers in the 65 years since its defeat in World War II.

Why such rapid turnover?
A "variety of reasons," according to Agence France-Press. These include a political system that does not allow enough time between elections for a leader to establish himself, a culture that quickly turns against prime ministers who impose unpopular policies, and an influential media that piles on in opposition to politicians newspapers regard as weak. Many of Japan's recent leaders have also been brought down by political scandals — Sosuke Uno was forced out of office in 1989 after just 69 days over his affair with a geisha girl.

Will the new prime minister be different?
Only time will tell. But Prime Minister Naoto Kan came to the post with "the backing of party heavyweights" and no significant opposition, says Rosie Kusunoki Jones in The Wall Street Journal. One thing's for sure, says the Associated Press. With his "ordinary upbringing" and "straight-talking populist" manner, Kan will be a break from the hereditary politicians who have paraded in and out of Japan's top job in recent years.

Sources: New York Times, CNN, Guardian, Telegraph, London Times, AFP, AP, Wall Street Journal