Which Leaders Resign, Like Jacinda Ardern? Often, the System Decides.

When Jacinda Ardern announced this week that she would step down as New Zealand’s prime minister, her decision caught the world by surprise. She called leading a country “the most privileged job anyone could ever have,” but said she would leave office by February.

It was particularly striking to see a leader voluntarily relinquish power at a moment when the world’s strongmen — and even some elected presidents — are clinging ferociously to theirs.

Jair Bolsonaro of Brazil, for instance, has disputed the election results that recently removed him from office, with some of his supporters storming the country’s legislature in an apparent mimicry of the United States’ own attempted insurrection in 2021.

Ms. Ardern framed her resignation as a personal decision based on no longer having “enough in the tank” to fulfill the responsibilities of being prime minister. Some supporters have also praised her move as embodying the democratic ideals on which she spoke passionately.

But what separates leaders who step down from those who do not often turns less on that leader’s ideology or personal life than on the simple nature of their political system.

In parliamentary systems like New Zealand’s, it is the norm for leaders to step down when it is thought that doing so will best serve their party’s electoral prospects. Sometimes such a resignation is voluntary, and sometimes it comes amid quiet internal pressure from party members. Usually it is a mix of both.

Though Ms. Ardern has said that she is stepping down for personal reasons, her party is facing its worst poll numbers in years and a national election in October.

Parties in parliamentary systems often nudge a leader to step down in such circumstances because they can elevate a new prime minister from within their ranks to win back voters before the next election. (In New Zealand, another member of Ms. Ardern’s Labour Party was nominated on Saturday to take over as prime minister.)

In such situations, the party’s incentive is to keep this process quiet, so as not to air internal divisions or project political weakness. This often creates the appearance of a graceful and voluntary resignation.

Angela Merkel, Germany’s longtime chancellor, stepped down voluntarily in 2021, also several months before national elections in which her party faced difficult poll numbers. She presented the choice as hers, preserving her political stature and her party’s show of unity. Her party carefully orchestrated Ms. Merkel’s handoff to a handpicked successor. But the party nonetheless lost power in that year’s election.

And because any intraparty maneuvering in parliamentary systems typically plays out mostly behind closed doors, such leaders might not appear to be clinging to power even when they fight to do so. For instance, Justin Trudeau, Canada’s prime minister since 2015, has repeatedly survived grumbling from within his party amid sliding poll numbers.

Still, internal party disputes over leadership do sometimes explode into the open. In Britain, for instance, Boris Johnson as prime minister feuded openly with challengers within his party. But Britain operates a bit differently from most parliamentary systems: Its parties hold public leadership primaries open to rank-and-file members. And the country’s intraparty politics have grown especially acrimonious amid the tumult of Brexit.

But in most parliamentary systems, prime ministers, unlike presidents, are elected by their party’s lawmakers. Those lawmakers typically also have the power to replace them at will, or at least to trigger votes that might remove them. As a result, power handoffs, even chaotic ones, are overwhelmingly likely to proceed peacefully.

“The vast majority of the stable democracies in the world today are parliamentary regimes, where executive power is generated by legislative majorities and depends on such majorities for survival,” Juan Linz, a prominent political scientist who died in 2013, once wrote.

Presidential democracies, Dr. Linz and others have found, are unusually likely to collapse into coups or other violence. Scholars have identified several reasons for this. One is that these systems are set up in a way that makes removing a leader far more difficult and gives it higher stakes, while also effectively discouraging leaders from stepping down voluntarily. The separation of legislative and executive branches means that a ruling party cannot simply change out an unpopular leader with a replacement as it can in parliamentary systems.

Rather, that party must use the legislature to pry the president from office via public impeachment proceedings. Even in the rare instances when this succeeds, it tends to open deep and damaging fissures within the president’s party, as well as grinding the government to a halt, which is why lawmakers rarely do it.

Even when they do, it can bring a constitutional crisis or worse. Peru, for instance, has been mired in chaos ever since its president dissolved the legislature in December to prevent it from holding an impeachment vote, which led to that president’s removal from office and weeks of nationwide unrest.

Presidents also know that resigning or declining to run for re-election would hurt their party’s prospects of holding power. Party allies in the legislature know this, too, giving them powerful incentive to urge even a president they see as dangerous to the country to stay in office.

These disincentives also apply to presidents who lose power in an election or impeachment.

Donald J. Trump’s efforts to hold onto power after losing the 2020 presidential election may have been shocking and unprecedented for the United States, but they were well in line with the sorts of crises that play out in presidential systems worldwide.

But the deterrents to stepping down in a presidential democracy pale in comparison to those in an autocracy — especially one in which power is concentrated around a single strongman leader.

It is not just that autocracies grant their paramount leaders a level of power that makes them often unwilling to step down, while empowering them to remove any threats to their rule.

Power transitions are uncertain moments in any authoritarian system, inviting power grabs and bureaucratic infighting. This gives everyone invested in that system’s survival a reason to keep the leader in power, even if they are seen as imperious or corrupt.

Autocracies built around an institutionalized power center — a vast ruling party, for example, or a family monarchy or a military dictatorship — are typically better able to force and to survive a leadership transition.

Those leaders, after all, derive their power from the institution that elevated them, which also makes them subject to it. And those institutions typically have the ability to install a replacement.

Communist states like the Soviet Union, Vietnam and China, for instance, have all outlived most other dictatorships in part for their ruling party’s capacity to manage power transfers that might have felled other systems.

That makes leaders in such countries perhaps a bit more inclined to step down voluntarily, knowing that their system has a good chance of surviving and of protecting them in retirement. China’s last leader, for instance, stepped down voluntarily in 2013, even helping to hand off power to his replacement, Xi Jinping.

But Mr. Xi has steered China toward a sort of autocracy in which leadership transfers are often dangerous and voluntary retirements rare: what scholars call a “personalist” system, built around a single leader, colloquially known as strongman rule.

Other examples include Vladimir V. Putin’s Russia, Recep Tayyip Erdogan’s Turkey and Nicolás Maduro’s Venezuela.

Such leaders tend to make themselves into a kind of keystone at the center of the political system, holding it all together. They also have a habit of vanquishing potential rivals, leaving their government, by design, less able to nudge them out or to elevate a viable replacement.

This makes stepping down extremely dangerous even when such a leader might wish to do so. Since the end of the Cold War, two out of three personalist dictatorships have collapsed outright on their leader’s departure from office, according to research by the political scientist Erica Frantz.

As a result, dictators who step down voluntarily often find themselves quickly imprisoned or even killed amid the tumult surrounding their government’s collapse. So few ever do, instead waiting to die on the throne.

So while Ms. Ardern can step down without having to worry about anything graver than her party’s electoral prospects, the ruling power centers in a place like Russia remain all but stuck with a leader who has plunged their country into disaster, as in Ukraine.

It is a reminder that while the world’s dictators have presented their systems as bulwarks of stability in contrast with unruly democracies, it is arguably stability that is among democracy’s greatest advantages.