Why public dissatisfaction boiled into all-out protest in Chile, Brazil, and Hong Kong

In Chile, bubbling frustration boiled over into deadly, full-blown protests over the weekend after the government announced what amounted to a four-cent subway fare hike in the country's capital.

In Brazil in 2013, it was a 20-cent bus fare increase that finally brought growing unrest to the surface.

In Hong Kong, protests that have been ongoing for more than 100 days began over an unpopular bill.

In isolation, each of the events that set off the protests is relatively mild. After all, what's another few cents to ride the train?

But each served as a "triggering event" for the region's citizens, tipping long-simmering frustration toward violent protests, according to Thomas Vicino, who is a professor of political science, public policy, and urban affairs at Northeastern.

The events "focused the public attention to a particular issue," says Vicino, who studies the political, economic, and demographic factors that shape cities and suburbs around the world.

After the increase was announced in Chile, thousands of students protested by jumping turnstiles, according to news reports. Chilean President Sebastián Piñera responded by declaring a state of emergency, deploying police and military units to Santiago, and putting a curfew into effect.

Far from quelling the conflict, such extreme measures only escalated it, Vicino says. Violent clashes between protesters and law enforcement officials have resulted in at least 15 deaths. International human rights advocates—including the human rights chief for the United Nations—are investigating allegations of excessive force, according to a news release by the U.N.

It's impossible to predict when a protest such as the ones in Chile, Brazil, or Hong Kong will happen, Vicino says, but there are certain ingredients, the presence of which make something like a transportation fare increase or an unpopular bill, more likely to trigger a protest.

Across the world, those ingredients are: A shift toward a more populist form of government, increasing economic inequality, and fast-paced changes to demographics, says Vicino, whose research on the 2013 protests in Brazil was recently published in the Journal of Urban Affairs.

The protests in Chile over the weekend—the worst unrest the country has seen since the days of Augusto Pinochet's dictatorship, according to news reports—may have started with a fare hike, but quickly came to represent the growing chasm between the have- and the have-nots in the country, Vicino says.

"It was a modest increase, but it represents something much larger to middle class Chileans who feel like they've been left behind in the global economy," he says.

So, the main ingredient for unrest here? Increasing economic inequality.

Chile has the highest level of post-tax income inequality among democratic countries, according to data from the Organization for Economic Cooperation and Development, an international organization that advocates for more equitable economic policy.

This widening wealth gap, combined with the country's recent history, was enough to trigger protests, Vicino says. An entire generation of Chileans still remember what it was like to live under Pinochet's dictatorship, which ended in 1990.

And, when these people started to see the images of the clash between protesters and the military, they were likely galvanized to protest as well, in an effort to protect the country's young democracy, Vicino says.