Over the past decade, Turkey and Brazil have been widely celebrated as star economic performers—emerging markets with increasing influence on the international stage. Yet, over the past three months, both countries have been paralyzed by massive demonstrations expressing deep discontent with their governments' performance. What is going on here, and will more countries experience similar upheavals?

The theme that connects recent events in Turkey and Brazil to each other, as well as to the 2011 Arab Spring and continuing protests in China, is the rise of a new global middle class. Everywhere it has emerged, a modern middle class causes political ferment, but only rarely has it been able, on its own, to bring about lasting political change. Nothing we have seen lately in the streets of Istanbul or Rio de Janeiro suggests that these cases will be an exception.

In Turkey and Brazil, as in Tunisia and Egypt before them, political protest has been led not by the poor but by young people with higher-than-average levels of education and income. They are technology-savvy and use social media like Facebook and Twitter to broadcast information and organize demonstrations. Even when they live in countries that hold regular democratic elections, they feel alienated from the ruling political elite.

In the case of Turkey, they object to Prime Minister Recep Tayyip Erdoğan's development-at-all-cost policies and authoritarian manner. In Brazil, they object to an entrenched and highly corrupt political elite that has showcased glamour projects like the World Cup and Rio Olympics while failing to provide basic services like health and education to the general public. For them, it is not enough that Brazil's president, Dilma Rousseff, was herself a left-wing activist jailed by the military regime during the 1970s and leader of the progressive Brazilian Workers Party. In their eyes, that party itself has been sucked into the maw of the corrupt "system," as revealed by a recent vote-buying scandal, and is now part of the problem of ineffective and unresponsive government.

The business world has been buzzing about the rising "global middle class" for at least a decade. A 2008 Goldman Sachs report defined this group as those with incomes between $6,000 and $30,000 a year and predicted that it would grow by some two billion people by 2030. A 2012 report by the European Union Institute for Security Studies, using a broader definition of middle class, predicted that the number of people in that category would grow from 1.8 billion in 2009 to 3.2 billion in 2020 and 4.9 billion in 2030 (out of a projected global population of 8.3 billion). The bulk of this growth will occur in Asia, particularly China and India. But every region of the world will participate in the trend, including Africa, which the African Development Bank estimates already has a middle class of more than 300 million people.