Moreover, given street protests involving the middle classes in Brazil, Turkey, China, or Chile, it might look like the same kinds of anxieties that gnaw at the American and Western European middle classes are also pushing their peers in poor countries into to the streets.

But while members of the middle class in some Western European countries and the United States may be fighting to preserve their economic, social, and political positions in their own societies, in parts of the rest of the world they have recently made a dramatic emergence. Surprisingly, however, economic progress and more prosperity do not always buy more political stability. Even though things are improving for hundreds of millions of people in Asia, Latin America, and Africa, that doesn’t mean the people there are content.

Researchers and institutions such as the World Bank define the middle class within a large income range anywhere between $11 and $110 a day. And the convulsions within this income bracket are not new. Back in 2011 I wrote that "The main cause of coming conflicts will not be clashes between civilizations, but the anger generated by the unfulfilled expectations of a middle class, which is declining in rich countries and booming in poor countries."

My argument was that the middle classes in the United States and other higher-income countries would see their standard of living decline, while in China, Turkey, Colombia, and other emerging countries, the economic situation of the poorest would improve. I warned that both the decrease and the increase in income might fuel social and political instability. In other words, the rising incomes of people in poor countries might actually make them less content.

"Inevitably,” I wrote, “some politicians in developed countries are blaming the economic decline on the rise of other nations." And I concluded that the international consequences were not yet obvious.

Unfortunately, in some cases, they are now.

Many people have been able to leave the ranks of the poor over the past three decades or so. But the size and speed of the growth of the middle classes in poor countries has been truly dramatic.

Homi Kharas, an expert on the global middle class, estimated in a recent study that 3.2 billion people, or 42 percent of the total world population, are now in the global middle class. The group’s size increases by 160 million people a year, and assuming current rates of growth, in a few years most of humanity will live, for the first time in history, in middle-class homes or better.

But the middle class has grown at different rates in different places. While in the United States, Europe, Japan, and other advanced economies, the middle-class market grows at a meager 0.5 percent each year, in China and India that market expands annually at 6 percent. Although the middle class is now bigger than ever in countries like Nigeria, Senegal, Peru, and Chile, its expansion is primarily an Asian phenomenon. According to Kharas, the overwhelming majority (88 percent!) of the 1 billion people who will join the middle class in the next few years will live in Asia.