In their foundation’s just-released annual letter, Bill and Melinda Gates attempt to debunk three pervasive myths in development economics:

“Poor countries are doomed to stay poor.” “Foreign aid is a big waste.” “Saving lives leads to overpopulation.”

From the letter’s introduction:

We hear these myths raised at international conferences and at social gatherings. We get asked about them by politicians, reporters, students, and CEOs. All three reflect a dim view of the future, one that says the world isn’t improving but staying poor and sick, and getting overcrowded. We’re going to make the opposite case, that the world is getting better, and that in two decades it will be better still.

When it comes to poor countries’ prospects for escaping poverty, Bill Gates, who wrote the section addressing the first myth, is particularly optimistic:

By 2035, there will be almost no poor countries left in the world. Almost all countries will be what we now call lower-middle income or richer. Countries will learn from their most productive neighbors and benefit from innovations like new vaccines, better seeds, and the digital revolution. Their labor forces, buoyed by expanded education, will attract new investments.

Poor in this case means low-income, by the World Bank’s current definition: any country with a gross national income per capita of $1,035 or less in 2012 dollars. There are 36 such countries in the world today. How much would incomes have to rise over the next two decades for there to be no more low-income countries?​

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In some cases, it won’t take much additional income at all: In the Kyrgyz Republic, income per capita only needs to rise by 5% over the next two decades for it to be a middle-income country.

In Burundi and the Democratic Republic of the Congo, however, income per capita will have to increase by a factor of five. But Gates believes that not all low-income countries will make it to middle-income status by 2035: