The Organization for Economic Cooperation and Development says the gap is growing even in countries seen as more equal, such as Finland, Norway and Sweden. The Paris-based OECD is made up of 34 major economies, including the United States.
The study says the top 1 percent of earners in the United States earned 47 percent of all income growth over the past three decades.
It warns that the gap will continue to grow "without concerted policy action." Ideas it is mooting include reducing tax breaks and havens that reward the rich more than the poor.