The closer that economists look at the rise in income inequality, the more they find one cause may be the rise of another inequality: The least productive firms are falling further behind the most productive firms. This point was made in a recent study spanning 16 countries by the Paris-based Organization for Economic Cooperation and Development. It found the “productivity gap” between firms in the top 10 percent by productivity and those in the bottom 10 percent rose by about 14 percent from 2001 to 2012.