The researchers studied how different types of industries affected the growth of productivity in U.S. cities from 1880 to 1930. They looked at data from the Census of Manufactures to see how cities specialized in different industries. They found that cities with more specialized industries tended to see faster productivity growth. Diversity in industries only helped productivity in big cities. This suggests that when cities have industries that focus on specific things, they can grow faster. The researchers think that with better transportation, cities became more specialized in certain industries, leading to better productivity for workers.