  Why do some economic activities agglomerate more than others? And, why does
the agglomeration of some economic activities continue to increase despite
recent developments in communication and transportation technologies? In this
paper, we present evidence that complex economic activities concentrate more in
large cities. We find this to be true for technologies, scientific
publications, industries, and occupations. Using historical patent data, we
show that the urban concentration of complex economic activities has been
continuously increasing since 1850. These findings suggest that the increasing
urban concentration of jobs and innovation might be a consequence of the
growing complexity of the economy.
