Deviations from these scaling laws, illustrated
by the spread of data in the figure,
measure how each city over- or under-performs
relative to expectations for its size6.
Relatively large deviations (as much as 30%)
ing
economies of scale have two profound
consequences. They constrain both the pace
of life (big mammals live longer, evolve slower,
and have slower heart rates, all to the same
degree9), and the limits of growth (animals
generally reach a stable size at maturity10). In
contrast, cities are driven by social interactions
whose feedback mechanisms lead to
the opposite behaviour. The pace of urban life
systematically increases with each expansion