India, July 11 -- World Population Day usually turns public attention to the population size. Is India 1.4 billion, 1.45 billion or 1.5 billion? Has fertility fallen? Which states are growing faster? Which regions will gain or lose political weight?

These questions matter. But for an urbanising country, another question is equally important: where are people counted?

Population is not only a matter of births and deaths. In demography, the population of any place changes through four components: births, deaths, in-migration and out-migration. A city grows not only because children are born there, but because workers move into it. A village shrinks not only because people die, but because people leave. Migration is therefore not a side st...