One study has suggested that the tax system alone reduces the population density in urban areas by 15%.

ECONOMIST: Urban sprawl

Probably this worry derives from the difference in population density between urban and rural or small town areas, but there is a tacit implication that non-urban dwellers are more expendable than urban dwellers.

FORBES: 1st Person | Facing an Unsettled Future After Radiation Exposure

Population density not only increased the vulnerability of urban centres but magnified the impact of natural disasters, delegates heard.

BBC: NEWS | Science/Nature | Losing sight of the countryside