The ratio of a population to a given unit of area. Lagerlöf and Basher (2006) MPRA Paper 369 make the bold claim that population density correlates with per capita income in Canada and the United States. On firmer ground is Sato (2007) J. Urban Econ. 61, 2, who argues that migration from areas of lower to higher population density in Japan maintains regional variations in fertility. Crude population density is simply the number of people living per unit area and is not necessarily a development indicator.