This may be spontaneous, as when a particular age–sex group constitutes most of the migrants; ‘age-selective migration tends to dichotomize areas between younger and older, with higher and with lower natural increase’ (Morrill (1995) Annal. Reg. Sci. 29, 1). ‘Age-selective migration is systematically draining the entry labour pool in rural America’ (W. Kandell and D. L. Brown 2006). Boyle (2004) PPG 28, 6 argues that selective migration ‘will alter our interpretation of health and mortality inequalities’.

A leading cause of selective migration is the increasing skill-focus of immigration policy in a number of industrialized countries—‘a trend that is likely to intensify as rich countries age, and competitive pressures build in knowledge-intensive sectors’ (Kapur and McHale (2005) Center for Global Development). Vigdor (2002) Rev. Econ. & Sta. 84, 4 considers selective migration and economic outcome. See Hjort (2009), Gerum Kulturgeografi (2009) 1, notably p. 18.