Opening ParagraphThe causes of migration are a major theoretical issue in migration studies. Some writers, such as Udo, see migration as involving a permanent or semi-permanent change of residence. It is usually considered to be a rational, that is an economic, man's reaction to economic differentials between regions (Udo, 1978: 124). Hill adds that in rural West Africa the propensity to migrate does not necessarily correlate at all closely with population density as demographers and economists so often assume; it may be the result of lack of lucrative non-farming occupations, because farming is not regarded as ‘work’ by some West Africans (Hill, 1978: 25–6).