Policy costs and benefits related to the provision of social security, social welfare and other public services are also considered. Two kinds of local government, neither seeing any expected loss in local welfare, have changed their negative attitude towards migration: fast-growing cities that have significantly benefited from the ample supply of migrant workers, and local governments whose local budget is so reduced and whose provision of public services is so limited that they are not affected by the inclusion of migrants. For example, Knight and Song (1999) depicted a distorted rural-urban relationship, which is characterised by large income gaps conditioned by an unlimited supply of labour. Yang and Cai (2003) examined the changes in institutions influencing labour mobility and the impact on the rural-urban income gap.