Behind the migration policy changes in OECD and other countries in recent years have been a number of factors. In some cases, policies have been changed to deal with the effects of the global economic crisis, often through more restrictive measures. In other cases, policy changes reflect a shift in paradigm, due to changes in governments or other shifts in the social and political situation in countries. While long-standing concerns about the impact of ageing populations and the need to attract skilled workers remain in the background, and even in crisis drive some reforms, this has been counteracted by a souring of public opinion regarding migration in some countries. Finally, many reforms are a response not to external factors but to efficiency issues, such as bottlenecks and backlogs in processing, or to refinements made based on operational experience, such as tweaks to admission criteria to prevent abuse.