Causes:
1. Economic factors: People migrated from their home countries to other countries in search of better economic opportunities, such as higher wages, better working conditions, and greater job security.
2. Political factors: Migration was also driven by political instability, persecution, and war in many parts of the world, including Europe and Asia.
3. Social factors: Some people migrated to join family members who had already settled in other countries, or to escape social discrimination and inequality in their home countries.
4. Changes in immigration policies: Immigration policies were often shaped by political and economic factors, including concerns about national security, public health, and labor competition. These policies could be restrictive or permissive, depending on the country and the time period.
Effects:
1. Demographic changes: Immigration led to significant demographic changes in many countries, including the United States, Canada, Australia, and New Zealand, which became more diverse and multicultural as a result.
2. Economic effects: Immigration had both positive and negative effects on the economies of host countries. Immigrants contributed to economic growth and innovation, but also competed with native-born workers for jobs and resources.
3. Social effects: Immigration could have positive social effects, such as cultural exchange and diversity, but also negative effects such as discrimination and social tension.
4. Political effects: Immigration policies and debates about immigration often became highly politicized, with different groups advocating for different policies based on their political and economic interests.
In conclusion, the changes in immigration patterns and policies between 1900 and 1945 were driven by a range of economic, political, and social factors. These changes had significant effects on the demographics, economies, societies, and politics of many countries, and continue to shape global migration patterns and policies today.