Concerns about low fertility have been present in many countries for at least 100 years. A large population was considered essential to national power. But the issue is never simply a shortage of warm bodies: overall the world population has increased dramatically over this period and untold numbers would immigrate, if allowed. The issue is the number of the 'right sort' of people, defined as those having preferred national, religious, racial, ethnic, or language characteristics. Fertility levels are below replacement in many economically advanced countries. As a result, these countries are aging; medical and retirement costs are increasing. Countries must either raise fertility, accept immigrants, or adapt to a smaller, older population. Policies to raise fertility have not been very effective, except in severe dictatorships. To keep the ratio of working age people to dependents constant, hundreds of millions of immigrants would be required such that 70-80% of the population of receiving countries would be immigrants and their children. Adaptation is probably best, but the required changes (raise retirement age, tax the pension benefits of the wealthy, etc) are politically difficult.