Underpopulation is usually defined as when a countries population has declined too much to support its current economic system. A simple example would be if all the retired people were supported through a sort of social security system, and then a generation only had an average of 2 children, compared with the last generations average of 3 children. In this case, the younger generation may not be able to support the older generation, and problems could erupt.
The essence of underpopulation is that it is defined and a result of the system in place in the country. It does not have to do with the population numerically, but only relatively, and is not usually opposed to overpopulation, which deals with the total possible amount of population desired.
It is widely believed that there will be an underpopulation crisis in Japan by 2014, which is unsolvable except by massive immigration. It is also believed that there could be the same crisis in Western Europe, however immigration there may help to offset the crisis. Problems could arise in Eastern Europe and Russia too, where the natality fell abruptly after the end of the communism. Underpopulation is also believed to be the root cause of the possible future failure of the United States Social Security system, but, until now, the constant flow of immigrants delayed the problem.
