http://en.wikipedia.org/wiki/Underpopulation
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 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
http://news.bbc.co.uk/1/hi/world/americas/3560433.stm
The growth rate of the world population has slowed down, according to the US Census Bureau.
Its report says there were 74 million more people in 2002 - well below the 87 million added in 1989-90.
The rate of growth peaked 40 years ago, when it stood at about 2.2% a year. The bureau partly attributes the drop to women having fewer children.
In 1990 women around the world gave birth to 3.3 children on average, the report says.
By 2002, the average had dropped to 2.6 children - slightly above the level needed to assure replacement of the population.
The bureau's projections show the level of fertility for the world as a whole descending below replacement level by 2050.
It forecasts there will be nearly 9.1bn people by 2050, just under a 50% increase from the 6.2bn in 2002.