There is an underpopulation problem, but it's not a global one. There's no danger to the human race if we enter a prolonged period of population decline. Howeve

There is an underpopulation problem, but it's not a global one. There's no danger to the human race if we enter a prolonged period of population decline. However, the local demographic problems can be severe. Many countries, particularly ones that don't permit a lot of immigration, are seeing their pension programs becoming insolvent because there aren't enough working people to support the retired people. And those who are worried about racial purity are always concerned with population decline, because it's always the immigrants who have higher birthrates.