From the First Things Blog in response to the call by an environmental organization that couples limit themselves to two children each:
First, as is well known, demographers say that for a constant population, the fertility rate averaged over all women should be 2.1 children per woman, not 2.0, since not all children survive to adulthood. Second, and much more important, there are many people who are unable to have children for one reason or another. About 15% of couples suffer from fertility problems; many people are unable to find a mate; and many who do find a mate marry too late to have children. Altogether about 19% of women in the United States in the 40-44 age bracket are still childless, which means that they will probably remain childless. This implies that in order to have a constant population, those women who are able and willing to have any children should have on average 2.6 children, not 2.0. If we also take into account the fact that many women who are able and willing to have a child are unable to have more than one, one finds that those women who are able and willing to have more than one child must actually average almost 3 children just to keep the population stable. Instead of the canonical "family of four" that has been held up for so long as the ideal, it should be the "family of five," or four and three-quarters, perhaps.For the full post, see http://www.firstthings.com/blog/2009/02/02/green-guru/
To put it another way, if no one had more than two children, as the green guru would want it, the fertility rate could probably not be gotten above 1.4. In twenty generations the world population would plunge to less than 2 million.