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A headline in the Sunday Times yesterday reads ” Two Children Should be Limit, Says Green Guru. ” The guru in question is Jonathon Porritt, who chairs the British government’s Sustainable Development Commission. According to the Times article, Porritt says that couples who have more than two children are being “irresponsible” by creating an unbearable burden on the environment.

I guess elementary math is not part of the training of green gurus. The idea of two children per couple is obviously premised on the notion that each generation should produce only enough children to replace itself. But even if one accepts that premise, the mathematics is wrong, for several reasons.

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.

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. Given the enormous division of labor and degree of specialization required by an advanced economy, such an economy could not be sustained, and the human race would reduced to a primitive economic level. Without advanced technology, infant mortality and mortality in general would shoot up. The Porritt two-child maximum would go by the wayside, since women would have to bear many children just so that enough would survive to keep the human race in existence. Yes, the world would be very green indeed.

Judging from remarks that colleagues have made in my presence, there are a remarkably many otherwise intelligent people who agree with the green guru that it is irresponsible to have more than two children, and who pride themselves on having stopped at two, as though they were benefiting society thereby. I suspect that this widespread attitude, based on an elementary mathematical error, may be one reason for the woefully low birthrates in economically advanced countries. Not the only reason, or the most important reason, but a contributing factor.