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So while governments focus on pronatalist policies, groups including Population Matters and Optimum Population Trust have reported a sharp uptick in interest in their advice, which is to only have one or two children, or none at all.

However, two key factors are strangely absent from the debate up to this point — the global context and the "optimum population size" for the United States.

Rather, it's an appeal for the United States to embrace a smart immigration policy — emerging from a sense of the country's optimum population size.

A serious discussion of immigration policy requires a robust examination of what might constitute the "optimum population size" for the United States, where optimum size is not a static concept.

If one makes an effort to calculate the “optimum population” for Japan given its natural-resource endowment and the likely impact of global warming over the next century, a number well below the current 127 million could be the answer.