In the United States, the birth rate has fallen steadily since the baby boom, from 3.6 births per woman in 1960 to 2.0 today, or just under the replacement level, at which a population replaces itself from one generation to the next. Yet even at that rate, demographers estimate, the country will grow from 311 million people now to 478 million by the end of the century, because of both births and immigration.

The highest birth rates — from five to more than six births per woman — are occurring in a handful of nations in Africa and Asia, including Nigeria and Yemen. Yet among large economies, the United States is second only to Australia in the amount of carbon dioxide it emits per capita, according to the latest figures from the federal Energy Information Administration.

“Every person you add to the country makes all these tremendous demands on the environment,” said Joel E. Cohen, chief of the Laboratory of Populations at Rockefeller University and Columbia University.

But experts are reluctant to suggest an ideal birth rate. “There isn’t any magic number,” Dr. Cohen said.

As recently as the 1970s, the subject of population control was less controversial, partly because the baby boom years had given rise to concerns about scarcity of resources, some population experts and environmentalists said. Then came China’s coercive one-child policy and a rise in social conservatism in the United States, combined with the country’s aversion to anything perceived as restricting individual freedoms, be it the right to bear arms or children.