Sustainability tied to pregnancy rates.

• Position: Childbearing

Unwanted childbearing is a greater demographic force than the desire for large families, and has been for centuries, suggests Robert Engelman, vice president at the Worldwatch Institute, Washington, D.C., in More: Population, Nature, and What Women Want. Expanding the capacity of all women to choose when to bear children thus is the surest route to achieving an environmentally sustainable population.

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In countries that make effective personal control of reproduction possible for all, women invariably have two children or fewer on average, relates Engelman. Such low fertility levels eventually lead to gradually declining populations in the absence of net immigration. "It makes sense that those who bear children and do most of the work in raising them should have the final say in when, and when not, to do so," Engelman argues. "By making their own decisions based on what's best for themselves and their children, women ultimately bring about a global good that governments could never deliver through regulation or control: a population in balance with nature's resources."

More explores the link between population and the environment through the lens of sexual relations and women's efforts to influence the timing of their reproduction. Engelman, a former newspaper reporter who has worked extensively in the population and family-planning field...