Unwanted childbearing is a greater demographic force than the desire for large families, and may have been for centuries, suggests Robert Engelman, Vice President at the Worldwatch Institute, in his new book More: Population, Nature, and What Women Want. Expanding the capacity of all women to choose when to bear children is thus the surest route to achieving an environmentally sustainable population.

In countries that make effective personal control of reproduction possible for all, women invariably have two children or fewer on average, according to More. 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 said. "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.

The author, a former newspaper reporter who worked in the population and family-planning field before joining Worldwatch in 2007, interviewed women in Africa, Asia, and Latin America over a period of more than 25 years. Interspersing stories from these conversations with wide-ranging research across history and the social sciences, More delves into the roots of sexuality and procreation to discover how women's lives and status have influenced cultural evolution, history, and modern society.

The answer to "what women want," Engelman writes, is not "more children, but more for their children, and we can be thankful for that.

" Women have been so intent on reproducing at a time that is best for their child's survival that they have hidden their contraceptive use from their husbands and religious leaders, or have risked their lives to manage their fertility with dangerous or ineffective herbs or unsafe abortions.

Similarly, societies have at times been so intent on rooting out the use of contraception that it was banned in parts of the United States from 1873 to 1965. In Europe, the role of midwives in helping women plan births may have made birth attendants prime targets of the witchcraft hysteria of the 16th and 17th centuries.

Based on this record and contemporary findings, societies that make it easy for women and their partners to safely plan the timing of births will experience stable or gradually declining populations, Engelman contends. And that, in turn, will ease the staggering challenge of building environmentally sustainable and socially just societies.

Since its founding in 1974, the Worldwatch Institute has demonstrated how important the stabilization or gradual decline in population is for long-term environmental sustainability. That record drew Engelman to the Institute, where he directs the research strategy and continues his work on population.

"With its accessible analysis and innovative solutions to environmental problems, Worldwatch offers a perfect perch for me and a strong partnership with Island Press in helping to launch this book," he said. Engelman is posting regular blogs that appear weekly both at the Institute's Web site at http://www.worldwatch.org and a dedicated Web site maintained by Island Press, http://morethebook.org.