Not tonight, sweetie; no energy: a neo-Malthusian looks at fossil fuels and fertility.

• Author: Abernethy, Virginia Deane

The "fertility opportunity hypothesis" holds that parents want more children when they perceive forthcoming opportunities for a better life, but have fewer children if they anticipate hard times ahead. Perceptions of a coming global oil scarcity could result in population growing less than the UN expects.

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The Reverend Thomas Malthus (1766-1834) was notorious for the view that "positive checks"--meaning poverty, famine, and premature mortality--are the only means of keeping population size in balance with resources. But his second edition of An Essay on the Principle of Population is far from pessimistic: it develops the idea that "moral restraint" (encompassing social rules as well as personal decisions) often depresses the fertility rate, thus slowing or stopping population growth before calamities occur.

Worldwide, the dynamics of self-restraint are causing fertility rates to fall much more rapidly than generally anticipated, vindicating Malthus's foresight. Projections of an ultimate population size of 12 billion have been forgotten. In 2003, the United Nations offered a middle projection of 8.9 billion as the ultimate peak world population. A March 2004 report by the U.S. Census Bureau projected a most-likely scenario of 9.1 billion by 2050, with average fertility below replacement level and with hotspots of elevated mortality.

My own view is even more optimistic: world population is unlikely to rise above 8 billion (from approximately 6.4 billion today), and the fertility rate will fall from the Population Reference Bureau's 2003 estimate of 2.8 children per woman to below replacement level within the next dozen years. With population size peaking at a level lower than either the Census Bureau or the United Nations project, much excess mortality may be avoided.

Fertility and Opportunity

Why do I believe this? Because my data show that people faced with real or perceived deprivation typically exercise reproductive caution. Whether in hunter-gatherer or agrarian societies, or developing or industrialized countries, intimations of scarcity (with respect to wants as well as needs) encourage restraint. And in the near term, expensive fossil fuels could trigger an acute sense of scarcity. Put another way, this fertility opportunity hypothesis proposes that people usually have as many children as they think they can afford, and that the motivation to have more or fewer arises from perception of economic prospects. Perceptions crystallize through comparisons to past experiences or a reference group.

Where straying above carrying capacity would terminally threaten life-support systems, individual evaluation of relative well-being often gives way to rigid, culturally embedded rules. The hierarchical social structure of indigenous peoples living in the Amazonian blackwater ecosystem is illustrative. With sparse game, few fish, and nutrient-poor soils, population size remains in balance with resources because only the few fortunate heirs to fishery and residential sites expect to marry and reproduce. Defined real estate rights are unusual in primitive societies but a less restrictive system might founder as population overshot resources.

The fertility opportunity hypothesis suggests that rules atrophy, reproductive decisions become individualized, fertility rates rise, and populations explode when communities encounter new opportunities, such as those created by potato cultivation in Ireland around 1740, or by nitrogen fertilizer and foreign aid to underdeveloped countries during the twentieth century. Self-restraint is not rediscovered until economic prospects sour.

Anecdotal support for this idea is plentiful. For example, a Cairo slum dweller says, "We're just surviving.... Certain days we don't eat ... I don't understand how people with seven or eight children survive." Pondering what she would do differently if she were rich, an Ethiopian mother of five says, "If I were wealthy, say if I had horses and a better house, I'd have more children." A Russian mother of two who has relied on abortion to limit family size muses, "I would have had more children if life were better." Near Mexico City, a Roman Catholic mother of two defends her use of contraception...