William G. Moseley

is an associate professor of geography at Macalester College

Today is World Population Day, a time when the United Nations calls on us to reflect on global fertility. Environmentalists have often framed the size and growth of the world's population as a problem. But if the question is defined as how many people the world can sustain, then facts suggest that American overconsumption is the real culprit.

Ever since the British philosopher Thomas Malthus wrote "An Essay on the Principle of Population" in 1798, we have been concerned that human population growth will outstrip available food supply. In the early 1970s, the burgeoning American environmental movement came to see overpopulation as one its key issues.

But human impact on the environment is not just a question of population numbers. At least two other factors - efficiency of technology and levels of consumption per capita - have as much, or more, influence in determining the environmental impact of a given population.

Conventional wisdom used to suggest that the wealthiest countries would employ the most efficient technologies. This assumption was shattered by the proliferation of gas-guzzling SUVs in the 1990s. In fact, the average fuel economy of American cars and trucks grew worse after the mid-1980s, even as per capita wealth grew.

In fact, no other population on Earth consumes at the same rate as the United States. With 4.6 percent of global population, Americans consume 24 percent of its energy. While China, Brazil and Ethiopia may have population growth rates that are, respectively, the same and 2.3 and 4 times higher than our own, Americans consume, on average, 6.8 times as much energy as the Chinese, 7.3 times as much as Brazilians and 28 times as much as Ethiopians.

In other words, in terms of environmental impact, our already high and exponentially growing per capita energy consumption far outweighs any population growth in the developing world.

So why focus on controlling population numbers when environmental impact is the result of three factors, not one?

As a college professor, I have watched students debate this issue for years. While students recognize the importance of all three factors, they invariably argue that it just isn't practical to try to control overconsumption. They suggest that the pragmatist must focus on what can be done - for instance, developing energy-efficient technology worldwide and supporting education and distribution of family planning methods in the developing world.

I am perplexed by the assumption that encouraging families in the developing world to have fewer children is more doable than reducing U.S. consumption. Having fewer or no children may be easy for a middle-class person in the United States, where raising children is expensive and most of us expect no economic return from children as they grow older. In fact, one could argue that having children in the American context is economically irrational.

It's true that millions of families in the developing world desire access to modern contraceptives, and filling this unmet need is important. However, for millions of others, children are crucial sources of farm labor or important wage earners who help sustain the family. Children often act as the old-age social security system for their parents. For these families, having fewer children is not an easy decision.

We also have misconceptions about overconsumption - that it's synonymous with human well being and development. But booming rates of childhood obesity, depression and environmental degradation contradict those connections.

While individuals can and should be encouraged to reduce consumption on their own, patterns won't change unless we address the underlying causes of interrelated consumption and development patterns. For years, government policy has promoted inefficient vehicles and auto-friendly suburban development to the detriment of mass transit. Subsidies also favor energy-intensive industrial agriculture over more efficient local farms.

It's time population control came off the top of the environmental agenda. While we should help those who want access to better family planning abroad, the real focus should be on controlling wasteful consumption at home.

William G. Moseley is coauthor of "The Introductory Reader in Human Geography: Contemporary Debates and Classic Writings."