How Zero Population Growth Works

By: Dave Roos
A crowd of people waits to cross the street in midtown Manhattan on Oct. 31, 2011, in New York City, the day the U.N. declared the world population had hit 7 billion.
Spencer Platt/Getty Images

In 1968, Stanford University biology professor Paul Ehrlich published a controversial best-seller titled "The Population Bomb." In it, Ehrlich – and his wife Anne, uncredited – prophesied imminent environmental catastrophe and human suffering caused by overpopulation.

"The battle to feed all of humanity is over," Ehrlich pronounced in the book's dramatic opening line. "In the 1970s the world will undergo famines – hundreds of millions of people are going to starve to death in spite of any crash programs embarked upon now" [source: Ehrlich and Ehrlich].


In 1968, there were 3.56 billion people living on Earth [source: U.S. Census Bureau]. Earlier in the decade, American women were giving birth to more than 3.5 babies on average, and the global fertility rate was five babies per woman worldwide [source: Suzuki].

When Ehrlich did the math back in the 1960s, he saw worldwide population growing at an exponential rate. Global food production, on the other hand, was stagnated by droughts and decreasing fertility of farmland [source: IFPRI]. Combining too many people with too little resources, the obvious result for Ehrlich would be a world ravaged by food shortages, mass famine and even the collapse of civilization itself.

The same year Ehrlich published "The Population Bomb," he cofounded Zero Population Growth (ZPG), a grassroots organization that joined forces with the burgeoning environmental movement to raise awareness about the threat of overpopulation.

The central tenet of the zero population growth movement is not that humans should stop having babies. Rather, we should stop having more babies than we want. When women have control over their fertility – through education, contraception and improved women's rights – birth rates inevitably go down.

The goal of zero population growth is to reach a sustainable global birth rate at or below "replacement level." This is the fertility rate at which population is maintained, but not grown. Replacement level is affected by many factors, notably the average life expectancy. The longer people live, the fewer babies you need to replace them. In the U.S., the replacement level fertility rate would be 2.1 babies per woman. In certain developing countries where there are higher death rates and shorter lifespans, it could be 3.0 babies per woman.

From the day it was published, "The Population Bomb" drew angry criticism from groups opposed to contraception and skeptical of Ehrlich's doomsday rhetoric. Nearly 50 years later, there are twice as many people on Earth (7.2 billion), and it's hard to ignore the fact that England has not devolved into a "small group of impoverished islands, inhabited by 70 million hungry people," as Ehrlich predicted, or that 4 billion people have not, in fact, died of starvation [source: Chivers].

But do Ehrlich's failed predictions mean that zero population growth is itself a dumb idea? Keep reading to explore the real threat of overpopulation, proposed solutions and whether it's an exercise in futility.


Causes and Effects of Overpopulation

Over the past 110 years – a blink of the eye in human history – the world population has exploded from 1.6 billion to 7.2 billion [source: Graf and Bremner]. This exponential growth has many causes, most of them positive and directly related to improvements in health care [sources: CDC, Wattenberg, WHO]:

  • Global life expectancy increased from an average of 31 years in 1900 to 70 in 2012.
  • The global infant mortality rate in 1900 was 165 deaths per 1,000 live births; in 2013 it was 34 deaths per 1,000 live births.

Still, the vast gains humanity has made in terms of health, economic security and human rights have not been distributed equally among the world's 7.2 billion inhabitants [sources: FAO, UN]:


  • An estimated 1.4 billion people worldwide in 2010 lived on less than $1.25 a day, down from 1.8 billion in 1990.
  • Worldwide, an estimated 805 million people went to bed hungry every night in 2014, more than half of them living in Asia.
  • One in four people in sub-Saharan Africa in 2014 was chronically malnourished.

Global population growth also puts a strain on natural resources like clean water and forests [sources: WHO, WWF].

  • In 2014, 750 million people worldwide lacked access to clean water, contributing to 842,000 deaths each year from diarrhea-related illnesses
  • Millions of hectares of forest and jungle are cleared each year for increased agricultural production to feed soaring demand for soybeans, palm oil and grazing land for beef cattle

In 2014, more than half of the world's population lived in cities [source: UN]. While cities can provide improved economic opportunities for people in developing countries, they are also home to slums and sweatshops. In the poorest nations, more than half of city-dwellers live in slums with limited or no access to clean water, sanitation or permanent shelter, let alone education or health care. In sub-Saharan Africa, 61.7 percent of the urban population lives in slums. The United Nations predicts that almost all of future population growth will happen in cities.

Even if Ehrlich's original predictions were wrong, there are clearly regions of the world in which too many people are struggling to survive on too few resources. Next we'll look at some of the solutions to overpopulation proposed by the zero population growth movement.


Proposed Solutions to Overpopulation

Mothers and their newborns share space on a bed after giving birth in the maternity ward at an overcrowded government hospital in the Philippines. Manila has a population of about 20 million people, and it rises by 250,000 every year.
Paula Bronstein/Getty Images

Zero Population Growth, the organization cofounded by Paul Ehrlich in 1968, is now called Population Connection. Population Connection is deeply concerned about the threat of unfettered population growth to the environment, human rights, and the health and safety of women and girls.

For Population Connection, the most effective solution to dangerous levels of population growth is to increase women's access to modern methods of contraception. According to statistics collected by the organization, there are 222 million women in the developing world who have an "unmet need for family planning" [source: Population Connection].


Population Connection defines this "unmet need" as women who are sexually active, fertile, and not using contraception, but who don't want a child in the next two years. Population Connection believes that simply by eliminating unintended pregnancies we could reduce population growth by 33 million unplanned births a year.

Improving access to contraception is only part of the solution to overpopulation. Education and family planning counseling is also key, especially in developing countries.

In Uganda, for example, a third of married women report an unmet need for family planning. Relatively few of these women – only 8 percent those surveyed – cited lack of access as their top reason for not using contraception. Instead, they expressed fears about side effects; confusion over the need for contraception after birth; opposition from their husbands and others; and a belief that you don't need to use contraception if you have sex infrequently [source: Megquier]. Similar results are reported for other countries in the developing world.

A woman's right to take control of her own fertility is closely tied to other human rights. Women who have access to better education, better health care, and better employment opportunities have fewer children on average than women who don't enjoy those same rights.

Despite its good intentions, the zero population growth movement has many vocal and passionate critics. Keep reading to hear their side of the population growth story.


Critics of Zero Population Growth

A woman walks with her child in a stroller by a Family Allowance Fund (FAF) office in France. The FAF gives cash assistance to families, particularly those with three or more children; it's one way France encourages childbearing.

Some of the loudest critics of the zero population growth movement are critics of Paul Ehrlich himself.

"If I were a gambler," Ehrlich wrote in "The Population Bomb," "I would take even money that England will not exist in the year 2000." He made similar predictions about India, which would dissolve in a slew of food shortages, disease and riots [source: Chivers].


Something that Ehrlich failed to foresee was the "Green Revolution" in agriculture. While Ehrlich was making his doomsday predictions, an international consortium of plant scientists began breeding higher-yielding varieties of rice that would thrive in difficult growing conditions. These seeds, in combination with widespread use of chemical fertilizer and modern growing practices, led to vastly larger harvests in the developing world [source: IFPRI].

Critics of Ehrlich and the zero population growth movement use the example of the Green Revolution to assert that people – far from being "parasites" on the planet – are one of its greatest resources. People create and innovate, designing technologies and systems that have increased overall quality of life since 1968, even as the global population doubled.

Other critics of zero population growth believe that low birth rates are bad for the economy. If fewer people are born each year, then a nation's population gets progressively older on average. The fear is that fewer working-age people will result in less consumption of goods and services, slowing the economy. Also, without enough young working people, there won't be enough tax revenue to support social programs for the elderly.

Another criticism of the zero population growth movement is that countries with lower fertility rates actually do more damage to the environment. The two greatest producers of climate-changing greenhouse gasses are China and the United States, with relatively low fertility rates of 1.7 babies per woman and 1.9 babies per woman respectively [sources: UCS, World Bank]. Carbon emissions closely track per-capita income levels, not population growth rates.

As of 2015, population growth worldwide is slowing significantly.

More than 70 countries are now categorized as "low fertility" with rates of two babies per woman or less. Still, more than 60 "high-fertility" countries (i.e., more than three children per woman), mostly nations in Africa and Asia, are expected to make great economic gains over the coming decades [source: UN]. As these growing populations develop into global consumers, they will pose new threats to the environment.

When Paul Ehrlich founded Zero Population Growth, the belief was that world population would continue to grow exponentially, greatly outpacing our ability to feed ourselves. The modern population growth movement, led by groups like Population Connection, Population Institute and the Population Reference Bureau, place a greater emphasis on women's reproductive rights and improving access to family planning resources.

The critics may scoff at Ehrlich, but by dropping his "bomb" of wild predictions, he launched a movement whose very goal was to prove himself wrong.


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Author's Note: How Zero Population Growth Works

Nearly 50 years later, it's easy to look back at Paul Ehrlich's dire predictions about population growth and scoff. Yes, England and India still exist, and the world has managed to feed itself – just barely in some places – while doubling in population since 1968. But Ehrlich's call to action, as overstated as it was, had its desired effect. The issue of population growth entered the national and international conversation. Today, the United Nations has a dedicated Population Division that monitors population trends as they relate to sustainable development worldwide. Ehrlich's original Zero Population Growth organization has spawned other global nonprofits dedicated to eradicating poverty, protecting the environment and promoting greater reproductive rights for women.

Related Articles

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