1. The world is overpopulated.
Sure, 7 billion is a big number. But most serious demographers, economists and population specialists rarely use the term "overpopulation" - because there is no clear demographic definition.
For instance, is Haiti, with an annual population growth rate of 1.3 percent, overpopulated? If it is, then was the United States overpopulated in 1790, when the new country was growing at more than 3 percent per year? And if population density is the correct yardstick, then Monaco, with more than 16,000 people per square kilometer, has a far greater problem than, say, Bangladesh and its 1,000 people per square kilometer.
Back in the 1970s, some scholars tried to estimate the "optimum population" for particular countries, but most gave up. There were too many uncertainties (how much food would the world produce with future technologies?) and too many value judgments (how much parkland is ideal?)
Even considering resource scarcity isn't all that helpful. During the 20th century's population explosion - when we went from 1.6 billion people to more than 6 billion - real prices for rice, corn and wheat fell radically, and despite recent spikes, real prices for food are lower than 100 years ago. Prices, of course, are meant to reflect scarcity; by such reasoning, the world would be less overpopulated today than a century ago, not more.
2. Rapid population growth keeps poor countries poor.
In 1960, South Korea and Taiwan were poor countries with fast-growing populations. Over the two decades that followed, South Korea's population surged by about 50 percent, and Taiwan's by about 65 percent. Yet, income increased in both places, too: Between 1960 and 1980, per capita economic growth averaged 6.2 percent in South Korea and 7 percent in Taiwan.
Clearly, rapid population growth did not preclude an economic boom in those two Asian "tigers" - and their experience underscores that of the world as a whole. Between 1900 and 2000, as the planet's population was exploding, per capita income grew faster than ever before, rising nearly fivefold, by the reckoning of economic historian Angus Maddison. And for much of the last century, the countries with faster economic growth tended to be the ones where population was growing most rapidly, too.
Today, the fastest population growth is found in so-called failed states, where poverty is worst. But it's not clear that population growth is their central problem: With physical security, better policies and greater investments in health and education, there is no reason that fragile states could not enjoy sustained improvements in income.
3. For all its ethical problems, China's one-child policy boosts its economy.
"It would probably take a catastrophe of biblical proportions to prevent global population growth over the next several decades."
China's economic boom has coincided with the promulgation of its one-child policy, which has used state muscle in an effort to limit births. Both this restrictive policy and the Chinese tilt toward pro-market reforms began in the late 1970s, and since then China's per capita income has risen more than eightfold. But that doesn't mean the two are linked.
Just before the one-child policy was enacted, China's total fertility rate (births per woman per lifetime) was about 2.7; today it is believed to be around 1.6, or roughly 40 percent lower. But between the late 1960s and the late 1970s , China's total fertility rate fell from about 5.9 to 2.9 births per woman per lifetime - a sharper drop. Yet China's per capita economic growth was much slower back in the pre-Deng Xiopeng decade of 1968-78. (Small wonder: Maoist economic formulas did not do wonders for material progress.)
China's fertility trajectory in the one-child era does not look strikingly different from those of many other East Asian and Southeast Asian societies. Much poorer countries, such as Burma, have very low fertility rates nowadays, even without state birth restrictions. Thus the demographic impact of China's policy remains uncertain. But some Chinese demographers suggest that it is responsible for much of the surplus of baby boys in China in the past generation; if so, a growing army of essentially unmarriageable young men is hardly auspicious for social stability or economic progress.
4. If your population declines, your economy does, too.
Between the 1840s and 1960s, Ireland's population collapsed, spiraling downward from 8.3 million to 4.3 million. Over roughly that same period, however, Ireland's per capita gross domestic product tripled.
More recently, Bulgaria and Estonia have both suffered sharp population contractions of close to 20 percent since the end of the Cold War, yet both have enjoyed sustained surges in wealth: Between 1990 and 2010 alone, Bulgaria's per capita income (taking into account the purchasing power of the population) soared by more than 50 percent, and Estonia's by more than 60 percent. In fact, virtually all of the former Soviet bloc countries are experiencing depopulation today, yet economic growth has been robust in this region, the global downturn notwithstanding.
A nation's income depends on more than its population size or its rate of population growth. National wealth also reflects productivity, which in turn depends on technological prowess, education, health, the business and regulatory climate, and economic policies. A society in demographic decline, to be sure, can veer into economic decline, but that outcome is hardly preordained.
5. The world will have 10 billion people by 2100.
No one can know how many people will be alive in 2100 because demographers have no techniques for accurately projecting our long-term population. The United Nations did project a population of 10.1 billionin 2100 earlier this year - but that was just its "medium variant" projection; it also put out a "high variant" projection (exceeding 15 billion) and a "low variant" (6.2 billion, lower than the world's population today).
Further complicating matters, we are seeing unprecedented declines in birth rates in some low-income countries. In just two decades, for example, total fertility in Oman is estimated to have fallen by 5.4 births per woman, from 7.9 in the late 1980s to 2.5 in recent years. And just a few years ago, the United Nations' "medium projection variant" for Yemen in 2050 exceeded 100 million - now it is down to 62 million.
It would probably take a catastrophe of biblical proportions to prevent global population growth over the next several decades. But we do not know with confidence just how big the world's population will be in 2030, much less 70 years after that.
Nicholas Eberstadt is the Henry Wendt scholar in political economy at AEI and the author of "The Poverty of the Poverty Rate."