How Population Works

The most basic (though not necessarily easiest or most accurate) way to measure population is simply to count everyone. This is known as a census and is usually undertaken by government officials. In the past, religious organizations carried out censuses, but usually on a local or regional level. The Roman Empire conducted censuses in order to measure the pool of military-age men and for taxation purposes, but these were limited because Romans had to report to government officials in their hometown to be counted. People who were poor or otherwise unable to travel were seldom counted [source: Weinstein & Pillai]. The U.S. government conducted the first true census in 1790 and has conducted a full census every 10 years ever since. A full census is sometimes known as complete enumeration -- every single person is counted either through face-to-face interviews or through questionnaires. There are no estimates.

Even a full census has limits. In countries with very remote areas, it can be impossible for census takers to count everyone. The 1980 U.S. census suffered from a documented undercount in part because census takers were afraid to go into some inner-city neighborhoods [source: Weinstein & Pillai]. A census also has trouble collecting information on rare populations. A rare population is one that is small or not reflected in standard census data. The United States isn't allowed to collect religious information in the national census, for example, so American Muslims could be considered a rare population. People who participate in a particular hobby or own a certain model of car are other examples of rare populations.

One alternative to a complete enumeration census is sampling. You might be familiar with this as the method used by market research companies and political analysts to conduct their research. Statisticians use a mathematical formula to determine the minimum number of people who must be counted to constitute a representative sample of the total population. For example, if the total population is 1,000 people, researchers might only need to survey 150 of them directly. Then they can take the data from the sample and extrapolate it to the full population. If 10 percent of the people in the sample are left-handed, it can be assumed that 100 out of a population of 1,000 are left-handed.

Sampling can actually return more accurate results than full enumeration, but there are some caveats. All samples have a margin of error, because there's always a chance that the sample selected for the survey differs from the total population in some way. This is expressed as a percentage of possible variation, such as "plus or minus four percent." The larger the sample size, the lower the margin of error. In addition, samples must be chosen as randomly as possible. This can be harder than it sounds. Let's say you want to survey a sample of everyone in France. One method used in the past was to select names at random from the phone book. However, this eliminates certain classes of people from the possibility of being selected for the sample: poor people with no phones; people who use cell phones and thus don't appear in the phone book; people with unlisted numbers; and most college students.

­Gathering population data for places that don't conduct censuses, or from historical periods before censuses became common, is accomplished by piecing together whatever demographic information is available. There may be partial censuses, local population data or information gathered by church or civic groups. Examining birth and death records provides other clues.

There's a lot more to know about populations than just how many people there are.

Age - The age of a population can tell us a lot about what the population is doing, as well as what it will be doing in the future. A sudden increase in the birth rate (such as the post-World War II "Baby Boom" in the United States) creates a "bulge" in the population. A larger than normal percentage of the population is then concentrated into a certain age group. As those people age, the bulge moves through the population and can have enormous societal effects. As Baby Boomers moved into middle age and started their own families, their tremendous purchasing power helped fuel the U.S. economy. As they move into old age, they will exert immense pressure on the health care industry and Social Security.

Location - Finding out where people live is one of the biggest reasons the United States conducts its census. Legislators in the House of Representatives are allocated to each state based on that state's population. Many government programs base their funding on population patterns as well. Location data also tells us about the movement of people. U.S. census data shows that Americans have been moving less and less often since the 1940s and that in the last 15 to 20 years, Americans have been moving away from the Northeast and into the Southeast [source: Population Estimates, Census 2000 Special Reports].

Socio-economic data - Computer mapping software combined with population data can show us patterns that might provide clues to underlying problems. Such a map might show high concentrations of poor people in certain urban areas, or high concentrations of people with cancer near certain industrial sites.­

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­Race - The demographic study of race is very controversial. Scientifically, there's no such thing as different "races" of human beings. The difference between Asian people and black people is the same as the difference between people with brown eyes and people with blue eyes. However, the idea of race still plays an important role in our societies. Many of us self-identify as being part of a certain race for cultural reasons. Demographers can study racial populations for information on issues that might be emphasized within a racial group, such as a medical problem. The U.S. Census Bureau explains the race data they collect as "generally reflect[ing] a social definition of race recognized in this country. They do not conform to any biological, anthropological, or genetic criteria" [source: U.S. Census Bureau Question and Answer Center].

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The human population has increased almost continually throughout history. Since no solid records exist for most historical periods, scholars have to estimate worldwide populations based on whatever demographic information they can piece together. In 10,000 B.C., there were between one and 10 million humans. By 1,000 B.C. there were 50 million. In A.D. 600, worldwide population had reached 200 million. At the dawn of the 20th century, 1.5 billion humans lived on the planet [source: Historical Estimates of World Population].

Our population seems to increase with greater and greater speed as the centuries go on. The primary reason for this is simple -- each increase in population creates more people able to reproduce. Population grows exponentially. If one million people have enough children to double the population (taking into account mortality rates), then the next generation will have twice as many giving birth to children. Doubling the population then results in four million people. This is sometimes known as the Malthusian Growth Model, named after one of the earliest researchers of population, Thomas Malthus.

So it should come as no surprise that in the 100 years between 1900 and 2000, the world's population quadrupled, topping six billion. The U.S. Census Bureau projects that it will exceed 10 billion by 2050 [source: World Population Information].

Spikes and Bottlenecks

­The more or less steady climb of human population is interspersed at various points with spikes (sudden, rapid jumps in the rate of increase that eventually level off) and bottlenecks, sudden drops in total population.

Worldwide population spikes in the past can only be estimated due to incomplete historical records, but evidence of sudden population growth coincides with the discovery of tools, the domestication of food crops and the Industrial Revolution. Each of these major changes in the way humans lived their lives resulted in a vastly increased capacity to produce food, goods or labor. They also freed up some people to take on specialized jobs and improved the overall quality of life. These conditions allowed humans to flourish and increase their populations. Generally speaking, periods of increased population growth coincide with periods of prosperity.

­Plagues and some wars represent population bottlenecks, also known as genetic bottlenecks. When the population suddenly and dramatically drops, a limited number of humans are able to continue reproducing. Although the population ultimately rebounds and grows beyond its pre-bottleneck levels, every human who is subsequently born can trace his lineage directly to one of the handful of reproducers in the bottleneck. This greatly limits genetic diversity.

As a population grows, it's put under pressure. This pressure can come from a lack of resources to feed, house and provide services; disease; war; or lack of enough space. The pressure can be relieved by migration. Wars, diseases and famine also reduce the pressure by killing off a portion of the population. In fact, the basis for Thomas Malthus' famous population theories is that human population will inevitably grow beyond the ability of the Earth to sustain it, resulting in self-correcting (and unpleasant) pressures.

Malthus' idea is sometimes known as "the Population Bomb" (or Malthusian Population Theory), and it gained popularity with the growth of the environmental movement in the 1970s. Fears of worldwide overpopulation are based on several factors:

Our vulnerability to these factors is bas­ed on population density, the number of humans per unit of area. Since the Industrial Revolution, urbanization has caused a tremendous increase in population density in cities. The highest population density ever probably occurred in the Kowloon Walled City area of Hong Kong. At one point, about 50,000 people lived in a megablock that was about 150 meters by 200 meters in area [source: Tofu Magazine]. The virtually lawless district was evacuated and torn down to make way for a park. Today, the areas of greatest population density are obviously in major urban areas. India and China have large areas of intensely high population density [source: NASA Visible Earth].

As population density in a given area increases, it approaches what's known as the carrying capacity. This is the maximum number of humans an area is capable of supporting in terms of available resources. For animals, this is easy to calculate. A goat, for example, might need 50 square yards of grass to survive. An area of 200 square yards therefore has a carrying capacity of four goats. Calculating carrying capacity for humans is much more complex. We can use technology to improve our resource production. We can ship in resources from other areas. We can create sanitation systems and other infrastructure to support higher density.

What happens when we reach carrying capacity in an area? There are several options:

­Humans are ­also capable of voluntarily controlling their own populations. This can occur on a large scale, like a ­government program or law, or at the individual level. Individuals have had far greater access to birth control since the 1960s. Governments can control populations by enforcing penalties for having too many children, by making it more advantageous to have fewer children, or by sterilizing people so they're unable to reproduce. Unfortunately, some governments have attempted to reduce or eliminate certain populations they view as undesirable by killing them off en masse -- this is known as genocide.

Since the 1970s, China has had an official policy forbidding most couples to have more than one child. Faced with tremendous population pressure, China levies hefty fines against anyone who violates the rule. The policy can be said to have achieved its goal, preventing an estimated 250 million births [source: BBC].

There are negative side effects, however. A cultural and religious preference for male children has led to abortions of many female fetuses, which in turn has led to a growing imbalance in China's male-to-female ratio. Shrinking populations in some areas have also caused economic problems. Dissidents and defectors claim that China engages in brutal human rights violations in the enforcement of the One-Child Policy [source: CNN].

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You might be surprised to learn that not everyone thinks a growing human population is a bad thing. In fact, some people think we're facing the opposite problem -- that our population isn't growing fast enough and might start shrinking.

How can that be? The simple answer is: birth control. Since the 1960s, when birth control pills became widely available to women in industrialized nations, the rate at which the world's population grows each year has dropped steadily [source: World Population Growth Rates]. This is becoming a problem in some countries, particularly if their population has been reduced by other factors such as disease or war. Russia is planning a program that would pay women cash grants for having children. Australia, Japan and several other nations have similar programs [source: New York Times].

­Why would a shrinking population be a bad thing? Wouldn't it be better if we used fewer natural resources and did less damage to the environment? It probably would be better in some ways. But it's also important to have a healthy world economy, and continual worldwide economic growth is largely fueled by population increase. People are consumers. More consumers equals more money. More money equals a healthier economy. ­

Population loss isn't a worldwide problem. There are plenty of humans, overall. It's only a problem in certain places where external factors have driven down the population. In these places, the population might get so low that it suffers from population collapse. This is the point at which the population is no longer large enough to support a functional economy. Any people that are left simply leave if they're able. Those who are too poor to move end up living in extreme poverty.

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