# Unit 5: Human Population Dynamics // Section 2: Mathematics of Population Growth

Population experts can make demographic predictions with more confidence than many other social scientists. Several basic truths apply to the demographics of all human societies:

• Everyone who is alive one year from now will be one year older at that time than s/he is today.
• Ages 15 to 49 are humans' prime childbearing years, biologically speaking (although resource constraints and social and political factors shape childbearing decisions differently from one country to another).
• Human mortality is relatively high among infants, children, and adults over age 60, and relatively low at other parts of the life cycle.

Putting these observations together, population analysts can develop a reasonably accurate map of how a society's population size, births, deaths, and age structure are likely to evolve in the next several decades.

Birth and death rates are the most important determinants of population growth; in some countries, net migration is also important in this regard. To calculate population growth rates, demographers take the difference between births and deaths in a given time period, add the net number of migrants (which for the world as a whole is 0), and divide that number by the total population. For example, there are now about 136 million births and 58 million deaths worldwide annually, adding a net of 78 million new inhabitants to a global population of 6.7 billion, a growth rate of nearly 1.2 percent.

Until the mid-19th century birth rates were only slightly higher than death rates, so the human population grew very slowly. The industrial era changed many factors that affected birth and death rates, and in doing so, it triggered a dramatic expansion of the world's population (Fig. 2).

Figure 2. Past world population growth
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Source: Based on data from The World at Six Billion (1999). United Nations Secretariat, Department of Economic and Social Affairs.

How did industrialization alter population growth rates so sharply? One central factor was the mechanization of agriculture, which enabled societies to produce more food from available inputs. (For more information about increasing agricultural productivity, see Unit 7, "Agriculture.") As food supplies expanded, average levels of nourishment rose, and vulnerability to chronic and contagious diseases declined over succeeding generations. Improvements in medical care and public health services—which took place more in urban than in rural areas—also helped people to live longer, so death rates fell. After several decades of lower mortality, people realized that they did not have to have so many children to achieve their desired family size, so birth rates began to fall as well.

In addition, desired family size tended to decrease. As women found many more opportunities to enter the labor force, they were less inclined to devote resources to childrearing rather than paid work, and the jobs they had were not conducive to having children beside them as they worked. The costs of raising children also increased, as slightly wealthier families living in urban areas faced higher expenses for a larger array of physical and social necessities.

This phased reduction in death and birth rates is a process called the demographic transition, which alters population growth rates in several stages (Fig. 3).

Figure 3. The demographic transition
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Because death rates fall before birth rates, population growth initially speeds up (a phase sometimes referred to as the mortality transition), adding a large cohort of young people to society. This group in turn will have children, although probably fewer per family than their parents did, and because this group of childbearing-age people is large, population will continue to grow in absolute numbers even though on a per-capita basis birth rates will decline—a phenomenon that demographers call the fertility transition. Population momentum (i.e., continued population growth after a fall in birth rates) accounts for a significant portion of world population growth today even though the global fertility rate has declined from about 5 children born per woman in 1950 to a little over 2.5 in 2006.

Developed nations have passed through the demographic transition, and most developing countries are at some point in the process today. As a result, a "bulge," or baby-boom, generation, distinctly larger than those preceding or following it, is moving through the age structure of the population in nearly all countries. These large cohorts create both opportunities and challenges for society. Expanded work forces can help nations increase their economic output, raising living standards for everyone. They also can strain available resources and services, which in turn may cause shortages and economic disruption. (For more details, see section 7, "Other Consequences of Demographic Change.")

The demographic transition is a well-recognized pattern, but it has shown many variations from country to country. We cannot predict when specific demographic changes will occur in particular countries, and it is hard to specify precisely which factors will shape a given society's path. Looking forward, a major question for the 21st century is what happens after the demographic transition, and whether some countries in areas such as western Europe, where birth rates are very low, will start striving to raise fertility (footnote 3). More important in terms of environment and health, however, is the question of how to help countries that are lagging on the transition path.