Demography is the study of the factors that affect rates of birth, death, and growth in populations. We begin our discussion of human demography with the observation that there are currently about 6 billion people on the earth. The three most populous nations are China, India, and the United States. Worldwide, the majority of people live in less-developed countries. Because of their higher population growth rates, less-developed countries will make the greatest contribution to world population growth over the next 100 years.