Population is defined as a group of individuals of the same species living in a given area. Population growth is the increase in the number of individuals in a population. Some key causes of rapid population growth are decreases in death rates due to factors like antibiotics and immunization, as well as increases in birth rates. Population growth is characterized by exponential growth over time, varying doubling times depending on growth rates, differences in infant mortality rates between developing and developed countries, and total fertility rates that are higher in developing countries. Problems caused by rapid population growth include increased demands on resources, inadequate services, environmental pollution, and unemployment.