Final answer:
A high rate of natural increase occurs when there are high fertility and low mortality rates, leading to rapid population growth.
Step-by-step explanation:
High fertility rates and low mortality rates result in a very high rate of natural increase. This pattern signifies that more individuals are being born than dying within a given population, thereby accelerating population growth. Fertility rates are understood as the number of children born to a woman during her lifetime, and mortality rates measure the number of deaths within a population. Sociologists use measures such as crude birthrate and crude death rate, which indicate the number of live births and deaths per 1,000 people per year, respectively. Developing countries often have higher birth rates along with declining death rates due to improvements in health care and reduction in infant mortality, which leads to a rapid increase in population size. Conversely, developed countries usually have balanced low birth and death rates, leading to minimal population growth.