The theory of demographic transitions is a model that explains the evolution of population growth and decline over time. It describes how changes in birth and death rates lead to shifts in population size and structure. The basic argument of this theory can be explained in the following points:

1. The Demographic Transition Model: The theory is based on the demographic transition model, which is a graph that shows the changes in birth and death rates over time. The model consists of four stages that represent different phases of population growth and decline.

2. Stage 1: The first stage is characterized by high birth and death rates, which result in a stable population. This stage represents the pre-modern era when people had large families to compensate for high infant mortality rates.

3. Stage 2: The second stage is marked by a decline in death rates due to improvements in sanitation, nutrition, and medicine. However, birth rates remain high, leading to a rapid increase in population size. This stage represents the beginning of modernization.

4. Stage 3: In the third stage, birth rates begin to decline as people adopt smaller family sizes and women gain access to education and employment. This shift in behavior leads to a slower population growth rate.

5. Stage 4: The fourth stage is characterized by low birth and death rates, resulting in a stable population size. This stage represents the post-industrial era when people have access to advanced healthcare and family planning services.

6. Implications: The theory of demographic transitions has several implications for social and economic development. For instance, countries in stage 2 and 3 are likely to experience a demographic dividend, which is a period of economic growth resulting from a large working-age population. However, countries in stage 4 face the challenge of an aging population and declining workforce.

In conclusion, the theory of demographic transitions provides a framework for understanding how population dynamics change over time. It highlights the importance of changes in birth and death rates in shaping population size and structure. The theory has implications for policy-making and planning in areas such as healthcare, education, and economic development.