Which of the following research questions would most likely fall within the domain of social demography rather than formal demography?

Why do fertility rates differ significantly between social groups with similar income levels?

The correct answer is option 3: Why do fertility rates differ significantly between social groups with similar income levels?

Formal demography focuses on the measurement and statistical analysis of population processes such as births, deaths, and migration. Social demography goes further by examining the social, cultural, economic, and political factors that influence these demographic processes. Understanding why different social groups display different fertility behaviour falls within social demography.

Option 1: How many births occurred in a state during a particular year? Incorrect. This is a measurement-oriented question belonging to formal demography.

Option 2: What was the crude death rate of a country over the last decade? Incorrect. This involves statistical measurement rather than social explanation.

Option 3: Why do fertility rates differ significantly between social groups with similar income levels? Correct. This investigates social influences on demographic behaviour.

Option 4: How many people were added to the population through natural increase in a given year? Incorrect. This concerns demographic measurement.