According to census 2011, general fertility rate in Iran was 1.6 children. The United Nations published a low population growth scenario for Iran in 2010, and if Iran continues to experience population replacement and does not have a plan to balance it, it will experience a population of 31 million, with a high percentage of elderly people in the next 80 years. This study was conducted to identify the causes of a decrease in population growth rate. This was a secondary study conducted by reviewing the scientific texts, papers, and upstream documents. The upstream documents contain all national documents related to population decline in Iran. Causal layered analysis (CLA) was used for data analysis. The 9 most important identified causes for a decrease in population were litany (child mortality, maternal mortality, diseases burden, fertility rate, marriage squeeze, abortion, marriage age, high-risk behaviors, and badly supervised and neglected children. Also, 5 causes in structural layer were urbanization, education rate, economic participation rate and unemployment rate, new structures, a change in family structure, and intergenerational gap. Moreover, three causes in discourse layer included welfare, materialism, individualism, and 2 causes in metaphor layer were changing the perception of life and family formation, and women as workforce. It seems that the decrease in population growth in Iranian society is less the result of social planning and population control and more the result of the value and structural changes that have been occurred due to modernization in the society. It is recommended that policymakers primarily address the discourse and metaphor layers to solve the problems.