Investigating the Role of Social Capital in the Interaction between Physical Capital and Iran's GDP using the Smooth Transition Regression (STR) Model

Economic growth models have long grappled with fundamental questions regarding the relationship between different types of capital and real growth rates. While numerous empirical studies have demonstrated the independent effects of physical capital and social capital on the quantity and quality of economic growth, less attention has been given to the interactive effects of these investment types. This study examines the influence of social capital on the interaction between physical capital and Iran's economic growth using the Smooth Transition Regression (STR) model. The analysis covers the period 1346–1400 (1967–2021) and employs four indicators inspired by Fukuyama's framework to measure social capital. The findings indicate that the impact of physical capital on Iran's gross domestic product (GDP) varies across different levels of social capital. Periods of greater fluctuations in social capital have been associated with increased variability in the effect of physical capital on economic growth. Specifically, stronger social capital enhances the productivity of physical capital, while weaker social capital reduces its effectiveness. This study highlights the critical role of social capital in shaping the returns on physical capital investments, suggesting that strategies to stabilize and strengthen social capital could lead to more consistent and sustainable economic growth outcomes.