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Moving out the parental home and partnership formation as social determinants of low fertility

The study of fertility across societies has been a core subject of demography, and is one of the main areas of theory development. As countries went from having fertility well above the replacement level to having fertility near the replacement level (2.1 children), one of the most widely accepted theoretical frameworks in demography emerged: the demographic transition. This transition was primarily driven by the social, health, and economic transformations associated with the general process of modernization (Notestein 1953; Lee 2003). But in many countries, fertility continued to decrease, falling to below the replacement level. The second demographic transition (SDT) was proposed to explain these changes (Van de Kaa 1987; Lesthaeghe 2014). The SDT placed more emphasis on ideational and cultural changes than on structural factors. Today, fertility below the replacement level is observed in virtually all high-income countries, and is spreading rapidly to other areas of the world. In 2017, 96 out of the 217 countries covered by the World Bank data reported total fertility rates of below 2.1 children per woman. Understanding the variations in fertility trends across these countries poses a new challenge for demography. To meet this challenge, we stress the need to identify and quantify the relative importance of the proximate social determinants of low fertility. We suggest that researchers investigating this issue pay particular attention to the effects of moving out of the parental home and partnership formation. We argue that the broad social, cultural, and economic forces that were cited to explain the occurrence of the first and second demographic transitions do not necessarily suffice to explain the variations in fertility across countries and time after these transitions have taken place. Neither the idea that further development will eventually reverse the declining trend in fertility (Myrskylä et al. 2009), nor the abundance of micro-level studies on the many factors that are associated with childbearing (Balbo et al. 2013), have yielded convincing explanations for why