BIOSFER · Untangling the biologic and social causes of low fertility in modern societies

High-income countries are experiencing both unprecedentedly low and increasingly polarized fertility with growing social gradients in childbearing. Key theories on fertility patterns are based on the empirical observation that until recently fertility remained comparatively high in gender-egalitarian countries with strong support for families. Since 2010 many of the countries that provided evidence for such theories have reached record-low fertility. This confronts the scientific paradigm of the key drivers of fertility.BIOSFER investigates how social, biological and psychological factors work together to produce the observed patterns, levels and variation in fertility among young adults, and to what extent the fertility decline and the related polarization can be attributed to social vs. biomedical factors. Our multi-theory approach leverages ideas from several disciplines and proposes that the existing theories must be complemented with concepts of risk aversion and information, intergenerational transmission of fecundity, epigenetics and beyond, in order to understand modern fertility behaviour. We develop theoretically informed, falsifiable hypotheses that we test against the two richest population-based longitudinal pregnancy and pubertal cohorts in the world, MoBa in Norway and the DNBC in Denmark.We offer a uniquely integrative life-course-based approach that is neither social or biomedical, but combines central ideas from both, and evaluates the biosocial determinants of the key transition points from fetal life through puberty and partnering into planned, unplanned, partnered and unpartnered childbearing. We study the social, biomedical and psychological forces, their interactions, and intergenerational forces as they operate throughout the life-course to produce the modern low-fertility landscape. The results will help to provide a novel, bio-social framework for understanding the life-course processes that drive contemporary fertility patterns.