Classic demographic transition theory assumed that fertility would decline from high levels and stabilize at the replacement level of around 2.1 children per woman. Yet nearly half of the global population now lives in a country with a period total fertility rate (TFR) below 2.1 children per woman. Traditional development and geographic boundaries have been blurred with all major world regions, except sub-Saharan Africa, now firmly set on a decline towards low fertility. By 2050, more than two-thirds of the global population is projected to live in a country with fertility below a period TFR of 2.1 children per woman.

Low fertility, particularly at very low levels, is seen by some governments as a source of concern. In particular, policymakers note concerns about the accelerated pace of population ageing and associated pressure on the labor market, healthcare and social security systems, which are largely supported by contributions from the working age population. Another set of concerns relates to eventual population decline and its attendant threats to economic growth, military power and national pride. In 2015, two-thirds of governments in more developed countries considered fertility to be too low and were pursuing policies to boost it.

Against this background, this report aims to review the theoretical and empirical evidence of the determinants of low fertility. First, it presents an overview of the main trends and patterns of low fertility, followed by a discussion of the role of fertility postponement and the associated tempo effect. Subsequently, it explores three broad sets of distal social, cultural and economic determinants: 1) ideational change and the Second Demographic Transition, 2) economic constraints, including labor market uncertainty and the direct costs of raising children, and 3) constraints affecting women’s ability to combine paid work and childbearing, namely gender (in)equity in the domestic sphere, workplace conditions, and the availability of childcare services.