This report by McKinsey Global Institute explores how falling fertility rates and ageing populations are reshaping economies worldwide. The analysis highlights that two-thirds of the world’s population now live in countries with fertility rates below replacement level, leading to shrinking workforces and rising dependency ratios. The report warns that these demographic changes could slow economic growth, strain public finances, and challenge social support systems. A combination of higher productivity, more work per person, effective migration and higher fertility rates can ensure global prosperity for the future. This will require society to rethink existing systems for work and retirement in ways that may compel a change in our social contract.