Dependency is the notional support burden placed on the working-age population by younger (0–14) and older (65+) age groups. The dependency ratio measures the number of young and older people relative to the working-age population who have to support them. Dependency ratios were high in the 19th century because high fertility rates meant there were many children to care for. As these children grew into adults, and fertility rates declined, dependency ratios became low. The post-Second World War baby boom meant dependency ratios increased again, then dropped after the boom finished in the early 1970s. Dependency ratios remained low in the early 21st century, but are projected to increase as the population ages, though not to the extent seen in previous periods of high dependency.