Life expectancy at birth is the average number of years a newborn would live if current death rates held steady. It is one of the most telling measures of a society's health and development.

Global life expectancy has risen from about 30 in 1900 to over 72 today — the greatest improvement in human well-being ever recorded. The gains came from cutting child mortality: vaccines, clean water, antibiotics, and better nutrition.

The gap between countries remains large. Japan, Switzerland, and Singapore exceed 84 years; Chad, Nigeria, and the Central African Republic are below 55. Most of the difference is driven by child mortality and infectious disease, not old-age care.

Within countries, life expectancy varies by income, education, and geography. In the United States, there is a gap of more than a decade between the richest and poorest counties.

Life expectancy can fall. It dropped globally during the COVID-19 pandemic, and in the United States it declined for several years due to the opioid epidemic and "deaths of despair." It is not a number that only goes up.