Short answers, pulled from the story.
Life expectancy is a statistical estimate of the average remaining years of life at a given age. The most common measure is life expectancy at birth, written e0 in demographic notation. National figures reported by agencies and international organizations are estimates of period life expectancy, the mean lifespan of a hypothetical group exposed to one year's mortality rates.
A low life expectancy at birth is driven heavily by high infant mortality, which pulls the average down even when survivors live long. In a society with a life expectancy of 30, it can still be common to have a 40-year remaining timespan at age 5. Demographers use life expectancy at age 5, written e5, to strip out the effect of infant deaths.
The longest verified human lifespan is that of French woman Jeanne Calment, who lived 122 years and 164 days between the 21st of February 1875 and the 4th of August 1997. This is referred to as the maximum life span, the upper boundary of human life.
Human remains from the early Bronze Age indicate a life expectancy at birth of around 24, while world life expectancy reached 73.3 in 2019. In the early 19th century no country had a life expectancy at birth longer than 40 years, and during the 20th century the average lifespan in the United States rose by more than 30 years.
Modern female life expectancy is greater than that of males despite women having higher morbidity rates, a pattern known as the health survival paradox. Most of the gap is accounted for by differences in death from cardiovascular disease among people aged 50 to 70. Proposed causes include behavioral factors such as higher male tobacco and alcohol use, and genetic explanations such as the unguarded X hypothesis.
Average life expectancy at birth in the United States reached 79.0 years in 2024, a record high, with 76.5 years for men and 81.4 years for women. The 0.6-year increase that year was attributed mostly to a significant decline in fatal drug overdoses.
The heritability of lifespan is estimated to be less than 10%, meaning the majority of variation in lifespan comes from environment rather than genetic variation. A genome-wide association study of 1 million lifespans found 12 genetic loci that influenced lifespan, with the largest effect at the APOE locus.