The word "longevity" is sometimes used as a synonym for "life expectancy" in demography - however, the term "longevity" is sometimes meant to refer only to especially long lived members of a population, whereas "life expectancy" is always defined statistically as the average number of years remaining at a given age. For example, a population's life expectancy at birth is the same as the average age at death for all people born in the same year (in the case of cohorts). Longevity is best thought of as a term for general audiences meaning 'typical length of life' and specific statistical definitions should be clarified when necessary.
Reflections on longevity have usually gone beyond acknowledging the brevity of human life and have included thinking about methods to extend life. Longevity has been a topic not only for the scientific community but also for writers of travel, science fiction, and utopian novels.
There are many difficulties in authenticating the longest human life span ever by modern verification standards, owing to inaccurate or incomplete birth statistics. Fiction, legend, and folklore have proposed or claimed life spans in the past or future vastly longer than those verified by modern standards, and longevity narratives and unverified longevity claims frequently speak of their existence in the present.
A life annuity is a form of longevity insurance.
Longevity and lifestyle
Evidence-based studies indicate that longevity is based on two major factors, genetics and lifestyle choices. Twin studies have estimated that approximately 20-30% of an individualís lifespan is related to genetics, the rest is due to individual behaviors and environmental factors which can be modified. Recent studies find that even modest amounts of leisure time physical exercise can extend life expectancy by as much as 4.5 years.
In preindustrial times, deaths at young and middle age were common, and lifespans over 70 years were comparatively rare. This is not due to genetics, but because of environmental factors such as disease, accidents, and malnutrition, especially since the former were not generally treatable with pre-20th century medicine. Deaths from childbirth were common in women, and many children did not live past infancy. In addition, most people who did attain old age were likely to die quickly from the above-mentioned untreatable health problems. Despite this, we do find a large number of examples of pre-20th century individuals attaining lifespans of 75 years or greater, including Benjamin Franklin, Thomas Jefferson, John Adams, Cato the Elder, Thomas Hobbes, Eric of Pomerania, Christopher Polhem, and Michaelangelo. This was also true for poorer people like peasants or laborers. Genealogists will almost certainly find ancestors living to their 70s, 80s and even 90s several hundred years ago.
For example, an 1871 census in the UK (the first of its kind) found the average male life expectancy as being 44, but if infant mortality is subtracted, males who lived to adulthood averaged 75 years. The present male life expectancy in the UK is 77 years for males and 81 for females (the United States averages 74 for males and 80 for females).
Studies have shown that African-American males have the shortest lifespans of any group of people in the US, averaging only 69 years (Asian American females average the longest). This reflects overall poorer health and greater prevalence of heart disease, obesity, diabetes, and cancer among African-American men.
Women normally outlive men, and this was as true in pre-industrial times as today. Theories for this include smaller bodies (and thus less stress on the heart), a stronger immune system (since testosterone acts as an immunosuppressant), and less tendency to engage in physically dangerous activities.
Study of the regions of the world known as blue zones, where people commonly live active lives past 100 years of age, have speculated that longevity is related to a healthy social and family life, not smoking, eating a plant-based diet, frequent consumption of legumes and nuts, and engaging in regular physical activity. In another well-designed cohort study, the combination of a plant based diet, frequent consumption of nuts, regular physical activity and not smoking accounted for differences up to 10 years in life expectancy. The Alameda County Study hypothesized three additional lifestyle characteristics that promote longevity: limiting alcohol consumption, sleeping 7 to 8 hours per night, and not snacking (eating between meals).