The word "longevity" is sometimes used as a synonym for "life expectancy" in demography. However, this is not the most popular or accepted definition. For the general public as well as writers, the word generally connotes "long life", especially when it concerns someone or something lasting longer than expected (an "ancient" tree, for example).

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 lifespan ever by modern verification standards, due to inaccurate or incomplete birth statistics. Fiction, legend, and folklore have proposed or claimed lifespans 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.

History

A remarkable statement mentioned by Diogenes Laertius (c. 250) is the earliest (or at least one of the earliest) references about plausible centenarian longevity given by a scientist, the astronomer Hipparchus of Nicea (c. 185 — c. 120 B.C.), who, according to the doxographer, was assured that the philosopher Democritus of Abdera (c. 470/460 — c. 370/360 B.C.) lived 109 years. All other accounts given by the ancients about the age of Democritus appear, without giving any specific age, to agree that the philosopher lived over 100 years. This possibility is likely, given that many ancient Greek philosophers are thought to have lived over the age of 90 (e.g., Xenophanes of Colophon, c. 570/565 — c. 475/470 B.C., Pyrrho of Ellis, c. 360 — c. 270 B.C., Eratosthenes of Cirene, c. 285 — c. 190 B.C., etc.). The case of Democritus is different from the case of, for example, Epimenides of Crete (7th, 6th centuries B.C.), who is said to have lived 154, 157 or 290 years, as has been said about countless elders even during the last centuries as well as in the present time. These cases are not verifiable by modern means.

Present life expectancy

main: List of countries by life expectancy Various factors contribute to an individual's longevity. Significant factors in life expectancy include gender, genetics, access to health care, hygiene, diet and nutrition, exercise, lifestyle, and crime rates. Below is a list of life expectancies in different types of countries:CIA World Factbook

• First World: 77-83 years (e.g. Canada: 80.1 years, 2005 est.)
• Third World: 35-60 years (e.g. Mozambique: 40.3 years, 2005 est.)

Population longevities can be seen as increasing due to increases in life expectancies around the world:Fact: date=January 2008