World population
Number of humans who have ever lived

Pre-modern infant mortality rates are another critical factor for such an estimate; these rates are very difficult to estimate for ancient times due to a lack of accurate records. Haub (1995) estimates that around 40% of those who have ever lived did not survive beyond their first birthday. Haub also stated that " life expectancy at birth probably averaged only about ten years for most of human history", which is not to be mistaken for the life expectancy after reaching adulthood. The latter equally depended on period, location and social standing, but calculations identify averages from roughly 30 years upward.
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