Image: National GeographicToday, the United Nations announced that the world's population will reach an historic 7 billion people on Oct. 31, 2011. World population hit 1 billion people in 1804. It took 123 years to add the next billion, but less than a century to cruise past the next four billion -- from 2 billion people in 1927 to 6 billion people in 1999. The U.N.'s Population Division also projected that the world will reach just over 10 billion by 2100, a number based on many uncertain assumptions about the future. Demographic projections are often mistaken for predictions, but they …
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Elizabeth Leahy Madsen
Elizabeth Leahy Madsen is a senior research associate at Population Action International. She leads PAI's research on the connections between population dynamics and broader development issues, including demographic security, and she directs the research components of projects on reproductive health and maternal health supplies.