This was one of FDR's favorite statements of the Allied Aims. They consisted of: freedom of speech, freedom of warship, freedom of want, and freedom from fear. The actual idea came from a picture made by Norman Rockwell, portraying different values of American life.
Follow @TIME. For our happiness issue, TIME conducted a poll to find out what Americans do to find joy. Overall, 59% of respondents said they were happy most of the time, about the same percentage of those surveyed by TIME in 2004 when 61% said they were happy. But fewer people said they considered themselves optimists in 2013, 50%, as compared to 2004 when 79% called themselves optimists.
This formula is now applied to a system of time intervals from 1,000,000 BC to the present. The sum of the total person-years for all these intervals gives the total person-years for all human life. By dividing by the average lifespan, one obtains an estimate of how many people have ever lived. The data at ends of intervals are taken from a recent textbook on population, except for the first data point of 2 people at -1,000,000. That data point was proposed by Keyfitz.