For ten thousand years, life expectancy barely changed. Between 1800 and 2000, it doubled. From AD 1 to 1800, global population grew less than 0.1 percent a year. By the mid-twentieth century it was growing at 20 times that rate. The world’s average economic output per person was nearly constant for two thousand years—between $450 and $650, in 1990 dollars. Since 1800, it has increased by a thousand percent.
