Economists frequently talk about annual GDP growth and whether it will be 2% or 2.5% or 3%. These may seem like very small differences, but over time they are huge. Assuming population growth is 1%/year, it takes an excruciating 70 years for living standards to double if GDP growth is 2%/year, only 35 years if GDP growth is 3%/year, but just 23.5 years if GDP growth is 4%/year.