Russia, which implemented its flat tax in 2001, provides one of the best longer-term examples. Between 2000 and 2008, Russia’s GDP (in U.S. dollar terms) grew at an average compounded rate of 26% per annum. The end result was that GDP in 2008 was 546% higher than in 2000.
Russia’s tax revenue/GDP ratio was 31.4% in 2000, and 31.6% in 2008. With this ratio stable, you can see that the Russian government’s tax revenue also grew right alongside the growth in the economy as a whole, increasing to more than six times its 2000 amount in less than a decade.
The funny thing is, between 2000 and 2008, Russia’s population actually declined from 146.7 million to 141.4 million. Blaming economic stagnation on population, as is common regarding Japan today, is a waste of time.