The debate about introducing a flat tax regularly flares up, but rarely is it based on a serious case study. The problem is that this debate is overly politicized and camps are formed on ideological lines, not an actual evaluation of the flat tax. Yet, there is data out there, as a flat tax has been introduced in Russia in 2001 and subsequently in six other Eastern European countries.
The flat tax in Russia was quite revolutionary. It is set at 13%, and replaced a system with marginal rates of 13%, 21% and 31%. So just looking at this, tax revenue has to decrease, yet it increased massively. This is not due to a Laffer curve, but rather to a drastic reduction in tax evasion, argue Yurly Gorodnichenko, Jorge Martines-Vazquez and Klara Sabirianova. Using micro data, they find that by measuring in a micro dataset tax evasion by the gap between reported income and expenses. They attribute most of the gain in tax revenue to changes in tax compliance, not to increases in economic activity. Russia's GDP grew by 5% in 2001, nothing unusual for the period.
What does this mean for a flat tax in OECD countries? First, the efficiency gains of such a reform do not seem to be that important. Second, tax compliance is already relatively high, for example in the US, which makes the gains less important in this respect. In the case of the US, a flat tax has, however, still the merit of simplifying an incredibly complex tax system that is the bread and butter of too many accountants.