Vteckidd hit on an important point that many people often lose sight of. Economic data is usually buried in so much noise that it is very difficult to draw any meaningful conclusions from it. You can't just compare two variables (taxation and GDP) for the last 50 years and prove cause and effect relationships. There are countless other factors like the ones Vteck mentioned (wars, natural disasters, technologic advances). These can't be ignored. This is why you can find evidence to support just about any position you happen to prefer.