I think Fareed Zakaria makes a good argument from this article in 2011: http://www.washingtonpost.com/wp-dyn/content/article/2011/03... . Basically, the share of taxes that go to productive investment (like the Interstate Highway System, GPS, ARPANet, basic research, etc.) is shrinking compared to entitled consumption spending (Social Security, medicare). The current political system in the US makes it exceedingly difficult to either (a) increase government investment spending or (b) decrease entitlement spending.