“Tax revenue as a percentage of gross domestic product is expected to be 16.5 percent next year. The long-term average in a full-employment economy is 18.5 percent of GDP; if revenue were at that level for the coming decade, debt would be $3.2 trillion lower and the 10-year fiscal gap would be halved. Returning to past revenue levels, however, will be inadequate over time, because an aging population will increase Medicare and Social Security costs. This need not pose a problem: Revenue was roughly 19 percent of GDP in the late 1990s, and economic conditions were excellent.”

– Former U.S. Treasury Secretary Richard E. Rubin, writing in The Washington Post