The Instant Economist, Timothy Taylor, Penguin Group, 2012, page 151:
https://books.google.com/books?id=E4K4nXvAu1EC&pg=PT143&dq=%22High-income+countries+have+historically+tended+to+run+trade+surpluses%22&hl=en&sa=X&ved=0ahUKEwj0k9LgkKLJAhUEKyYKHSziDpoQ6AEIHTAA#v=onepage&q=%22High-income%20countries%20have%20historically%20tended%20to%20run%20trade%20surpluses%22&f=false
"High-income countries have historically tended to run trade surpluses, and thus have been net investors abroad, investing in low-income countries. But in recent decades, the rest of the world has been investing in the U.S. economy on net. There is no precedent for this situation, and it doesn't seem likely to continue in the long run. At some point, the U.S. economy will need to repay this money. The question for the rest of the world is, how much in U.S. assets are you willing to hold? At some point the world won't be willing to keep adding to its portfolio of U.S. assets, and something will adjust. If less foreign financial capital flows into the United States, either we will have to lower the budget deficit, which means higher taxes and/or less spending; or we will need a higher domestic savings rate, which means less consumption; or firms will have to invest in their own growth. None of these options are attractive. But if the United States keeps running high trade deficits, one of these options—and maybe all three—must come to pass."