President Barack Obama heads to the G-20 Summit in Seoul this week with a major goal: limiting China's and other countries' current account surpluses.

China is being singled out for a reason. Its current account surplus with the U.S. this year may run to $250 billion, far more than any other country. To many, this shows China is manipulating its currency, destroying jobs and limiting growth in the U.S.

Nothing could be further from the truth. But the truth is much harder to find these days than scapegoats. Fortunately, economics can move the debate beyond finger pointing.

Countries that run current account surpluses save more than they can fruitfully invest at home and invest the difference abroad. Countries with current account deficits do the opposite. They save less than their economy's investment needs and attract investment from abroad.

Surplus countries take some of the seed corn they've saved and plant it in deficit countries. This physical movement of the seeds, or capital, is recorded as an export of the surplus country and an import by the deficit country.

Nations with current account surpluses are net exporters and have trade surpluses. Those with current account deficits are net importers and run trade deficits. Indeed, apart from the net income foreigners earn in the U.S. and invest here, their current account surplus equals their trade surplus, and their trade surplus is, apart from a minus sign, our trade deficit.