The Commerce Department announced the latest U.S. trade deficit figures on Thursday. Although the monthly deficit for September was down from previous months, Americans are still on track to run a record merchandise trade deficit for all of 2006 that will reach nearly $900 billion by the end of the year.

The trade deficit numbers are sure to provide fodder for the incoming 110th Congress, which because of Tuesday’s election will probably take a more belligerent tone toward global trade than the previous, Republican-controlled Congress.

How worried should we be about the large and growing trade deficit? I’ve written extensively about what the trade deficit means, and what it doesn’t mean. (See here and here for details.) One unappreciated aspect of the trade deficit is the offsetting capital surplus that flows into the U.S. economy year after year.

Think about it for a moment: What on earth are the Chinese, Japanese, Canadians, and Mexicans doing with those hundreds of billions of dollars they earn each year exporting into the American market? They are not stuffing them in cookie jars and under mattresses. Almost all those dollars come back to the United States to buy U.S. assets — real estate, stocks, corporate and Treasury bonds, and bank deposits. In other words, they invest those dollars in America.

According to the basic rules of supply and demand, the surplus of global savings flowing into the United States each year to finance the trade deficit puts downward pressure on U.S. interest rates. A new study from the National Bureau of Economic Research, “International Capital Flows and U.S. Interest Rates,” by Francis Warnock and Veronica Warnock, confirms the positive effect of international capital flows on long-term U.S. interest rates. “Large foreign purchases of U.S. government bonds have contributed importantly to the low levels of U.S. interest rates observed over the past few years,” the authors concluded. Specifically, they found that current inflows of foreign capital reduce long-term U.S. interest rates by about 100 basis points, or one percentage point.

If you are among the tens of millions of American families that are paying off a home mortgage, you can thank the trade deficit and the offsetting foreign capital surplus for saving you thousands of dollars a year in interest payments.