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Why do 10 year Treasury bond yields affect mortgage interest rates?
The simple answer is that, even though mortgages can go for 10, 15, 20 and 30 year terms in the U.S., they're typically backed by bonds sold to investors that mature in 10 years, which is the standard term for most bonds. These bonds, in the open market, are compared by investors with the 10-year Treasury note, which is the gold standard for low-risk investment; the U.S. Government has a solid history of always paying its bills (though this reputation is being tested in recent years with fights over the debt ceiling and government budgets). The savvy investor, therefore, knows that he or she can make at least the yield from the 10-year T-note in that time frame, with virtually zero risk. Anything else on the market is seen as being a higher risk, and so investors demand higher yields (by making lower bids, forcing the issuer to issue more bonds to get the money it needs up front). Mortgage-backed securities are usually in the next tier above T-debt in terms of risk; when backed by prime-rate mortgages they're typically AAA-rated, making them available to "institutional investors" like banks, mutual funds, etc. This forms a balancing act; mortgage-backed securities issuers typically can't get the yield of a T-note, because no matter how low their risk, T-debt is lower (because one bank doesn't have the power to tax the entire U.S. population). But, they're almost as good because they're still very stable, low-risk debt. This bond price, and the resulting yield, is in turn the baseline for a long-term loan by the bank to an individual. The bank, watching the market and its other bond packages, knows what it can get for a package of bonds backed by your mortgage (and others with similar credit scores). It will therefore take this number, add a couple of percentage points to make some money for itself and its stockholders (how much the bank can add is tacitly controlled by other market forces; you're allowed to shop around for the lowest rate you can get, which limits any one bank's ability to jack up rates), and this is the rate you see advertised and - hopefully - what shows up on your paperwork after you apply.