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Why do 10 year Treasury bond yields affect mortgage interest rates?
Different bonds (and securitized mortgages are bonds) that have similar average lives tend to have similar yields (or at least trade at predictable yield spreads from one another). So, why does a 30 year mortgage not trade in lock-step with 30-year Treasuries? First a little introduction: Mortgages are pooled together into bundles and securitized by the Federal Agencies: Fannie Mae, Freddie Mac, and Ginnie Mae. Investors make assumptions about the prepayments expected for the mortgages in those pools. As explained below: those assumptions show that mortgages tend to have an average life similar to 10-year Treasury Notes.  100% PSA, a so-called average rate of prepayment, means that the prepayment increases linearly from 0% to 6% over the first 30 months of the mortgage. After the first 30 months, mortgages are assumed to prepay at 6% per year. This assumption comes from the fact that people are relatively unlikely to prepay their mortgage in the first 2 1/2 years of the mortgage's life. See the graph below.  The faster the repayments the shorter the average life of the mortgage. With 150% PSA a mortgage has an average life of nine years. On average your investment will be returned within 9 years. Some of it will be returned earlier, and some of it later. This return of interest and principal is shown in the graph below:  The typical investor in a mortgage receives 100% of this investment back within approximately 10 years, therefore mortgages trade in step with 10 year Treasury Notes. Average life is defined here: The length of time the principal of a debt issue is expected to be   outstanding. Average life is an average period before a debt is repaid   through amortization or sinking fund payments. To calculate the   average life, multiply the date of each payment (expressed as a   fraction of years or months) by the percentage of total principal that   has been paid by that date, summing the results and dividing by the   total issue size.