The article explores how changes in interest rates affect mortgage prepayments. Different types of mortgages react differently to interest rate increases, impacting the economic value of the mortgages. By analyzing a sample of 4,000 mortgages in California, the researchers found that a sudden 5% increase in interest rates led to a 23% loss in the economic value of fixed interest rate mortgages and a 28% loss for assumable mortgages. This resulted in a lengthening of the average time to repay the mortgages, from 6 years to 9 years for fixed interest rate mortgages and 13.5 years for assumable ones.