Ten years later, the Smiths and the Joneses want to send their oldest child to college. Their homes have appreciated nicely, so they want to refinance their mortgages and use their home equity to pay the tuition.

The Smiths call their lender and find out the Wall Street firm that bought their loan 10 years ago is no longer buying loans in their market because investors don’t like the foreclosure rate in that city.

They have a hard time finding a lender willing to help them and after a time-consuming search, they find an adjustable-rate mortgage from a local bank. This makes them nervous: Rates will inevitably rise and their mortgage costs will go up.

The Joneses are able to refinance into a 15-year loan backed by Fannie Mae or Freddie Mac, which guarantees loans in all markets and whether home values are rising or falling.