Pay Off the Mortgage or Invest

You have money left over at the end of the month, or a lump sum from a bonus or an inheritance, and you cannot settle the one question that keeps coming back: throw it at the mortgage, or invest it? You have already looked it up. One camp swears a paid-off house is freedom and debt is a trap. The other swears a cheap fixed-rate loan is the best money you will ever borrow and paying it early is a m...