[steven777400]

This is an oversimplification. The average household has several kinds of debt, and generally debt should be paid down in descending order of interest rate.

More significantly, investment income opportunities need to have their interest rate (or equivalent) assessed. For example, in the past 12 months, the DIA has risen 18%. Thus, if 12 months ago I had money to spare, it would have been better to put the money into DIA rather than make an extra principal payment on the mortgage, unless my mortgage is 18% or more.

With the exception of bonds and CDs, it's not possible to know the investment growth in advance, so that creates some risk of course.

[crackered]

Further, because of the amortization schedule, applying that extra principal payment only shaves off the last month of the amortization schedule, which is the smallest fraction of interest of all payments.

One would still be better off investing that money in something until that last month, then apply the payment to save the very small amount of interest.

This assumes the mortgage is like most (all?) mortgages out there that follow an amortization schedule -- which are unlike credit cards or student loans, where early payments have a big benefit.