In late 2009, a Wall Street Journal reporter wrote that "questioning the sanctity of the 30-year fixed home loan is tantamount to proposing that the White House should be repainted pink."

With mortgage rates near historic lows, buyers may be tempted to lock in that long-term certainty. It's not difficult to see why the 30-year rate is so appealing: loans 30 years in length, with a fixed interest rate. Typically, the home-buying borrower will put roughly 20% down and then borrow the remaining 80% from a financial institution (the average down payment on all mortgages has been around 25% of late, a 20-year high).

But the popularity of the 30-year fixed mortgage could be a bad thing, Robert Shiller told me recently in an interview in front of a live audience at Motley Fool Headquarters.

Shiller, a Yale professor who just published his 10th book, Finance and the Good Society, is a leading expert on housing; in the video below, see why he thinks these types of mortgages represent "conventional thinking," a pet peeve of his. (Run time is 1:16; a transcript is below.)