The real estate research firm Zillow.com released a sobering statistic, and it took some by surprise: more than one-third of homeowners who bought in the past five years have negative equity in their homes.

Negative equity is owing more on your mortgage than the market value of your home. On the heels of the United States' greatest residential real estate boom in the modern era, how did the above occur?

Two factors, so says economist Peter Dawson.

First, many regions of the U.S., particularly the west, experienced abnormal gains during the 2003-2007 real estate boom. "Total appreciation rates over 300% were not unusual during the period. It was an amazing run, fueled by adequate national GDP growth, and low mortgage rates," Dawson said. "But as we've seen, ultimately the appreciation rates proved to be unsustainable, everywhere they occurred."

Dawson says a 7-9% annual increase in the U.S. median home price is normal, and his models label a 10% annual increase or higher as "outsized" -- a deviation from the mean that calls for a correction at some point in time. "But during the boom, it was not uncommon to see 30%, 40%, 50% annual increases over multiple years," Dawson said. "Clearly unsustainable. Downright frothy. But these conclusions were largely ignored during the boom, on the fallacy of 'what has occurred will continue.' "

Second, a financial habit shifted, Dawson said. Way, way back in the twentieth century, Dawson recalled, the biggest stigma when he grew up in a typical neighborhood in White Plains, N.Y., a suburb about an hour north of New York City, was... Not gaining acceptance at a good college? No. Not getting the hottest date for the high school senior prom? No. "We learned that the Smith's [not their real name] down the street had to take out...a second mortgage," Dawson said.