Millions of Americans make 5x leveraged bets on their homes

You wouldn't make a 5x leveraged bet on the S&P 500 — not unless you were an extremely sophisticated financial arbitrageur, or a reckless gambler.

Even then you wouldn't put substantially all of your net worth into such a bet. Stocks are just too volatile. But millions of Americans make 5x leveraged bets on their homes — that's what it means to borrow 80% of the value of the house and put just 20% down.

By the numbers: Unison, a housing-finance startup, has crunched U.S. house-price data in a paper to be released tomorrow. Over the long run, any given home is likely to experience price volatility of about 15% per year; during the height of the crisis, that number spiked to more than 35%. That's broadly in line with the kind of volatility you see in the stock market.

• Unison's results are in line with public data from the Federal Housing Finance Agency, which show annualized house-price volatility, over the past 10 years, ranging from 12% in Alaska to 17% in Hawaii, New York, and the District of Columbia.

Be smart: Annualized house price volatility is much greater than the amount you can expect a home to rise in value over the long term. That number is closer to about 4%. While homes are much less volatile than individual stocks, they're just as volatile as the kind of diversified stock indices most people invest in.

The bottom line: Any given home has roughly a 30% chance of ending up being worth less in five years' time than it is today. If you can't afford that to happen, you probably shouldn't buy.