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Picture a mortgage, and you're likely imagining a down payment of 20 percent of the price of the house. 

"I think the 20 percent down payment has become the default, no pun intended," says Jonathan Miller, president of Miller Samuel Real Estate Appraisers and Consultants. "To many homeowners, I think it symbolizes a commitment."

The requirements of Fannie Mae and Freddie Mac — the government-backed entities that support the vast majority of new mortgages — are the most obvious reasons the standard applies today.

"Under Fannie and Freddie's rules, you can get a lower down payment mortgage, but that then requires extra payment in the form of mortgage insurance," says Susan Wachter, professor of real estate and finance at the University of Pennsylvania's Wharton School. 

The history of that requirement dates back to the Great Depression. According to Wachter, before the 1930s most mortgages were short-term and non-amortizing: a home buyer had to either pay off the whole house in a lump sum after a few years, or roll over the loan at a new interest rate. Down payments, on the other hand, were typically more than 30 percent.

After the resulting foreclosure crisis and construction halt — similar to what happened after the recent financial crisis — the government created the Federal Housing Administration, which backed mortgages, but required a 20 percent down payment. After World War II, the Department of Veterans Affairs and the FHA adopted a 30-year, fixed-rate standard. By the mid '50s, most mortgages fit that description.

But 30-year, fixed-rate, 20 percent-down loans weren't strictly the result of government-sponsored enterprises, or GSEs.

"When I bought my first home it was $22,000 and I had to put 20 percent down, and it was a conventional loan," says Chris Polychron,  president-elect of the National Association of Realtors. "The conventional lenders mimicked what the GSEs did."

Since the 1950s, 20 percent has remained the average down payment — with the exception of the run-up to the financial crisis in 2008. But how did 20 percent become that dividing line in the first place, back in the 1930s? As with so much of our economic life, it’s anybody’s guess.

"I would speculate if you scored 80 percent, you’re a B-minus student, and I guess that means you’re above average," says Miller. "So maybe that has something to do with it."