In 1997, housing prices began to diverge substantially from rental costs. Between 1997 and 2002, the average compound rate of growth in housing prices was 6 percent, exceeding the average compound growth rate in rentals of 3.34 percent. This, incidentally, contradicts the widely held idea that the last housing bubble was caused by the Federal Reserve’s monetary policy. Between 1997 and 2000, the Fed raised interest rates, and they stayed relatively high until almost 2002 with no apparent effect on the bubble, which continued to maintain an average compound growth rate of 6 percent until 2007, when it collapsed.

Today, after the financial crisis, the recession and the slow recovery, the bubble is beginning to grow again. Between 2011 and the third quarter of 2013, housing prices grew by 5.83 percent, again exceeding the increase in rental costs, which was 2 percent.

Many commentators will attribute this phenomenon to the Fed’s low interest rates. Maybe so; maybe not. Recall that the Fed’s monetary policy was blamed for the earlier bubble’s growth between 1997 and 2002, even though the Fed raised interest rates during most of that period.

Both this bubble and the last one were caused by the government’s housing policies, which made it possible for many people to purchase homes with very little or no money down. In 1992, Congress adopted what were called “affordable housing” goals for Fannie Mae and Freddie Mac, which are huge government-backed firms that buy mortgages from banks and other lenders. Then, as now, they were the dominant players in the residential mortgage markets. The goals required Fannie and Freddie to buy an increasing quota of mortgages made to borrowers who were at or below the median income where they lived.

Through the 1990s and into the 2000s, the Department of Housing and Urban Development raised the quotas seven times, so that in the 2000s more than 50 percent of all the mortgages Fannie and Freddie acquired had to be made to home buyers who were at or below the median income. To make mortgages affordable for low-income borrowers, Fannie and Freddie reduced the down payments on mortgages they would acquire. By 1994, Fannie was accepting down payments of 3 percent and, by 2000, mortgages with zero-down payments. Although these lenient standards were intended to help low-income and minority borrowers, they couldn’t be confined to those buyers. Even buyers who could afford down payments of 10 to 20 percent were attracted to mortgages with 3 percent or zero down. By 2006, the National Association of Realtors reported that 45 percent of first-time buyers put down no money. The leverage in that case is infinite.