“Whatever goes up must come down.” That was my father’s favorite law of physics, and he never tired of quoting it. He had been deeply traumatized by the calamitous crash in the value of real estate during the Great Depression – “You couldn’t give it away,” he used to say – and he spent the rest of his life waiting and watching for when the next crash would come.

It never did. Although there was a shallow and fleeting dip in realty prices during the early 1990’s, throughout almost all of the past 70 years people who have invested in residential real estate – that is, in their own homes – have been rewarded with rates of return vastly higher than anything commonly achieved on Wall Street.

When an aunt and uncle spent $6,300 for a brand-new brick house in suburban Forest Hills in the early 1940’s, my father thought it was an extravagance. Of course, when the war ended they were able to sell it for a huge profit. Someone I know bought an Edwardian colonial in New Jersey’s Bergen County for $50,000 in the early 1970’s and, sold it recently, pocketed a return of some 1,500 percent. (He turned around and spent almost all of that on a new and luxurious condo farther from the city.)

But it’s not just the size of the returns on residential real estate that’s so impressive; it’s also their extraordinary reliability. Just think: How many people do you know who have lost money by buying their own homes? And, for comparison, how many who have lost money investing in securities? It gives a whole new meaning to the expression “safe as houses.”

Over the past few years we have seen technology stocks go into the tank, taking most of the rest of the stock market with them. Interest rates paid by banks have dwindled to insignificance as the interest rates the banks charge for home mortgages keep going lower and lower.

For most people living through this difficult period, the only sensible conclusion was that home ownership was by far the only way to shelter their investment dollars. Consequently, the “home ownership rate” in America drifted steadily higher, settling record after record, until it now stands at an all-time high of 69.2%. That is, almost 7 out of every 10 homes are owned by the people who live in them. Surely that is a good thing, new proof that the American dream is alive and well.

And yet, and yet. The laws of physics have still not been repealed. Whatever goes up must – eventually, inevitably – come down. It is in their very nature for bubbles to burst, and who can deny that what we are witnessing now is yet another real estate bubble? House prices are at an all-time high. The level of consumer debt, including home mortgages, is likewise higher than ever before – which means that any downturn in values could have disastrous consequences. But rising prices take a toll as well. Increasingly we hear reports that home prices are rising so much faster than wages in many places that increasing numbers of people find they cannot afford to buy a house – no matter how good an investment it may seem.

But a house, of course, is more than just an investment; it is one of life’s necessities: a place to live. And, regardless of what the real estate market does, people will still need to seek new houses as the needs of their family change. But that raises a curious question. As people grow older- as the American population is growing older – they need less space, not more. But homes in America – even condos designed strictly for the over-55 set – keep getting larger; on average they are probably the largest homes, per resident, ever in the history of the world. It is hard to imagine that this trend can be sustained for very long, but huge new “McMansions” pop up on the horizon every day.

And as they say, everything that goes up …