Forty years is roughly the length of a working lifetime -- and long enough for history to have taken some unexpected turns. And to have proved that long-term forecasts based on extrapolations of existing trends usually end up wide of the mark.
The list of failed prophecies from the 1970s is rather long. The conventional wisdom of the time was more than usually unreliable.
Example: the Club of Rome's Limits to Growth report in 1972, which predicted that the world was running out of oil and other natural resources. For a while that seemed right, as the 1973 and 1979 OPEC oil price hikes led to gas lines in the United States.
But in the longer run, as the Club came to recognize, engineers and entrepreneurs found more oil and other natural resources and figured out how to get them to market. Capitalism works, and in ways planners don't expect.
Another common assumption in the early 1970s was that Britain was a fusty, antiquated country that had to join the modern, up-to-date Common Market (now the European Union). Europe's war-devastated economies had actually grown faster than Britain's in the quarter-century after World War II.
Fast forward to today. It is Europe that looks out of date, with zero economic growth and economies smothered by sclerotic regulation, overlarge welfare states and the poorly conceived euro.
Britain got rid of much of that under Margaret Thatcher and John Major. And thanks to Gordon Brown, it wisely avoided the euro. Now it's growing solidly while the continent lags.
A third bit of conventional wisdom from the 1970s is that Asia generally and China in particular could never grow because of the burden of overpopulation.
But Asia's state-led capitalism and Deng Xiaoping's adoption of that model in 1978 has made Asia the growth capital of the world. Hundreds of millions have risen from poverty.
As for the population bomb, the biggest problem for Asia and China today is low birth rates and a contracting work force. These stopped growth in Japan and may do so elsewhere.
A final thing taken for granted in the 1970s was the enduring strength of the Soviet Union. It was, after all, ramping up its military even as America was recoiling from defeat in Vietnam.
Many seers predicted that the Soviet and Western models would converge, and that Soviet living standards would approach America's.
It turned out that the very few leaders who predicted the demise of the Soviet Union -- Ronald Reagan, Daniel Patrick Moynihan -- got it right. America, once it got the will, could outclass the Soviet military, and economic stagnation and ethnic tensions brought down the "evil empire."
There are common threads running through these mistaken projections. One is the extrapolation of recent trends far into the future. History doesn't proceed like a straight line on a graph; sometimes the lines bend.
Another is the assumption that progress means ever-larger states and increasing superintendence by international elites.
But much unpredicted progress has occurred when nations freed markets from the grip of centralized states and private sectors produced innovation that the supposed experts failed to anticipate.
A third common thread builds on the insight of economist Herbert Stein, who said that anything that could not go on forever would some day stop.
This prompts a question: Which of the widely accepted prophecies of today will seem as invalid today as the Club of Rome report? I have my own nominations, made with some confidence since actuarial tables tell me I will not be here in 40 years.
One is that the Chinese Communist regime will remain in place. Remember that it seemed on the verge of tottering in Tiananmen Square in 1989. It's lasted 25 years since, thanks largely to robust economic growth.
But certain dates in history -- 1789, 1917, 1991 -- tell us that sudden upheaval is possible when a regime's legitimacy seems exhausted.
And will today's conventional wisdom that the planet faces inevitable warming seem as risible in 2054 as the Club of Rome's prediction of exhausted resources seems today?
We are told that "the science is settled," when it is in the nature of science never to be settled, but always to be subject to verification and revision. I think we're in for more of that.
Finally, those widely shared views that America's best days are over. That's never been a good bet and I suspect it never will be.