People fretted over falling prices in the 1920s and 1930s and rising prices in the 1970s and 1980s.
In the 1960s and 1970s demographers warned of over-population. They calculated that, at the rate of growth then prevailing, the weight of humanity would quite soon exceed the entire matter of the known universe.
But few people remember that in the 1930s economists worried about depopulation. Even fewer know that some demographers now forecast that world population will peak in 2040 and then decline by 25 per cent each generation until, presumably, humanity becomes an endangered species, like the African elephant.
Again, in the 1970s the doomsters used to prophesy a new Ice Age. Now, as we all know, the threat is global warming, though some scientists confusingly predict that the warming may trigger an Ice Age after all.
Perhaps we should all congregate on the Isle of Wight, which, you will recall, has always been able to accommodate the whole human race with a bit of a squeeze. Once there, we could reasonably assume that advancing glaciers would stop at the Hampshire coast and that tropical diseases would stop at Calais or Dunkirk.
And we could ponder the similar cycles that plague education. In the 1970s everybody worried about girls' under-achievement and how to tackle it. Now girls are ahead in every major subject at GCSE, including the traditional male strongholds in the sciences.
Stephen Byers, the school standards minister, opened 1998 by telling an international conference in Manchester that "the present level of under-achievement by boys will need to be tackled as a matter of urgency". I am sure that teachers and local authorities - to say nothing of those exciting business people being lined up to run schools - have learned to jump to it whenever Mr Byers wants something done urgently. But perhaps he could first explain exactly what they should do. In the past year, l have read of some schools compelling boys and girls to sit together, while others have introduced single-sex classes.
Academics, l fear, can't help because they are behind the game - the weight of research is almost entirely addressed to improving girls' learning. If you doubt me, read Equity in the Classroom: Towards Effective Pedagogy for Girls and Boys, edited by Patricia F Murphy and Caroline V Gipps, and published by the London Institute of Education last summer.
All but one of the 18 contributions are from women - imagine the outcry if it had been the other way round. Most are inclined to see boys' antipathy to learning as a problem only insofar as it obstructs girls' progress. Maths, technology and the sciences, we are told, are dominated by "male interests, experiences and examples", while boys' under-performance in English "is excused by downgrading that subject".
Well, l haven't heard any serving teacher argue that English is unimportant for at least 20 years. And if the sciences are indeed still masculine subjects, this can only make the boys' second-rate performance there even more alarming.
In fairness I should say that Ms Gipps, almost despite herself, concludes the volume with a shaft of common sense: "The simple answer to the question 'Is there a pedagogy for girls?' is 'No'. Although we can identify approaches which tend to enhance girls' engagement and performance, girls not only differ from boys, but from each other; different school subjects carry different messages and valences, and consequently interest pupils differently ... Effective pedagogy requires ... a focus on the pupil as learner."
And this is surely the nub of it. Tolstoy famously observed that "all happy families resemble one another, but each unhappy family is unhappy in its own way".
Many of the families of the 1950s and 1960s may not have been all that happy. But they provided a standard model, in which male and female roles were clearly delineated. Schools that wanted to raise girls' aspirations knew what they were up against. They knew that the large majority of their pupils came from homes where men were men and women, even if they went to work, were first and foremost housewives and mothers. The exceptions were still sufficiently exceptional to see that kind of home as the norm.
Now that standard model has disappeared, and no other model has replaced it. In the past 20 years Britain has undergone a social revolution greater than at any time since the Industrial Revolution. Tolstoy would have to revise his opinion. (And while we are about it, so should some of our politicians.) Now even the happy families have to learn to be happy in their own way.
In some the traditional roles still apply, and if they are Muslim, the roles may do so in a way that would have seemed extreme even to 1950s Britain. In others both parents' careers are valued equally. In others again, the woman is the bigger and steadier wage-earner. In yet others, men have completely disappeared and are perceived by young boys as distant, feckless and unemployable. And even that list doesn't exhaust the contemporary possibilities.
So what do we now mean when we talk about male and female role models? The Spice Girls? Paul Gascoigne? Norma Major? John Motson?
Teachers can no longer afford to make assumptions about the stereotypes that children bring to school. Some children may actually need those antique reading books that have Mum baking cakes while Dad tinkers with the car in order to see that, for all its imperfections, this was a perfectly serviceable way of life for many people of their grandparents' generation. It may (dare I say it?) still be so for a small minority in the 21st century.
I think, then, that it is time for the gender studies industry to moderate its fierce hunt for "negative" female stereotypes and "inappropriate" treatment of girls. We should pay more attention to boys if only because, as Ms Gipps and her colleagues grudgingly recognise, they will just smash the place up otherwise. And as the obstacles to female success continue to fall, we should acknowledge that, if a few girls want to aspire to full-time motherhood, it's their business.
But I also hope that Mr Byers doesn't have everybody going overboard in the effort to raise boys' self-esteem. Otherwise women will all be back in the kitchen before you can say Germaine Greer. Give a man an inch, and he'll take a mile. l should know.