Inscribed above the dais in the wood-paneled Gustave Tuck Theatre in University College London is a quotation from Deuteronomy: “Remember the Days of Old; Consider the Years of Each Generation.” It’s a poetic and even rather inspiring injunction but if you think about it too much it’s not obvious how to live up to it. Remembering the days of old is complicated enough, but what exactly are “the years of each generation”?

Professional historians don’t always deal well with the concept of generations. And that’s understandable because once you move beyond family history, it’s not obvious how a “generation” should be defined. How can the beginning and end dates for a generation be anything other than arbitrary? Nor is it clear whether one’s generation is a marker of “identity” comparable to class, race or gender. Should historians be in the business of trying to assign characteristics to a given generation? Should they give rising generations—or perhaps the clash between generations—a formative role in shaping historical development?

Both of these things have been done in great detail by two amateur historians, William Strauss and Neil Howe who, among other things, coined the term “Millennials” to describe the allegedly common characteristics of those born between 1982 and 2004. Their work has had a huge popular impact and helped sustain an industry of “generation consultants” running seminars for brand managers or university admissions teams.

Those Millenials (who are, by the way, “sheltered, special, confident, team-oriented, conventional, pressured and achieving”) are only the tip of Strauss and Howe’s elaborate conception of history as a series of generational cycles. The generation gurus explain in their books that about every 80 years — every four generations as they count them – society experiences a crisis, followed by a process of rebuilding, after which a rising generation rebel (think the baby boomers in the Sixties). Then there’s an unraveling and a new crisis. They then divide every generation into four archetypes, complete with romantic names–hero, artist, prophet, and nomad—who are shaped by the role they play in this cycle of human affairs. A generation is not just the product of circumstances, but in an almost mystical dynamic interacts with other generations to drive historical change in an endlessly repeating cycle.

Strauss and Howe’s schema seems to work cutely when you apply it in broad-brush terms to US history in the twentieth century (though they’ve rather entertainingly provided character descriptions for every generation going back into the fifteenth century). Their generational generalisations are plausible in much the same way as a horoscope: banal enough to be non-falsifiable, but chiming sufficiently with what we think we know to be true to be superficially compelling. Unsurprising, then, that while popular culture is suffused with generational stereotypes, historians generally give such ideas a wide berth.

But look deeper and it turns out that every generation of historians has confronted the question of generations in some way. The pioneering sociologist Karl Mannnheim influenced a generation of inter-war historians with his analysis of how age cohorts were shaped by how they experienced major events. If the event in question was a catastrophic conflict on the scale of the Great War, the gulf between those who fought and those who were too old or too young was too stark to ignore. In similar terms, the historian of French national memory Pierre Nora thought that the rupture of the French Revolution first created “generational consciousness”, and the student unrest of the Sixties reignited American historians awareness of how the same event was politicized in different ways depending (in part) on your age.

But to take that last example, the self-conscious appeal to a generational identity of the postwar New Left was always more rhetorical than real. If the baby boomers – as a generation — had all really interpreted the world so differently from their parents why did a majority of them vote for Richard Nixon in 1972? Generations in the large historical sense are not self-evident entities.

And yet, as members of families we know we’re part of a cycle of birth and death and so we also know intuitively that common life experiences do shape people’s values. In remembering the days of old it is salutary to try to think oneself into the years of each generation and to try to recapture what they must have seen and felt at each stage of their lives, elusive as that quest may seem.