The phenomenon we know as Beatlemania was unprecedented in world history, and it has never been duplicated. True, other popular performers had generated “hysteria’’ in young girls, from Rudolph Valentino to Frank Sinatra to Elvis Presley. But the public reaction to these charismatic performers was far removed from the kind of mass pathology that the Beatles inspired in both England and America, where uncountable thousands of teenage girls fainted, wept, and peed themselves en masse, even as battalions of police officers herded them behind fences and barricades.

These images have long ago been anaesthetizied into the highlight reel of 1960s nostalgia. But it was all very disconcerting at the time. Journalists compared the sounds made at Beatles concerts to the nerve-shredding cries of pigs being brought to slaughter or the screech that New York City’s subway trains make as they grind along the rails. When the Beatles played Shea Stadium in 1965, The New York Times reported that the crowd’s “immature lungs produced a sound so staggering, so massive, so shrill and sustained that it crossed the lines from enthusiasm into hysteria and soon it was in the classic Greek meaning of the word ‘pandemonium’ — the region of the demons.’’ Small riots broke out at many Beatles concerts, and some people feared a major riot. The Beatles worried about being torn to bits or causing a potentially fatal stampede.

The Beatles remain, of course, an evergreen source of public fascination; a Ron Howard-directed documentary (“The Beatles: Eight Days A Week — The Touring Years’’) due to be released next month is just the latest evidence. But even as Aug. 29 marks the 50th anniversary of the last proper Beatles concert (at San Francisco’s Candlestick Park), Beatlemania remains a singular phenomenon for which we still don’t have an adequate explanation. Many writers and academics have tried to provide one, with only limited success.

It has frequently been suggested that the Beatles were just what British and American citizens needed in response to the woeful year of 1963. The Beatles first arrived in the United States on Feb. 7, 1964 — 11 weeks after President John F. Kennedy was assassinated in Dallas, instigating a prolonged period of national mourning. A less dramatic but still profound trauma occupying British citizens was the Profumo Affair, a tawdry sex scandal (involving a 19-year-old showgirl, a Russian spy, and a suicide) that broke out in early 1963 and reached into the highest levels of government.

Speculation holds that the dour national mood present in both countries set the stage for the onset of Beatlemania. This view has some problems. The Beatles’ core audience back then was very young — pubescent girls, mostly. Children of that age would not have understood the Profumo Affair in all of its dimensions. Furthermore, the American public has absorbed devastating shocks before and after Kennedy’s assassination without a Beatlemania-like catharsis following months later.

In 1977, a trio of scholars — Barbara Ehrenreich, Elizabeth Hess, and Gloria Jacobs — observed that news footage from the early ’60s, showing fans chanting, screaming, and carrying signs, may look a bit like something from the protest culture of several years later. That is for good reason, they say, since there was an inchoate element of protest to Beatlemania.

Their provocative argument (laid out in the essay “Beatlemania: A Sexually Defiant Subculture?’’) goes like this: Society was becoming increasingly sexualized in the ’50s, yet girls were always asked to walk a delicate line. They were advised to make themselves appealing to boys and to get dates, but also to remain pure and chaste. To those ends, girls were expected to police their own ranks; a young teenager who was too sexually forthcoming risked being ostracized by her peers. In that context, “to abandon control — to scream, faint, and dash about in mobs — was, in form, if not conscious intent, to protest the sexual repressiveness, and rigid double standard, of female teen culture,’’ these scholars write.

Meanwhile, as the American economy surged in the late ’50s and early ’60s, advertisers began targeting teens as a niche market. Even before the Beatles arrived in America, promoters touted the group with a sophisticated publicity campaign. Teenagers had money, and they were looking to spend it.

This may have been a moment when baby boomers first began developing a sense of their generational potency. No previous generation in American history had occupied so large a role in the consumer culture or had so much attention showered upon it. These young girls were not yet ready for the women’s liberation movement, the argument goes, but they were willing to trespass on propriety by asserting “something’’ (it’s hard to say precisely what) about their sexuality, or their innermost desires, in a public and ritualized fashion. So Beatlemania was not just rebellious but also “in its own unformulated, dizzy way, revolutionary.’’

This focus on sexual defiance and consumer culture has a lot going for it. The majority of Beatles scholars have been men, few of whom have tried to imagine what Beatlemania was like from the perspective of the group’s early adolescent fans. But this argument is also American-centric. The dazzling array of consumer goods and leisure opportunities that so many Americans enjoyed during the prosperous ’50s and early ’60s were largely unknown to British teens. So, too, was the massive publicity campaign that preceded the Beatles’ arrival in America. (In England, the Beatles’ sudden fame had caught people off guard.) Finally, none of the above does very much to explain why it was the Beatles and not some other pop group that provoked, by far, the most convulsive reaction from teenage girls.

In his book “Beatlemania: Technology, Business, and Teen Culture in Cold War America,’’ André Millard suggests that the Beatles would never have experienced such phenomenal success if not for the trans-Atlantic diffusion of culture, and the operation of multinational business operations, that happened in the preceding decade. By then, advances in transportation regularly brought American entertainers and technology (including mass-produced phonographs, records, and films) all the way to Liverpool, where they shaped the tastes and sensibilities of British youths.

It is true that as the Beatles were growing up, they were profoundly influenced by American popular culture. So, too, was their manager, Brian Epstein, whose paramount contribution was to make the Beatles more visually appealing than they would have been otherwise. He was the one who got them into wearing distinctive, matching suits, and he choreographed their performances, teaching them to smile, bow, and act gracious while onstage. That was hugely important, because early in their career, the Beatles frequently had to sell themselves on television. Furthermore, the Beatles regularly recorded special sessions to be broadcast on various BBC radio shows, and they did so at a time when high-quality recorded sounds — transmitted by records, television, and radio broadcasts — were more widely accessible than ever before. The Beatles were a commercial product that could be promoted and distributed around the globe.

This attention to business and technology is valuable because it helps establish the necessary preconditions for Beatlemania. But it fails to establish the proximate cause. After all, many other British groups operated in the same context. Once again we have to ask: Why the Beatles and not some other group?

It should go without saying that the Beatles were handsome, charming, and incredibly talented. But it is also important to stress just how incredibly novel they were. The point is so obvious that it’s easy to overlook. When the Beatles started recording in 1962, rock ’n’ roll hadn’t been around for very long. With their presentation, harmonies, and peculiar range of musical influences, the Beatles were distinctive in ways that we sometimes fail to appreciate.

Younger fans would have noticed, for instance, that the Beatles behaved just as they saw themselves — as a tightly knit group. This was a new thing. Previously, pop and rock music had emphasized solo acts like Elvis or individuals who fronted backing bands such as Bill Haley & His Comets or Buddy Holly and the Crickets. British pop, especially, seemed beholden to a formula where a lead singer with a flashy stage name, like Billy Fury or Marty Wilde, performed in front of a generic backup group. As the Beatles were preparing to launch their recording career at EMI, their producer, George Martin, came very close to insisting that they follow custom and designate either John Lennon or Paul McCartney as the front man. But thankfully, he did not press the issue. When fans were introduced to the Beatles, they saw an indivisible group in which each member nevertheless had distinctive qualities.

The Beatles were also unusual in the ways they related to their female followers. Beginning in 1961, the Beatles started experimenting with the call and response vocals, skirling harmonies, and falsetto leaps that characterized the Motown girl groups they admired, like the Shirelles (and later the Ronettes and the Shangri-Las). No other group, on either side of the Atlantic, was anywhere nearly as steeped in the girl group genre or as comfortable embracing its clichés.

Unlike other groups from their era, the Beatles also had a habit of seeming to sweetly and directly address their girl fans through the use of personal pronouns. (“P.S., I Love You,’’ “With love from me, to you,’’ “I Want to Hold Your Hand,’’ and so on.) The Beatles also flavored some of their early songs with dashes of sexual innuendo. (Certainly it’s difficult, nowadays, to listen to “Please Please Me’’ as anything other than an exasperated plea for oral sex.) But few, if any, of the group’s contemporary listeners had much to say about the possible sexual insinuations in the Beatles’ work. The band projected sexual charisma, to be sure, but it was a charisma that was tamed and domesticated for their youngest female fans. More than anything, teenagers swooned at the tenderness and vulnerability that the Beatles expressed in their songs.

Finally, Beatlemania makes a bit more sense if it’s properly contextualized within the vapid and formulaic pop scene of the early ’60s. Before the Beatles came along, past-their-prime American acts like Gene Vincent — not to mention deceased stars like Buddy Holly and Eddie Cochran — still sold lots of records in both England and America. That was probably because nothing much else was going on musically. Syrupy ballads, formulaic dance numbers, and cheesy novelty songs dominated the airwaves. In contrast, the Beatles seemed refreshing and even anomalous. Their unrivaled capacity for making and presenting music that on the one hand seemed safe and familiar, and on the other hand was daring and inventive, goes a long way toward explaining their singular appeal.

But that probably is not the whole story. Causation in history can be hard to determine even in the best of circumstances, and Beatlemania may not be explainable by normal scholarly methods. Historians like myself are practiced at uncovering source material, determining its reliability, and weighing contradictory evidence. We are, perhaps, less well equipped to diagnose a mass pandemonium. If I’m correct, however — if the Beatles were largely the authors of their own phenomenon, by virtue of the novelty and charisma they injected into the very particular context of early ’60s pop and rock — then another thought emerges. In addition to being unprecedented, it seems increasingly unlikely that we will ever see something like Beatlemania again.

John McMillian is an associate professor of history at Georgia State University, in Atlanta. His most recent book is “Beatles Vs. Stones.’’