And the main piece of the brain puzzle is the simplest of all: repetition, repetition, repetition. In the grand tradition of everyone from Beethoven (and his hook-filled Fifth Symphony) to the dude who wrote "Who Let the Dogs Out?" ABBA songwriters Benny Andersson and Bjorn Ulvaeus recognized the power of telling us something as often as possible. Like the Beatles before them — think "She loves you/Yeah, yeah, yeah" — they also recognized the importance of making that something not so complicated: "Gimme, gimme, gimme a man after midnight ... "

"If you really want to know what makes a song powerful, I would say look at how the memory works," says physiologist Harry Witchel, a senior research fellow at the Medical School of the University of Bristol in the United Kingdom who ranked "Waterloo" as the all-time No. 1 Eurovision song contest winner for the BBC. "Memory works either through strong emotions or through repetition — that's how we normally teach. And ABBA songs allow for both of those things to occur."

We hear the words repeatedly, start to sing along, relate to the words and tunes emotionally with either a happy or sad reaction, and thus an earworm is born. He adds that the simplicity of the lyrics, the small number of syllables in the hooks, and the consistent backbeat all factor into the insidious nature of the tunes. That's how they extend their tentacles into a large swath of the public.

In "Musicophilia," his new book about music and the brain, Oliver Sacks supports this claim. "There are, of course, inherent tendencies to repetition in music itself," he writes. "Our poetry, our ballads, our songs are full of repetition; nursery rhymes and the little chants and songs we use to teach young children have choruses and refrains. We are attracted to repetition, even as adults; we want the stimulus and the reward again and again, and in music we get it."

All of these elements are in no way unique to ABBA says Levitin. Berklee's Aldrich, who's written or sung such regionally memorable jingles as "It's time to Stop & Shop" and "Tweeter, for times like these," agrees. "If you study it intimately, you will find there's a tremendous amount of repetition in song style and form that really hasn't changed much at all in 70 years," says Aldrich, citing Tin Pan Alley scribes like Cole Porter and even classical composers like Handel as using similar approaches.

Phyllida Lloyd, director of "Mamma Mia!" and a veteran opera director, doesn't need a scientist to explain why ABBA songs are so infectious.

"I think it's a combination of things," says Lloyd. "I think it's genius melodies by Benny Andersson and really quite deceptively complex and intricate orchestration. They were sort of masters of studio production, and they used every gizmo in the book at that time available to man, including a very ornate use of vocal harmony and words used partly as orchestration."

Lloyd is living proof that an inability to shake ABBA has no long-term side effects. Having had one or another of the songs in her head for the past 10 years as she shepherded "Mamma Mia!" onto both Broadway and the big screen, Lloyd says her sanity is perfectly intact.

"You wouldn't think so, would you? Questions ought to be asked," she says with a laugh. "I find that you just don't tire of them."