"If you look at the evolutionary biology of the species and the chemical reactions we have to events in the world, for tens of thousands of years when we as a species heard music we heard groups singing it, not an individual and not an individual standing on a stage," says Levitin. "So the ABBA model of the multiple voices or the Edwin Hawkins Singers singing 'Oh Happy Day' is much closer to stimulating these evolutionary echoes of what music really is, fundamentally — closer than, say, Frank Sinatra or Miley Cyrus."

In other words, if a caveman encased in ice were to be thawed out, revived, and immediately given a full iPod, he would respond more immediately to ABBA or a gospel choir than, say, free jazz. He might eventually dig Ornette Coleman, too, but the presentation of "Knowing Me, Knowing You" would sound more familiar.

The glossy production and compositional patterns of Sweden's fab four (or shall we say "fabelns fyra"?) also set off different neurological reactions that have medicinal powers. In the most upbeat of the group's songs, like "Money, Money, Money," the simplicity of ABBA's lyrics makes them easy to sing along to. In addition to the fizzy melodies, that participation, says Levitin, gives listeners "an even more powerful hit of happy juice in the brain from dopamine."

With sad songs in general, and in ABBA's case specifically with tracks like the more contemplative "The Winner Takes It All," listeners' brains produce an opposite but equally enjoyable reaction.

"You get the comfort hormone of prolactin when you hear sad music," says Levitin. "That's the same hormone that's released when mothers nurse their babies. It's soothing. And sometimes it's lyrics and sometimes it's music. I think it's most powerful when the two are well matched and you get what I would call an emergent property where the whole is greater than the sum of its parts."

Structurally, ABBA's songs, like most enduring pop songs, generally offer a straightforward verse-chorus format that satisfies our need for order.

"Whether they sat down and counted and said, 'This can't be nine measures; it has to be eight,' they probably didn't, but they probably wrote eight because in Western music we are used to that balance," says Jon Aldrich, associate professor and founder of the songwriting department at Berklee College of Music. To illustrate, Aldrich hums the melody of the sing-songy "shave and a haircut," leaving out the "two bits" conclusion. "Don't you want to hear the rest of it? You want to finish it, so with an eight measure or a 16 measure or even a 12 or a 24, the listeners feel balance and resolution."