Go to Kim’s Video & Music on 1st Avenue in Manhattan and you’ll find two used copies of Bjork’s “Homogenic,” two copies of “Medulla,” two of “Post,” two of “Telegram,” two of “Selma Songs,” and one copy of her 2001 opus “Vespertine.” Why is Bjork—her face in various stages of alien ecstasy—always in used CD bins like Kim’s? And why, for that matter, can the same be said of AC/DC or Sonic Youth or Coldplay or Beck?

If you’ve ever pondered such vast questions, congratulations! You shop at record stores. This is good news. And there’s more good news: There’s a fairly straightforward answer. It’s called a saturation point. Here’s how it works. Borrowing a little from 9th-grade chemistry, we can say that a significant number of any given artist’s records will end up in a used CD bin if the number of records in circulation (our “substance”) exceeds the capacity of that artist’s popularity (our “solution”) to absorb them within a specific regional economy.

It isn’t all about the health of a band’s reputation. It’s about their ability to balance their success with their fan’s ability to absorb that success. You can be the most beloved artist in the world, but if there’s too much of you out there, you’ll end up in the bin stuffed between Frank Black and The Replacements.

This is precisely what’s happened to Bjork over the last few years. Bjork has her detractors sure, but most folks think she’s a real good time, a total A-lister. And that hasn’t changed for 16 years. All seven of her records—not to mention her numerous singles and box sets—have become ubiquitous. Nearly everyone’s got at least one. Which means nearly everyone’s traded one in. But besides the Beatles, no artist’s fans can include “nearly everyone.” It’s not sustainable. There’s simply too much product for Bjork’s fans to take in. And the toss-offs, the CDs given to ambivalent boyfriends or yoga-obsessed retirees, end up repackaged and marked down.

Of course, the older you get the more likely you are to reach saturation point. Nearly all of the bands who’ve done so in a big way—Bjork, Sonic Youth, Talking Heads, Stereolab—have been around for 10 years or more. And bands that are just now reaching saturation point—My Morning Jacket, The Mars Volta, Clinic—have at least five records under their built. The trick is to postpone saturation by keeping album quality as high as possible, thereby expanding the volume of your fans’ love even as you flood them with more and more product. Turn your records into timepieces, family heirlooms, objects of nostalgia and adoration—things that cannot be parted with.

Radiohead had been one of those rare artists who, despite their age and immense popularity, were rarely seen in used CD stores . This began to change, slowly, a couple years ago. Now you can walk into Other Music down on E. 4th St. and find two copies of “OK Computer” and “Hail to the Thief,” along with “The Bends” and a semi-obscure EP. Sometime around “In Rainbows”—a phenomenal record, to be sure—and the attendant media hysterics, people began to relent, and Thom showed up in the bins.

Still, consistent bin appearances for established artists are ultimately points of pride—a sign they’re part of the cultural lexicon, a band people know about who don’t know much about music. By the same token, used CD bins are places where hype goes to die. There’s no better proof that a freshman band’s expectations far exceeded reality than to see their debut release hit used record stores in large numbers soon after its release.

Take San Diego’s Wavves, for example. Head down to Other Music, and you’ll find two copies of the duo’s self-titled debut (released last December) and three copies of their follow-up, “Wavvves,” released this March. Having your Pitchfork-approved record reach saturation point less than six months after its release is, shall we say, bad for business. Could this signal a backlash against frontman Nathan Williams’ much-chronicled onstage meltdowns and hipster-on-hipster violence? Maybe.

Or it could just be a symptom of New York’s unique music economy. Bands get recycled a lot faster here than say, Butte, Montana, or really anywhere else besides LA and Chicago. In fact, the specific regional economy in question has possibly the greatest impact on when (or whether) an artist reaches saturation point. This is because, beyond a region’s hunger for new bands and new scenes, economies can also differ dramatically in terms of listening habits and living arrangements. Folks in big cities are more likely to listen primarily to mp3s than anything else—CDs become expendable as soon as their ripped to their owner’s laptops. Similarly, people who live in larger apartments (or full homes, for that matter) and move less often are more likely to hold on to that Springsteen box set for a few years. Physical records are simply less of a burden.

All of which is to say that whether or not any given artist—be they Jay-Z, Chicago, or Christian Death—ends up crowding a used CD bin isn’t so simple after all. And I, most certainly, am no economist. Yet, I do think the mystery of Bjork is a little closer to being solved, at least where used CDs are concerned. Her music is a whole other matter…

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