Because humans are novelty-seekers. Emily Yoffe, my former colleague at Slate, has written at length about what motivates our desire for new information. The Internet taps into our insatiable desire for more—more pleasure, more distraction, more news. When we get this new stimulus, dopamine leaks into our brain, making us want to dive even deeper into the web. And diving deeper means clicking further—slide after slide after slide.

Jonah Lehrer, the author of How We Decide, furthered Yoffe’s work and noted that we especially want to know more about that which we already know. The slideshow format is designed to exploit exactly this. Once we see one slide, we have enough background knowledge to want to see them all.

The pictures also are key. In the developer and web-design community, it’s well known that web readers’ eyes linger longer on articles and headlines with images attached to them. That same rule applies with a slideshow.

Some sites have capitalized on our psychological vulnerability more vigorously than others. The Boston Globe advertises its slideshows on every article page. (These slideshows range from the serious to the seriously mundane. In September, quarterback Tom Brady’s minor car crash merited a thirty-page slideshow, including photos of broken glass.) Time links to photo galleries from within articles—while you’re reading, a red link asks you to “See the aftermath of the [Pakistani] floods.” Entertainment Weekly runs multiple slideshows, like “MTV Video Music Awards: 26 Years of the Good, Bad and Ugly,” every day, filling its home page’s top-story slots with anodyne top-twenty-five lists.

Of course, not all slideshows are born with original sin. Half-naked photo galleries have as much in common with a serious visual essay as Maxim does with The New Yorker. So to make sense of the new slideshow economy, I surveyed the field and devised a rough taxonomy:

The Gallery
Aesthetically intriguing but editorially empty, the gallery is photojournalism’s most valuable contribution to the economics of web journalism. All slideshows are, in a way, galleries, but the true gallery is defined by its simplicity. The photos do the talking, which means they’re usually hyperbolic in their beauty, horror, or strangeness. Typical example: “PHOTOS: Astronauts’ Spectacular Twitter Pictures From Space,” The Huffington Post.

The Listicle
A gallery with more of an editorial bent, listicles are an easy way to trap the completist reader into clicking through the whole thing. Listicle creators are essentially modern-day collectors, assembling and categorizing disparate items to make a larger point. Typical example: “A Complete Guide to Justin Bieber’s Dance Moves,” Vulture.com.

The Countdown
Little more than a listicle with an extra layer of arbitrary opinion, the countdown is an adaptation of every “Best of” list that appears in magazines and on cable TV at the end of the year. When slides are placed in descending order, the slideshow takes on a narrative momentum, ensuring its audience keeps clicking. Typical example: “The 10 Best Sports Movies of the 2000s,” Bleacher Report.

The Timeline
Once again, an organizational framework is applied to the classic gallery, and once again it makes it a more propulsive read. Great timelines are trips through a past the audience either vaguely remembers, or that informs the zeitgeist of the present. Typical example: “The Secret Origins of Clippy: Microsoft’s Bizarre Animated Character Patent,” Technologizer.

The Aggregator
A visual display of the kitchen sink. When there’s a loose scattering of things to be presented, and no good way to present them cohesively, they may as well be presented visually. It’s an unadulterated play for your clicks with little editorial value. Typical example: “VOTE: Where Should Arianna Stop on Her ‘Third World America’ Tour?” The Huffington Post.

The Sex Show
The most noticeable slideshow on the web, and also the most virulent. As scanty as it is shameless, it’s often organized around a theme, celebrity, or body part, the sex show can range from scandalous to staid, depending on the site’s editorial tenor. Typical example: “Blake Lively’s Breast Looks,” Vulture.com.

Chadwick Matlin is the former associate editor of Slate's The Big Money. He lives in New York and can be reached at Chadwick.Matlin@gmail.com.