In the mid-20th century, the Dutch biologist Nikolaas Tinbergen uncovered an odd quirk of animal behavior: Across species, the animals in his experiments seemed to prefer prettier, flashier, more attention-grabbing versions of their natural environments—“supernormal stimuli,” he called them—even when those stimuli were fake. Certain types of fish, he found, would become more violent towards dummy fish whose undersides were more vibrant than the species’ usual color; mother birds would ignore their own eggs to sit on a nest of larger, more colorful imitations, or divert food from their children to feed models of chicks with brighter beaks.

“The essence of the supernormal stimulus,” the psychiatrist Deirdre Barrett wrote in her book on the subject, “is that the exaggerated imitation can cause a stronger pull than the real thing.”

“We humans can produce our own,” she continued, like “candy sweeter than any fruit,” or pornography.

On its face, the comparison makes sense: People like sugar and people like sex; candy and pornography are both super-concentrated, turbocharged doses of more natural sensory experiences.

On closer examination, though, one of these things is not like the other: The pleasure of sugar is delivered the same way—taste—whether it comes from a strawberry or a piece of strawberry taffy. Pornography, on the other hand, is a different sensory experience than the real thing, relying on sight and sound in place of touch.

And between the two is another distinctly human supernormal stimulus: food porn, the carefully arranged, carefully filtered images that show a meal—homecooked or restaurant-served—at its most appealing.

Food porn is defined in part by the senses that it is a visual experience of something that other people can smell and taste. Food porn, as Amanda Simpson, the creator of the site Food Porn Daily, told The Daily Meal in 2010, is “anything that makes me drool”—something that, at its best, should manufacture a desire that it can’t satisfy.

What’s the appeal in ogling what you can’t have? In the case of food porn, at least, researchers still aren’t sure.

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The first documented use of the term “food porn” comes from the feminist writer Rosalind Cowards’s 1984 book Female Desire. It was referenced every so often for the next two decades or so by food writers and chefs, according to the site Know Your Meme, but didn’t take on its current meaning—food photos shared through social media—until the early 2000s. The photo-sharing website Flickr launched a “Food Porn” category in September 2004 (today, it has around three-quarters of a million photos).

And a few months later, in April 2005, it entered the Urban Dictionary lexicon. Definition: “Close-up images of juicy, delicious food in advertisements.” Used in a sentence: Oh, that McDonalds ad was like food porn. I want a Big Mac sooo bad.

Urban Dictionary, though, makes an assumption on something that research hasn’t yet been able to prove. A McDonalds ad for a Big Mac may look delicious, but what’s still murky is whether that necessarily translates into hunger for a Big Mac.

The chef’s maxim that people first eat with their eyes is backed both by common sense—food stylists exist for a reason, and a glistening grill-marked burger that oozes cheese is an easier sell than a limp, gray one—and by science. A 2012 study published in the journal Physiology and Behavior, for example, found that seemingly minute details about a dish’s appearance, like “gloss, evenness, and shape,” can alter how diners perceive its taste and smell.

But what happens when eating with the eyes is the only step, rather than just the first—when the image isn’t a bridge to smelling and tasting a dish, but the entire experience?

Some scientists believe—like Simpson—that images of food only trigger the desire for the real thing. A 2012 study, for example, found that just looking at pictures of food may be enough to cause an uptick in ghrelin, a hormone that causes hunger.

One reason may be that looking primes the brain for eating. “If you think about throwing a baseball, your brain reacts like you’re really throwing a baseball,” explained Gabriella Petrick, a professor of nutrition and food studies at George Mason University. “When we eat things, different parts of our brain light up in different ways. It’s not just taste—we evoke sight, we evoke hearing, we evoke lots of different [things] as our brain tries to construct what our food is.”

But other research has shown that when it comes to appetite, food porn may be a substitute for food itself. One 2011 study found that looking at pictures of food may turn people off from the real thing—but only if the food in the image has a similar flavor to whatever real item is about to be consumed. When volunteers viewed photos of salty snacks and then ate salted peanuts, they tended to enjoy the nuts less than people who had viewed photos of desserts.

And in 2013 study in mice published in the journal Nature Neuroscience, when researchers flooded the reward centers of the rodents’ brains with insulin, a hormone that triggers feelings of fullness, the mice lost interest in returning to places where they had previously been given food—suggesting, the authors said, that the brain’s reward centers may not respond as much to “food cues”—a feeding area for mice, a photo for humans—when the brain knows that the stomach is full.

Taken in sum, the research, with all its contradictions, doesn’t reveal much. And ideas about why people take so much pleasure in sharing their food-porn images are as varied as theories about why people view them.

In 2013, the psychiatrist Valerie Taylor delivered a presentation at the Canadian Obesity Summit arguing that posting photos of meals on social media is a sign of a disordered relationship with food. “We take pictures of things that are important to us,” she later told The Huffington Post, “and for some people, the food itself becomes central and the rest—the venue, the company, et cetera—is background.”

“A worthy hypothesis,” Jezebel’s Katie Baker countered, “but I’m pretty sure my friend who won’t stop Instagrammming photos of ramps this month just wants us all to know that she’s on trend.”

“There’s a kind of a performative aspect,” agreed Richard Magee, a professor of English at Sacred Heart University who has studied food writing. “I used to do this 55-mile ride where I’d stop and get cupcakes in the middle of the ride, and I’d post pictures of the cupcakes. And I think part of it was like, ‘I can do this, I can break the rules because I’m exercising.’ … I have friends who are really good cooks, and they’ll post pictures of the stuff that they do, and it’s kind of like a play in a way.”

And really, the tricks that make food porn what it is—the filters, and the kale arranged just so, and egg yolks cut to spill at just the right, gooey angle—are just watered-down versions of the elaborately staged performance that is professional food photography. In a professional shoot, the milk in a cereal bowl, for example, may actually be glue; a stack of pancakes may be propped up by hidden layers of cardboard; and blemishes on berries may be covered up with lipstick. Grill marks are meticulously drawn. Extra sesame seeds—each one carefully selected—are glued on to buns. Wherever the image may fall along the spectrum of fakery, from an Instagram filter to a bowlful of Elmer’s, the desired effect is to make something that seems real.

“It makes sense,” Megan Garber wrote of food journalism in The Atlantic last week, “that we would come to treat food not just as a source of sustenance, but also as a source of beauty that warrants intellectual engagement.”  But food porn is to food writing what images, in general, are to words: more immediate, more visceral. When choosing recipes for the Food Porn Daily Cookbook, Simpson recalled, she would “sit there for hours thinking, ‘What is porny? What turns your palate on?’”

“Those pictures and those websites draw us in,” Magee said, “because they do hit something really primal in us.” Hunger and craving and fuel and want and need come together in complicated ways, but an ad for a Big Mac still draws on a fundamental quirk of animal behavior: It’s prettier, flashier, more attention-grabbing than the real thing, and that in itself is reason enough to enjoy it.