Pumpkin spice is a lot like Halloween candy. Both are harbingers of fall, and both seem to push the appropriate seasonal boundaries by popping up earlier and earlier every year.

But unlike other food fads—two well-documented examples are cronuts and cake pops—pumpkin spice has endured. This is the 11th year that the Pumpkin Spice Latte (the PSL, in the parlance of aficionados) has graced the menu at Starbucks SBUX . With more than 200 million PSLs sold, it’s the company’s most popular seasonal beverage. Rival coffee purveyor Dunkin’ Donuts has more than a half dozen pumpkin-flavored items on the menu this year. The company says it spends time strategizing how it can ramp up the “pumpkin-osity” of its offerings.

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PumpkinsPumpkin Background: Getty Images

As most consumers are well aware, we are not speaking merely of coffee. Just about anything that can have a pumpkin variety now does, or soon will: Oreos, Milano cookies, M&Ms, yogurt, marshmallows, gum, oatmeal, Eggo waffles. There was even buzz last week, which turned out to be a hoax, about a pumpkin spice condom. The fact that this fake news gained so much traction is a sign of how sexy the subject has become.

Or perhaps it was an ironic commentary on the ubiquity of a once-humble member of the squash and melon family. Indeed, some have begun to complain of pumpkin spice fatigue (let’s call it PSF) out of one side of their mouths—but data on eating habits suggest they’re still sipping PSLs out of the other. Since 2006, industry tracker Mintel has found that pumpkin as a beverage ingredient has grown 130% on U.S. menus. Since 2004 the use of pumpkin as a flavor in food on menus has increased ten-fold.

So what’s driving the flavor to these new heights? Fortune decided to investigate the consumer psychology behind pumpkin spice.

Both Starbucks and Dunkin’ Donuts executives point to the seasonality and nostalgia evoked by the flavor. “It’s a beverage that captures the essence of fall,” says Peter Dukes, who led the PSL’s development as a Starbucks product manager. His colleague Cliff Burrows, group president of the U.S., Americas, and Teavana, thinks its success is linked to the combination of the return to school after the long summer holiday and the closure of the season with Labor Day. “There’s something about that whole turning point, which is so significant,” he says. “It’s a nice way to turn the corner on the year.”

That’s all well and good, but it doesn’t really explain why pumpkin spice outperforms other seasonal flavors. Starbucks’ eggnog latte and peppermint mocha both predate the PSL but haven’t entered the zeitgeist to nearly the same extent. Before landing on the PSL, Dukes said that Starbucks also considered a cinnamon streusel latte, a flavor that seems unlikely to have garnered as much attention or traction.

One reason pumpkin spice may be so popular is that it taps into a paradox of human desire: We like new things, but we also want things we know we’ll like, explains Harry Balzer of consumer market research company NPD Group. Balzer says that pumpkin pie is the second most-consumed pie in America (apple is No. 1), which is astonishing considering it’s essentially eaten only one day a year. That gives us the illusion that we’re eating a new flavor, and helps explain why Dunkin’ and Starbucks likely won’t ever offer pumpkin year round. Doing so would erode its mystique.

Alan Hirsch, founder and director of the Smell & Taste Treatment and Research Foundation in Chicago, thinks that there might be something physical going on. Hirsch, whose findings and research are not without controversy, has done work with Dunkin’ Donuts and sister brand Baskin-Robbins although not involving pumpkin. In one of his studies, he looked at the effects of 30 different scents on the sexual arousal of 31 male volunteers (yes, you read that correctly). He found that the scent causing the highest level of arousal was a combination of lavender and pumpkin pie. Doughnut and black licorice came in second, and the combination of doughnut and pumpkin pie came in third. Ironically, cranberry, the other big marker of Thanksgiving, came in last.

One of Hirsch’s hypotheses is that in our current culture, in which there’s been a loss of traditions, people are seeking out forms of comfort and security that they had as children. The smell of pumpkin spice brings them back to those moments. He also believes its ascent might stem from an aging society. As you grow older, you lose your sense of smell, but cinnamon, a major ingredient in pumpkin pie, is something that continues to cut through.

But part of pumpkin spice’s allure to consumer products companies is that it’s become a safe bet. “It’s a flavor line that’s appealing across all age groups,” says MaryAnne Drake, a professor of sensory analysis and flavor chemistry at North Carolina State University. “It transcends demographics.” Companies have caught on to the fact that Starbucks, a brand popular with millennials, has had major success with pumpkin spice, but the flavor is not too cutting edge that their parents won’t drink it. Drake notes that there’s been a push in food research to examine the role emotions play in what we eat. That has encouraged the proliferation of pumpkin spice offerings, since they evoke warm feelings.

Pumpkin spice’s popularity might not have anything to do with, well, pumpkin, argues Hedy Kulka, senior flavorist with International Flavors & Fragrances. “I don’t know if it’s so much the pumpkin part,” says Kulka, who admits to preferring a Starbucks eggnog latte to the PSL. “If you sold pumpkin people wouldn’t want it. It’s the background note.” What’s driving pumpkin spice’s appeal is the inclusion of cinnamon and nutmeg—two spices that coffee shops have been putting out on counters for customers to use for years, she notes. “They’re so familiar to Americans,” she says. “Nobody is putting cardamom out there for you to use.”

There are two possible futures for pumpkin spice: a world in which we max out on the flavor and the bubble pops. Or it might become even more ubiquitous to the point where we don’t even think about it. “I see this as something that is not necessarily a novelty anymore,” says Kulka. Soon, pumpkin may become so common that it’s just like plain old vanilla.