Forget paying extra for more legroom seats. These days I’d pay to sit next to someone who keeps his socks on. On recent flights I’ve had bare feet beside me, on the back of my armrest, on the bulkhead in front of me. Once, upon feeling something push through the sliver of space where the seat back and bottom meet, I reached behind me and grabbed a stranger’s toes.

Yet barefoot flying is merely one example of how public space, especially in airports and on airplanes, is rapidly transforming into more personal and intimate territory.

“Everybody on a long flight takes their shoes off now,” said David Huberman, who lives in Ashburn, Va., and flies about 20,000 miles a month for work. “And not everybody’s aware of how that smells.”

With the holiday travel season upon us, when patience will be as essential as a passport, the time seems right to seek some understanding about why we behave the way we do on airplanes.

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Psychological and anthropological scholarship suggests it comes down to this: Public space isn’t what it used to be. We’re living in an age when smartphones and tablets allow us to have our most personal conversations in extremely communal places. Centuries-old walls between what’s considered private and what’s considered public are crumbling.

“People are trying to come up with strategies to make themselves feel comfortable in a world of tremendous mobility,” said Setha M. Low, director of the Public Space Research Group at the City University of New York Graduate Center, an environmental psychology and anthropology professor and a former president of the American Anthropological Association.

“Gate lice” and other pains

While this phenomenon is happening everywhere — in parks, restaurants, shopping malls — it’s exaggerated when we travel. One could argue that at a time when flying is an exercise in contortionism, filling a seat pocket with your paraphernalia and putting your feet up is an attempt to create what the environmental psychologist John B. Calhoun referred to in the 1940s as “defensible space” — public territory you try to turn into personal space (like your office cubicle) to gain some measure of cognitive control.

Problems arise, though, when some travelers’ efforts to do that impinge on another’s sense of what public space ought to be.

Like most experienced travelers, Dawn Bozulich, a Los Angeles businesswoman who flies about once a week, has her own mental list of passenger infractions, like loud talking and “gate lice,” a term frequent fliers use for travelers who besiege the gate long before their zone is called.

Air travel has gone from being a social occasion in which fliers chatted and ate meals together, to a mass commoditized landscape where travelers shell out for seats and early-boarding privileges, eat their food when they feel like it and engross themselves in their iPads.

“It’s no longer a social situation,” Low said. “It’s a completely different experience. You put your headphones on and you’re in your own world.”

Except that we’re all still hurtling through space together. There have to be some basic principles, no?

In Emily Post’s 1922 book, “Etiquette,” she wrote that “to do nothing that can either annoy or offend the sensibilities of others, sums up the principal rules for conduct under all circumstances — whether staying at home or traveling.”

Still sounds good to me. But it’s been 90 years, so for a decorum update I turned to Lizzie Post, Emily’s great-great-granddaughter and the co-author of “Emily Post’s Etiquette, 18th Edition.” Naturally I asked her about feet.

“Personally I think the biggest difference comes from: Do your feet smell or not?” Post said. “Bare feet are a bit inappropriate.”

What about scents, like cologne and perfume? “Even the lightest perfumes can actually be amplified, especially when you’re in travel situations,” Post said. “Stress levels are elevated, and when that happens you do tend to perspire more.”

And personal grooming like hair brushing and nail filing? “They’re inappropriate when you have to be this close to someone,” Post said. “So take it to the restroom or wait until you get off the plane.”

So in the spirit of the season, let’s all be a little more mindful of one another. And remember: We don’t just take our newspapers and neck pillows onto the plane. We take on all kinds of cultural and social expectations, too. No one is perfect. Not even you.