There’s a full-body sigh that happens when you cross the threshold of your home for the first time after a long trip. And I do mean full-body: No sooner have your limp arms discarded your luggage on the floor and your lungs filled themselves with that sweet familiar home air than your gut feels the sudden, emphatic need to poop.

For me, it happens within minutes, if not seconds. And I’m not alone.

“This is indeed a very familiar story,” says Nick Haslam, a professor of psychology at the University of Melbourne and author of Psychology in the Bathroom. “Most people feel more comfortable going to the bathroom in familiar—and private—surroundings.”

It’s sort of the inverse of another bowel-related vacation experience: traveler’s constipation. Many people find themselves clogged while they’re away from home, often because they’re uncomfortable pooping in unfamiliar bathrooms. It can also be because they are eating less poop-friendly foods on vacation, or because unfamiliar bacteria in a new environment are throwing off their gut microbiomes, as Mollie Bloudoff-Indelicato reported for The Atlantic in 2015. (Of course, if you eat the wrong meal, you could suffer traveler’s diarrhea instead.)

“In my view the experience of ‘unburdening’ upon returning from a trip is largely a Pavlovian response: The home is a safety signal, signifying that this is the right place to go,” Haslam told me over email. This is likely to be the case whether or not you were particularly constipated during your trip. “If there has been any inhibition or retention at all during the trip the relaxation response is likely to kick in when you come home,” he says.

This is largely the explanation I expected for this phenomenon, which I shall henceforth be calling the “returnee’s release.” And it makes total sense. But there’s a deeper explanation here—one that involves the mysterious ways our bodies respond to changes in environment. And if you embrace the reasoning of one scientist, it may lead you to question the existence of your very soul.

That scientist is Jack Gilbert, a professor of surgery at the University of Chicago, and the director of the university’s Microbiome Center. I spoke with him in an attempt to understand whether there is a physical call-and-response between my home and my body that might trigger the need to make a deposit in the porcelain bank. Or is it simply that I feel more comfortable at home?

“What is ‘more comfortable?’” Gilbert asks. “Why do you feel more comfortable at home?”

Because it’s familiar, I guess, I say. All my stuff is there.

“Right, but that’s not you thinking that, remember that,” he says, meaning there is no mystical entity that makes up my self. “Unless you’re highly spiritual, then the soul doesn’t really exist. There’s no ghost in the machine. You’re just a sensory programming device.”

He devotes a large portion of this interview to really hammering home the point that the brain does not “store” memories, like a computer. “All you’re doing, when you try to recall something, is triggering sensory simulacra of that experience,” he says.

So back to the returnee’s release. When you enter your home, you get certain sensory inputs—smells, sights, a familiar creak in the floor, perhaps. All aspects of the environment, from the temperature of the room to the feeling of the doorknob in your hand to the microbes all around you, contribute to the sensation of “being more comfortable at home.”

“‘More comfortable’ is an emotional state, but emotions are physiological responses,” Gilbert says. “So ‘more comfortable’ is a physiological state. It’s a way in which your body responds to its environment.”

“When you get back into your home, your glucose tolerance will change,” he continues. “Your adrenaline pumping will change, and the energy sensors of your muscles will change, altering your actual respiration, how much energy your burn, and how much fat you deposit. When you get back into your home your sleep patterns will change, because the hormones that control sleep will be altered. All of these factors influence how quickly food moves through your gut.”

We don’t really know exactly how the body responds to certain sensory stimuli, he notes. Just that it does.

So it’s not only that people feel safer and more comfortable pooping at home, but that the complex sensory stew of the home environment may in some way be physically moving the mail along.

“We are essentially automata responding to environmental cues,” Gilbert says. “I’m pretty sure I can train you as a human being to pee when you smell peppermint. That’s an example of how much of an automaton you are. It would be technically possible to do that.”

And as Haslam noted, we’re already trained in a more mundane way. “You have a lot of experience defecating and urinating in your preferred toilet,” he writes, “which becomes strongly associated with those acts, so just being in its presence triggers the relaxation response that allows you to release the inhibitions that led you to ‘hold it in’ while in unfamiliar surroundings.” And it’s not just while traveling: As I’ve previously reported, some people find themselves inhibited in any public bathroom.

If nothing else, the returnee’s release is certainly a stark reminder of how much our bodies are held in thrall by their surroundings. “You came in saying your question is stupid,” Gilbert chastises me. “It’s not. Your question is paramount to the fact that we are not in control of our environment. There is no free will.”

Think about that the next time you drop the kids off at the pool.