Johannes Hofer would shudder at the thought. Hofer was a Swiss physician who, in 1688, diagnosed the symptoms of a potentially fatal mental illness he called “nostalgia.” This is the origin of our modern study of nostalgia.

In “Medical Dissertation on Nostalgia or Homesickness,” Hofer, nodding to Odysseus’ epic homesickness, combined the Greek words for “return” (nostos) and “pain” (algos), cited a story some guy told him, ascribed a number of intuitively plausible symptoms and gave name to a feeling that had been observed since ancient times, notably by Hippocrates and Caesar. The Bible also makes note of it.

“[Hofer] made the classic scientific error,” Green says. “Mistaking a correlation — an association — and surmising a causal link.”

Hofer did not excel at the scientific method. Around the time Isaac Newton figured out the math to launch stuff into orbit, Hofer observed Swiss mercenaries he saw as direly homesick. “A melancholy delirium,” he called it. Hofer thought it was a mental illness and probably caused by demons.

Today, Hofer couldn’t diagnose nostalgia if it took him by the hand and whispered in his ear. His proposed symptoms included burning fever, sad wandering, “scorn” of “foreign manners,” sleeplessness, enervation, hunger, thirst, heart palpitations, not being able to take a joke, “frequent sighs,” stupidity and making “a show of the delights of the Fatherland.”

As treatment, Hofer recommended bloodletting, mercury and public prayer. But the best remedy, he said, was just taking someone back home. Hofer — who, again, was observing mercenaries — considered those most at risk of nostalgia to be “young people and adolescents sent to foreign nations.” Others at the time studied this, too.

“They realized these people are homesick and they’re not in very good shape,” Green says. “There was a correlation between loneliness or depression and this homesickness or nostalgia. It turns out that it’s not that nostalgia causes these negative things; it’s actually when we’re in these negative states, we’re more likely to feel nostalgic. That’s going to spark nostalgia, which, in turn, helps ameliorate those states. In other words, nostalgia has this repair function or restorative function.”

Psychology, somehow, mostly agreed with Hofer for several hundred years. Into the 19th century, it was even believed that nostalgia bedeviled the Swiss and no one else.

“We have this thing called the hindsight bias,” Green says after I got haughty. “Which is, after we tell you something, you’re like, ‘Oh, yeah, well, obviously.’ [For example], when you’re talking about attraction, I’ll start out my lecture and I say, ‘When I tell you opposites attract …’ and then everybody’ll nod like, ‘Well, yeah, opposites attract. We talk about that all the time. We have that saying.’ And that’s actually a load of crap. Opposites do not attract. So it’s 98% the other saying we have, which is ‘Birds of a feather flock together.’”

Nostalgia’s rehabilitation came about during the “positive psychology” movement. In the late 1990s, after a century of focusing mostly on disorders and negative feelings, researchers looked deeper at the mental processes that kick in when we’re having trouble emotionally keeping it together. Nostalgia seems to be one of those processes.

Constantine Sedikides, Ph.D., was Green’s doctoral adviser at the University of North Carolina and among the first to publish specifically about nostalgia, though there are journal references to it sprinkled through the decades. Sedikides co-authored the first of his many, many academic papers on the subject in 2004: “Nostalgia: Past, Present, and Future.” 

At the end of the paper, the authors describe nostalgia as formerly a “psychological ailment” that’s now “emerging as a fundamental human strength” — a part of our psychological immune system. It’s the name for the feelings and mental processes, like nostalgia or hope, that help regulate our emotional well-being.

“It’s a bittersweet emotion, so it’s a more complex emotion,” Green says of nostalgia. “You could simultaneously think back about all those fun memories in college and you can still, at the same time, lament that you don’t see those folks as much, or that you lost touch with one or two of your closer friends there. 

“Nostalgia is a pretty unique state in that it also involves your personal, autobiographical memories. You can be nostalgic for an event that your close friend feels completely differently about. They had a different experience. They brought a different psychology to that experience.”

Green, who lobbies friends and loved ones to go to their class reunions (research says it’s beneficial), offered this keepsake from his alma mater, Dartmouth: the remains of a clay pipe. Observing tradition, he smashed it when he graduated.

“I actually saved these shards of that pipe. Even back then, I was maybe predisposed to and ready to be a future researcher,” he says. “I was the only one who did that.”