SOUTHAMPTON, England — Not long after moving to the University of Southampton, Constantine Sedikides had lunch with a colleague in the psychology department and described some unusual symptoms he’d been feeling. A few times a week, he was suddenly hit with nostalgia for his previous home at the University of North Carolina: memories of old friends, Tar Heel basketball games, fried okra, the sweet smells of autumn in Chapel Hill.

His colleague, a clinical psychologist, made an immediate diagnosis. He must be depressed. Why else live in the past? Nostalgia had been considered a disorder ever since the term was coined by a 17th-century Swiss physician who attributed soldiers’ mental and physical maladies to their longing to return home — nostos in Greek, and the accompanying pain, algos.

But Dr. Sedikides didn’t want to return to any home — not to Chapel Hill, not to his native Greece — and he insisted to his lunch companion that he wasn’t in pain.

“I told him I did live my life forward, but sometimes I couldn’t help thinking about the past, and it was rewarding,” he says. “Nostalgia made me feel that my life had roots and continuity. It made me feel good about myself and my relationships. It provided a texture to my life and gave me strength to move forward.”