Sometime in the late ’50s, Frieda Fromm-Reichmann sat down to write an essay about a subject that had been mostly overlooked by other psychoanalysts up to that point. Even Freud had only touched on it in passing. She was not sure, she wrote, “what inner forces” made her struggle with the problem of loneliness, though she had a notion. It might have been the young female catatonic patient who began to communicate only when Fromm-Reichmann asked her how lonely she was. “She raised her hand with her thumb lifted, the other four fingers bent toward her palm,” Fromm-Reichmann wrote. The thumb stood alone, “isolated from the four hidden fingers.” Fromm-Reichmann responded gently, “That lonely?” And at that, the woman’s “facial expression loosened up as though in great relief and gratitude, and her fingers opened.”

Fromm-Reichmann would later become world-famous as the dumpy little therapist mistaken for a housekeeper by a new patient, a severely disturbed schizophrenic girl named Joanne Greenberg. Fromm-Reichmann cured Greenberg, who had been deemed incurable. Greenberg left the hospital, went to college, became a writer, and immortalized her beloved analyst as “Dr. Fried” in the best-selling autobiographical novel I Never Promised You a Rose Garden (later also a movie and a pop song). Among analysts, Fromm-Reichmann, who had come to the United States from Germany to escape Hitler, was known for insisting that no patient was too sick to be healed through trust and intimacy. She figured that loneliness lay at the heart of nearly all mental illness and that the lonely person was just about the most terrifying spectacle in the world. She once chastised her fellow therapists for withdrawing from emotionally unreachable patients rather than risk being contaminated by them. The uncanny specter of loneliness “touches on our own possibility of loneliness,” she said. “We evade it and feel guilty.”

Her 1959 essay, “On Loneliness,” is considered a founding document in a fast-growing area of scientific research you might call loneliness studies. Over the past half-century, academic psychologists have largely abandoned psychoanalysis and made themselves over as biologists. And as they delve deeper into the workings of cells and nerves, they are confirming that loneliness is as monstrous as Fromm-Reichmann said it was. It has now been linked with a wide array of bodily ailments as well as the old mental ones.

Ariel Lee

In a way, these discoveries are as consequential as the germ theory of disease. Just as we once knew that infectious diseases killed, but didn’t know that germs spread them, we’ve known intuitively that loneliness hastens death, but haven’t been able to explain how. Psychobiologists can now show that loneliness sends misleading hormonal signals, rejiggers the molecules on genes that govern behavior, and wrenches a slew of other systems out of whack. They have proved that long-lasting loneliness not only makes you sick; it can kill you. Emotional isolation is ranked as high a risk factor for mortality as smoking. A partial list of the physical diseases thought to be caused or exacerbated by loneliness would include Alzheimer’s, obesity, diabetes, high blood pressure, heart disease, neurodegenerative diseases, and even cancer—tumors can metastasize faster in lonely people.

The psychological definition of loneliness hasn’t changed much since Fromm-Reichmann laid it out. “Real loneliness,” as she called it, is not what the philosopher Søren Kierkegaard characterized as the “shut-upness” and solitariness of the civilized. Nor is “real loneliness” the happy solitude of the productive artist or the passing irritation of being cooped up with the flu while all your friends go off on some adventure. It’s not being dissatisfied with your companion of the moment—your friend or lover or even spouse— unless you chronically find yourself in that situation, in which case you may in fact be a lonely person. Fromm-Reichmann even distinguished “real loneliness” from mourning, since the well-adjusted eventually get over that, and from depression, which may be a symptom of loneliness but is rarely the cause. Loneliness, she said—and this will surprise no one—is the want of intimacy.