The experimental psychologist Dorothy Tennov writes in her 1979 book Love and Limerence: The Experience of Being in Love, of the moment she began to take an interest in the systematic study of romantic love.

It was a fall afternoon in the mid-1960s when Dr. Tennov walked into her office from class to discover her student Marilyn Weber waiting for her. Ms. Weber was particularly impressive and bright, but that day, her posture was sloped, and her eyes were time-stamped red, marking hours of crying.

She’d come to apologize for having failed to hand in an assignment and vowed to get it done within a week.

Without detailing her distress, Ms. Weber said she couldn’t pull herself together, that she wasn’t sure why she was behaving the way she was, but wondered aloud whether life was just too hard for her.

Dr. Tennov flashed to a conversation about romantic love she’d had with two graduate students, a few weeks earlier. Both students described the pain of past breakups and how it rendered them utterly useless.

The emotions they expressed were ones Dr. Tennov had experienced, and she wondered if there was a universally shared progression to the stages of romantic love.

As Ms. Weber headed out the door, Dr. Tennov couldn’t help but ask whether her distress had “anything to do with a disappointment with love?”

Indeed, it had.

Dr. Tennov was curious about this distress caused by love that Ms. Weber was experiencing—indeed, her students had been describing something similar. She turned to her textbooks, journals, psychology papers, and other writing on romantic love but found nothing about the agonizing pain of loving another.

What was this state called?

The question gripped her until she decided to conduct the research herself.

Guided by the notion that this particular love-distress was not a pathological state, nor an aspect of neurotic personality (based on the psychological states of students who were known to her), she concluded that the pain from love must be a normal, universal experience.

From there, she would record the incidence of unhappy love and look for commonalities that induced love-distress.

She created a questionnaire comprising 200 true or false statements about romantic love, sex, and personal relationships. She solicited volunteers through ads, bulletin boards, newsletters, and in person. She interviewed over 500 people.

Yet, during a conversation about unrequited love, she realized she was closing in on the answer. Those who did not reciprocate love, who didn’t feel the same intensity of feeling, who weren’t gripped by agonizing ruminations about another, were the ones who helped her crystallize the state she was seeking to name.

The term “love” wasn’t quite right. Love doesn’t stay propped in such a heightened state as reported by those trapped inside unrequited longing.

Being in love also didn’t suffice, as the condition described included feelings one had sometimes to endure.

The state being described repeatedly was of an unrequited and agonizing longing for another, a yearning that seeks reciprocity above all else.

This is the state that Tennov coined “Limerence.”

In her book Love and Limerence, Tennov expressed the difficulty of conveying the specific craving a limerent has for their limerent object (LO) for full emotional enmeshment with another human being, she writes:

“Obsessed comes closer, but leaves out the aching,” Tennov writes.

This all-consuming state affects a staggering 5% of the population, indicating a genetic component.

There is also some scientific data to back up claims that limerence is a type of cognitive obsession—the state of experiencing “catastrophic misinterpretations of the significance of one's unwanted intrusive thoughts”—possibly caused by low serotonin levels in the brain.

To be in its thrall causes a wondrous euphoric high, but then leads a person to descend into the hot quicksand of despair, only to rise again into a halo of rabid, enthusiastic energy, strong enough to power a hundred suns.

Limerence, Tennov declared, is a mental activity, centered not only on the emotional sensations themselves but on the limerent’s interpretation of events.

For the limerent, the possibility of reciprocity is the driving force. Like drugs, it’s wildly easy to fall into its grip, and absurdly difficult to escape.