Before Bruce Willis was diagnosed with frontotemporal dementia (FTD) in 2023, he had begun stuttering again—a speech problem he had overcome as a child. In an interview with journalist Katie Couric, his wife, Emma Heming Willis, said the family initially dismissed the change and never imagined it could signal a devastating neurological disease.

Other early signs were just as easy to misread. His daughter Tallulah recalled his “vague unresponsiveness,” which she attributed to what she jokingly called “Hollywood hearing loss.” The family assumed that Die Hard had simply “messed with dad’s ears.”

As Willis’s cognitive abilities declined, Demi Moore—his former wife, who has remained close to him since their 2000 divorce—described the pain of watching someone “so vibrant and strong and so directed” gradually shift into unfamiliar versions of himself.

More than four years after his diagnosis, Willis now lives in a specialist care facility. Earlier this month, during World FTD Awareness Week, Heming Willis paid tribute to her husband, calling him a “legend” and saying the family exists in a “constant state of grief.”

FTD remains extremely rare, affecting an estimated 50,000 people in the United States. Yet one Pennsylvania family may offer researchers an important route toward a treatment. After decades of dead ends, scientists believe a mutation that has affected several members of the same family could help unlock new approaches to preventing and treating dementia—including Alzheimer’s disease.

In the 1970s, a family with nine children lived in a small town nestled in the foothills of America’s Rust Belt.

Their father, Ollie, was a successful civil engineer. Their mother, Jean, was a homemaker known for her optimism and energy. She spent her days reading, gardening and playing bridge.

Jean managed the family’s grocery shopping on a tight budget, keeping every calculation and list in her head. She cooked from scratch, making her own doughnuts and pizza, and served enchiladas at a time when the dish was still viewed as an exotic foreign specialty.

Her older children remember a dramatically different woman from the one she later became. Only in retrospect did they recognize what had been happening: Jean gradually withdrew, fading from the center of her children’s lives until she seemed almost like part of the household itself.

The qualities that had once made Jean so lively began to disappear. She played solitaire and watched television—Jeopardy, Wheel of Fortune and The Price Is Right. She stopped paying bills, caring about her appearance and shopping for clothes. A wife and mother who had once managed everything seemed to have lost the will to do anything for herself.

Her husband viewed her with contempt, while the children often responded with frustration and distance. They watched in silence as she stopped dressing up, abandoned the hair salon and began speaking less and less.

The woman once known as the neighborhood’s “library mom” was now often seen outside the local liquor store.

“Our community was pretty small,” a longtime family friend said. “She would be there before the doors even opened. People would whisper about it.”

Jean died of breast cancer at 60. In the aftermath, her children settled on a shared explanation for her decline. They believed she had failed to care for herself and might have lived longer had she not smoked or avoided doctors.

None of them understood why she had seemed so detached from the people she loved. Even then, they could not agree on a cause. They considered depression, alcohol and the overwhelming demands of raising such a large family.

It never crossed their minds that Jean might have had early-onset dementia.

Dementia is one of the conditions people fear most, although the term does not describe a single disease.

Advances in molecular and genetic brain research have enabled scientists to identify and distinguish an expanding range of dementias. Some, including Alzheimer’s disease, are widespread; others are exceptionally uncommon.

Each of Jean’s nine children was born with a 50 percent chance of inheriting the genetic form of frontotemporal dementia, or FTD. Geneticists have since confirmed that Jean also suffered from the condition during the years before cancer claimed her life.

Unlike more familiar disorders such as Alzheimer’s, FTD typically appears during midlife, often between 45 and 55. It progressively damages the areas of the brain responsible for planning, organization, language, social understanding and judgment.

Dementia is commonly associated with memory loss and declining thinking skills. FTD can look very different. It attacks behavior and personality, striking at the qualities people associate with identity itself. In Jean—and later in at least five of her adult children—it was as though a switch had been flipped, resetting their personalities.

FTD is extraordinarily uncommon. A few hundred thousand people may have it worldwide, although the true figure is difficult to establish because the disease is so often missed or misdiagnosed. Bruce Willis has FTD, but it remains unclear whether his case is inherited or sporadic.

Because FTD looks so unlike the dementia most people recognize, it is frequently mistaken for depression, schizophrenia or another psychiatric condition. Jean’s family—whose surname is being withheld for privacy—went years without an accurate diagnosis. Their painful history of missed signs reflects the broader struggle scientists have faced in understanding the many forms dementia can take.

Rare dementias create a striking paradox. Individually, they may seem unusual, but together they could account for more than 40 percent of all dementia cases. Researchers also suspect that some cases categorized as addiction or homelessness may involve undiagnosed dementia.

For doctors and families seeking answers, the priority is recognizing that dramatic behavioral changes may have a neurological cause. Too often, the explanation is assumed to be severe mental illness, menopause or ordinary depression. In some cases, the real cause is dementia.

Christy was the next sibling to change. The second oldest of Jean’s children, she had been one of the family’s standouts: a married mother of two and a senior marketing executive at a pharmaceutical company, traveling internationally and living in an affluent New York suburb.

Then, in her mid-thirties, she abruptly left her job. No one was sure whether she had been laid off or fired.

Christy said she planned to remain at home. But her husband, Don, had begun noticing troubling changes, including a striking lack of attention to their children. Their marriage deteriorated and ended in divorce. Christy became increasingly withdrawn, showing little interest in her children or in anything beyond the television.

At first, the family was willing to believe she was simply going through a difficult period. Serious illness seemed impossible to imagine. Christy had always been capable, and perhaps everyone hoped she would regain her footing and return to her former life.

Within months, her condition worsened. On one visit, Don and his brother found rubbish scattered across the floor. As they moved through the home, throwing things away, Christy barely acknowledged them and continued staring at the television.

She was soon hospitalized and diagnosed first with depression and later with schizophrenia. Medication did little to change the fact that Christy seemed emotionally absent and disconnected from the world around her.

Her brothers and sisters were bewildered. The confident, perceptive older sister they had admired seemed to have vanished, replaced by someone they barely recognized and could hardly reach through conversation.

Despite how unsettling the thought was, they began to see echoes of their mother in Christy’s decline.

Doctors did not diagnose Christy with FTD for several more years. By then, another sister, Mary, had developed similar symptoms and was also initially misdiagnosed.

Mary’s husband, Fred, had reached the end of his patience. She had never formed a strong bond with their two-year-old son, did not work, rarely showered and routinely slept until 11 a.m. The woman Fred had known seemed to be disappearing, with no recognizable personality taking her place.

Fred divorced Mary and sought full custody of their son. When Mary’s relatives argued that she had FTD, he rejected the diagnosis as a tactic for use in court. Her family eventually secured visitation rights, but Mary herself never accepted that she had the disease.

The tendency to dismiss behavioral changes as something other than dementia is so widespread that even scientifically minded members of the family struggled to accept what was unfolding.

Another sister, Peggy, refused to acknowledge her changes. So did their brother Dutch and another sister, Jenny.

By that point, no one could decide which fate was more painful: having the disease or being the only family member untouched by it, forced to watch the others disappear.

The family’s clearest source of hope lies in the genetic precision of its illness.

Researchers are entering an era of gene editing, raising the possibility of removing harmful mutations and eliminating inherited diseases. If scientists can overcome the financial and regulatory barriers involved in developing treatments and create a gene therapy for FTD, this family could benefit directly.

More compelling still, the family’s particular mutation is involved in several other forms of dementia, including Alzheimer’s. Many researchers believe that solving this family’s disease could provide a blueprint—or at least valuable clues—for treating other neurological disorders.

That possibility has become another thought the family carries into the night: their devastating illness could ultimately open a path toward treating the roughly 55 million people living with Alzheimer’s worldwide.

Robert Kolker is an award-winning journalist and the author of The Vanishing Family: Love, Fate, and the Quest to End Dementia, recently published by Quercus.