Imagine a genetic disease with the following characteristics. If you’re born into a family that has it, there’s a 50-50 chance you’ll have the following life course. In your 40s, you’ll start acting strangely, only you won’t recognize that it’s strange; everyone else will. You might lose your inhibitions, say mean things, and stop caring for yourself or your home. In time, you may begin isolating yourself, spending your days watching TV and chain-smoking. Eventually, you may lose the ability to speak and become a shell of yourself, needing help with the most basic functions.

This disease has a name: an inherited form of frontotemporal dementia, or FTD.

Robert Kolker, bestselling author of Hidden Valley Road, tells the story of one family with this disease in his new book, The Vanishing Family: Love, Fate, and the Quest to End Dementia. It’s a heartbreaking journey into a poorly understood illness that slowly chips away at our very selves.

I interviewed Robert Kolker about how he came to this family, why some members chose not to learn their genetic fate, and what FTD reveals about the self.

JG: Can you recount how you came across this family?

RK: This family found me. They had read Hidden Valley Road, and one member of the family contacted me and said, “We have a rare disease, and we have nine children in our family.” I had just spent several years researching schizophrenia, which, as you know, has no single known cause, genetic or otherwise. And this family was telling me that they had an incurable disease, but that they knew exactly what the cause was: a genetic cause, a mutation, one gene. And I said, “Aren’t we in the era of genetic editing? Couldn’t CRISPR or some other tool wipe out this problem by lunch tomorrow?” And she said, “That’s what we would like to know.”

Unlike Alzheimer’s, which affects cognition and memory, their illness hits personality first. The people they care most about, they suddenly don’t seem to care much about. And it’s heartbreaking, of course, but it also raises the question: if all it takes to change your personality is one gene that goes awry, what is a personality? Why are we the people we are?

And really, the question that I found most relatable: what is it about any of us that is real and lasting? I know, personally, I’m going to be a very different person at age 80 than I was at age 16. But this kind of drastic change raises the question of what it is about us that makes us us to begin with.

That really got me. It took my breath away, that question, because in short order I found a lot of researchers who are drawn to FTD for that same reason. They think FTD is the place where psychiatry and neurobiology bump up against one another and overlap a little bit, and it’s almost a key to understanding the roots of the self.

JG: You talk about family members who decided not to get tested, and I really found myself judging that. I dabble in medical ethics, and it seemed to me that if I knew there was a 50-50 chance I was going to be completely dependent on my family, I’d have an obligation to prepare as best I could. How did the ones who declined testing justify that to themselves?

RK: When I was starting to work on this, I thought that if I were in this position, I would want all the facts as soon as possible. I thought, “I’m a reporter. I want to know all the information. I want to know what I’m dealing with.” After a while, what started out seeming like rationalization started to feel like authentic, visceral reactions to what was going on in their lives.

There’s one person who had spent her whole childhood watching her mother vanish—disappear before her eyes. And by the time she found out that she could take this test, she was already married, a mother with kids who were becoming teenagers. So it was like three different nightmares all happening at once: she might have it, the kids might have it, and they might pass it down. So she suffers in silence for 10 years, and it takes a toll, until finally she’s ready to learn the truth.

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The one woman who went ahead and had kids, even though she knew she was positive, had tried selective embryo transfer, and it hadn’t worked. When she got pregnant, she decided, “Who’s to say what anyone’s future holds?” From our point of view, maybe she’s rationalizing, but she was choosing radical optimism. She was saying, “Who’s to say there won’t be a cure? Who is anyone to tell me that I’m unethical to have a child based on my genetic makeup? Why is that wrong?”

JG: I was thinking about how mental illness often comes in two kinds. One is just distressing to have, like depression. And others are not so much distressing for the person who has it, but really unpleasant for other people, like antisocial personality disorder. I know that FTD is not a mental illness, but as I was reading your book, I kept thinking it seemed closer to the second kind—that it didn’t seem inherently awful to have, but obviously awful for the caretakers and families watching this person, or losing this person.

RK: You’re right that in most cases the people are not suffering overtly and do not believe that they’re sick. They have anosognosia. The thing that’s most fascinating to me is that as soon as they had a real diagnosis, several members of this family were talking to each other about how they would want to end their lives if they learned that they were starting to change. And none of them have. One by one, many of them have changed, and none of them have opted to harm themselves. So I think you dread it in advance, but once you have it, it’s really more of a tragedy for the people around you.

And then you become enmeshed with this new person and even love them. You see a drastic version of this in The Vanishing Family with Sue caring for Christy for 30 years. This woman could not be more different because of her illness, and her sister loves her anyway and cares for her every day.

JG: You point out that FTD shares the fate of a lot of rare diseases. What would really move the needle treatment-wise?

RK: It’s an exciting moment right now, because there are a lot of researchers and pharmaceutical companies who are interested in the tau protein and how it plays out in lots of dementias, including Alzheimer’s. This family and a lot of FTD families have a tau issue. So they could get a trickle-down effect from all the energy going toward Alzheimer’s drugs.