Where lies the divide between an illness in the brain and an illness in the mind? Is there a divide? As famed Stanford psychology professor David Rosenhan asked, "If sanity and insanity exist, how shall we know them?"

These questions anchor Susannah Cahalan's The Great Pretender: The Undercover Mission That Changed Our Understanding of Madness, and the stakes involved in answering these questions are high. As Cahalan puts it, the ability to answer has wide-reaching effects: "from how we medicate, treat, insure, and hospitalize, to how we police and whom we choose to imprison."

Cahalan's investment in the topic is personal. She recounted the story of her own experience with mental illness in her 2013 runaway bestseller Brain on Fire, which Netflix adapted into a film the same year. At the age of 24, she found herself hospitalized with a disease whose symptoms aligned alarmingly with schizophrenia.

"Diseases like the one that set my brain 'on fire' in 2009 are called 'the great pretenders,'" she explains, "because they bridge medical worlds: their symptoms mimic the behaviors of psychiatric illnesses, but have known physical causes, autoimmune reactions, infection, or some other detectable interferences in the body." Cahalan has since traveled widely, telling her story to audiences across the world. But even she found herself slipping into a mode of thinking that distinguished the psychiatric from the physical--and she wanted to better understand why. Where Brain on Fire delved into Cahalan's experience of being a patient herself, The Great Pretender probes the systems that shape a patient's experience, exploring historical and contemporary frameworks for understanding them.

Cahalan starts with a whirlwind history of mental healthcare in the United States, highlighting moments of innovation and infamy and sketching its most controversial patrons and purveyors. She traces Nellie Bly's famed 1887 exposé on the women's asylum on Blackwell's Island; the impacts of activist Dorothea Dix in the mid-1800s; and the tragedy of Rosemary Kennedy and others whose families' approach to "treatment" ranged from ethically ambiguous to outright horrific. A sampling: Charles Dickens, trying to commit his wife, Catherine, so that he could have an affair with another woman; Sir Edward Bulwer-Lytton--of "It was a dark and stormy night" fame--similarly trying to have his inconvenient wife committed; 20% of the mentally ill in Switzerland kept restrained in their homes; a man chained by his neck to a wall in a London hospital for 14 years, unable to move more than a foot.

What makes many of the stories so shocking is the uncertainty in whether those committed to asylums in any way "belong" there. In the midst of her research, Cahalan learns of a study from the 1970s that gets at the very same issues: Dr. David Rosenhan's landmark 1973 report "On Being Sane in Insane Places." It was a study that had an explosive impact on the field of psychiatry. Rosenhan's starting point, in his own words, was the question "How many people, one wonders, are sane but not recognized as such in our psychiatric institutions?" As Nellie Bly had done almost a century before, Rosenhan decided the best way to investigate would be to get himself admitted to a mental hospital as a patient--and then to behave as he normally would. Would anyone notice?

To bolster his study, Rosenhan recruited others as well. In all, eight "pseudopatients" spent time in asylums across the United States, documenting their experiences with exhaustive notes. To get themselves admitted, all claimed to be hearing voices that repeated the words, "thud," "empty" and "hollow." All were committed. All were diagnosed. All were medicated. All, of course, weren't actually sick.

In the era of One Flew Over the Cuckoo's Nest, Rosenhan's experiment flew in the face of the cult of psychiatry. The renowned journal Science published the article. The enormity of its impact is hard to overstate. People reconsidered how they viewed psychiatry. Psychiatrists reconsidered how they saw themselves. The study led to a complete overhaul (one of several) of the Diagnostic and Statistical Manual of Mental Disorders (DSM). More than four decades later, Rosenhan's experiment is still widely taught in introductory psychology courses.

Rosenhan died in 2012. To learn more about him, Cahalan visits his longtime Stanford colleague and friend, Lee Ross. Rosenhan had entrusted many of his files to Ross, who shares them (with the notable exception of one file Ross deemed "personal") with Cahalan. She pores through Rosenhan's notes. And then she gets a bit concerned. As Cahalan pieces together the story of what may really have happened in the study--or, perhaps more accurately, what may not have really happened--inconsistencies arise. Then inaccuracies. Then outright lies.

What results is a fascinating, nuanced and engrossing journey to better understand the study that led so many in a field to question whether it really understood itself. Cahalan researched The Great Pretender over the course of five years, but the pages practically turn themselves. It's absorbing, sometimes sobering, sometimes seriously funny. Cahalan's narration makes the reading great fun, with an urgency occasionally akin to a thriller.

At its core, The Great Pretender is a multifaceted portrait of a study that, no matter its complications, fundamentally shook the foundations of psychiatry. Cahalan invites us to reconsider how many fallacies about the mind and brain we may all still simply be taking for granted. She raises more questions than answers, but along the way Cahalan helps us learn how to ask better questions about what madness is, how we should name it and how we might better care for those it afflicts. --Katie Weed

photo: Shannon Taggart

Journalist Susannah Cahalan (Brain on Fire) returns with The Great Pretender: The Undercover Mission That Changed Our Understanding of Madness (Grand Central Publishing), further unpacking how we (mis)understand mental health and why it matters. Cahalan is an award-winning journalist and author who lives in Brooklyn with her family.

As you yourself ask: where is the divide between brain illness and mental illness, and why do we even try to differentiate between them at all?

I think we try to differentiate out of fear. There's fear of the mentally ill "other"; there's fear that we ourselves may not be as "sane" as we believe; there's fear of the unknown, or what causes mental illness. I think it comes down to this fact: none of us are immune.

Naming is so important. As diagnostic terminology grows, you write, psychiatrists can "provide a name for their patients' suffering, something I personally would argue is one of the most important things a doctor can do, even if a cure isn't in sight." What is so powerful, or empowering, about a name?

I experienced this with my diagnosis of autoimmune encephalitis. When my family got a name for my suffering--even when it was unclear if I would recover--they immediately felt a sense of relief. Names give us clarity and focus. Without them, we feel lost. Unnamed illnesses remind us how little we know, and that's the most frightening concept of all.

On the flip side, you also acknowledge the danger of labels.

It's so hard. We need the labels because they not only provide comfort, but help decide what if any treatment is necessary, prognosis, etc. Still, psychiatric labels, I found, are less objective than we like to believe and sometimes this leads us down paths that may not be right for us. I think clinicians can combat this by being open and honest about the limitations of these labels--that one person with one label might not have the same experience, outcome, etc. as another person with the same label.

Your note is so honest about preparing to speak to a crowd of psychiatrists: "When I had packed for the trip I made sure to bring my most adult, sophisticated, not crazy ensemble." Do you still find yourself dressing or comporting yourself in certain ways to project to others that you're "not crazy"?

Of course! I wrote a memoir about having experienced hallucinations and paranoid delusions and having exhibited violent, aggressive behavior, so I am eternally aware of how I come off to a crowd of strangers. This was especially true when Brain on Fire was first published. I felt I had to prove myself constantly and was certain that people were looking at me and asking themselves, "Is she really recovered?" This likely came out of insecurity, but it was my reality for a while. I believe this impulse is starting to self-correct, hopefully.

You deem Robert Spitzer's critique of Rosenhan's work "delicious in its biting bitchiness... the drollest piece of academic literature I've ever read." What kinds of humor have you encountered in the realm of mental health?

I'm so glad you picked up on the humor! I just love that paper (written by Rosenhan's rival and DSM-godfather Robert Spitzer). There is so much levity and wit in Rosenhan's writing, and he loved to engage with people with similar sensibilities. I laughed a lot while researching this book, considering how dark some of the subject matter was.

Your writing is consistently accessible and entertaining. A favorite line: "The impossibility of distinguishing sanity from insanity ha(s) received the most mainstream of honors--its own reality show." Speaking of reality shows and entertainment, how do you feel about current representations of mental illness in popular culture? What was the experience like of having your own story adapted into a film by Netflix?

First off, thank you so much for your comment about my writing. It was important to me to make it as enjoyable and exciting as possible, while also diving into difficult topics.

In terms of my views on the current representations of mental illness in popular culture, I'll just say it straight: most of it is total rubbish. People still don't really understand what schizophrenia is or the range of behaviors and symptoms associated with it. They think of people in horror movies with butcher knives stalking people. That's not serious mental illness, and it's infuriating to see it portrayed that way. I wrote a piece about my experience watching my own story be adapted, but I think it can be summed up in one word: surreal.

The argument that the Stanford psychologist whose work grounds this book is perhaps a "great pretender" himself is fascinating. Is a movie on him next?

I would love to see David Rosenhan on screen as the Great Pretender! There's nothing in the works just yet, but I think he is such a compelling unreliable narrator and you really could have some fun with him.

What do you hope people will do after reading your book? How do you hope they might think differently about mental health and its treatment?

There are so many specific things I hope this book will do--better understanding of the limitations of medicine, the role of context in diagnosis, the (often unfair) distinctions between "mental" and "physical" illnesses--but in general I hope it sparks a conversation that people might not have been inclined to take on before reading the book. There's a lot to debate in it, and I'm sure many people will not agree with this or that, but that's the goal, to get people talking about these issues.

Do you have a stack of books to read for pleasure outside of your research, or do those lines blur?

Well, I just had twins (they're four months as of this writing), so I've been a bit derelict with my reading. I can tell you, though, that there are some remarkable books coming down the pipeline (or already here) that I was fortunate to have read and blurbed before the twins turned my life upside down, among them: Karen Rinaldi's It’s Great to Suck at Something (which totally gave me the confidence to try running again with a post-pregnancy body), American Predator by Maureen Callahan (the scariest book I've ever read), The Ghosts of Eden Park by Karen Abbott (a sexy bootlegging murder mystery mixed with The Great Gatsby) and Ada Calhoun's Why We Can't Sleep (a deeply reassuring look at the new midlife crisis that women face).

And in terms of those lines blurring, yes! It was so fun to reread One Flew Over the Cuckoo's Nest, Erica Jong's Fear of Flying, Joan Didion's The White Album and Tom Wolfe's essays and Susana Kaysen's Girl, Interrupted. I had so much fun immersing myself in the work of that era.

What's next for you?

I've worked on this book for five years. Next up? A massage.

Finally, if sanity and insanity exist, how shall we know them?

Oh, to answer this one in a short paragraph--an impossible task! --Katie Weed