Steve Silberman is an eclectic and big-hearted polymath: a respected writer on science and culture, a Deadhead, TED head, former restaurant critic, and assistant to poet Allen Ginsberg. His book is a wide-ranging and authoritative history of autism. That history echoes through the lives of the many people affected by autism, and it will resonate with anyone whose experience registers outside narrowing definitions of normal. Admittedly, I am one of those people. A few years ago I took a personality test favored by academic psychologists. I failed it. Apparently I’m more than a little ADD, someone autism advocates might call a “cousin,” our kinship defined by a shared strangeness. My mental wandering led me to work in two disciplines, social neuroscience and evolutionary biology, whose intersection reveals just how varied brains can be. I didn’t find much neurobiology in NeuroTribes, probably because researchers know so little about what makes a brain autistic. There is, however, a moving record of society’s awakening to diversity.

In the days before psychiatry’s Diagnostic and Statistical Manual, the DSM, diagnoses were done by ballot, with a group of doctors voting, for example, on whether a child was schizophrenic or simply “feebleminded.” When compared to the richness of human variety, their categories were few—as though sorting worlds of color into bins of blue and yellow—and doctors might not agree on how to label any particular hue of human experience. In the 1940s, Leo Kanner described autism as a distinct cluster of traits marked by an extreme isolation and adherence to routine. Thirty years later, a group of London researchers led by Lorna Wing, a brilliant physician and the parent of an autistic girl, would draw attention to the overlooked writing of the Austrian clinician Hans Asperger.

Wing and colleagues pushed for finer shades of diagnosis, broadening criteria until they could include the many children who had been misclassified or overlooked. As Silberman recounts with welcome clarity, these changes in diagnosis had consequences. In the late 1960s, the estimated prevalence of autism was roughly 1 in 2,000; in 2010, the Centers for Disease Control and Prevention published the most recent numbers: 1 in 68. This rapid rise was not an epidemic. It was a revision of what it meant to have autism, as new categories like Asperger’s syndrome and the euphonic “pervasive developmental disorder not otherwise specified” took hold.

The shifting boundaries of diagnosis bring NeuroTribes into one of the most contested territories of mental health: What defines a disease? As Silberman puts it, “The fact that [Asperger’s] syndrome shaded into subclinical eccentricity raised a question that cut to the core of the entire psychiatric enterprise: Was Asperger’s syndrome truly a mental disorder or a common personality type in its most extreme form?” A disease is easily mistaken for a physical fact, an enduring burden that can be spotted in an X-ray or on a chromosome. But diseases and disorders are ideas, medicine’s imperfect attempts to bring order to nature’s messy realities. Autism advocates coined the term “neurotypical” to label normalcy as a disorder of social obsession and chatty conformity; this playful inversion is a reminder that sometimes people suffer simply by the numerical accident of rarity. We imagine the healthy mind to be a geological feature whose contours we map to learn its landscape, so we can return if we get lost. Maybe it is more like a disputed land whose borders and flags need negotiation. For its part, the American Psychiatric Association recently stopped naming the varieties of autism. The DSM-5, published in 2013, now recognizes only varying degrees of Autism Spectrum Disorder.