I STARTED READING every news story about autism I could find and downloading journal articles by the score. It soon became clear that the mysterious rise in diagnoses was not restricted to Silicon Valley. The same thing was happening all over the world.

To put the rising numbers in context, I familiarized myself with the basic time line of autism history, learning the story of how this baffling condition was first discovered in 1943 by a child psychiatrist named Leo Kanner, who noticed that eleven of his young patients seemed to inhabit private worlds, ignoring the people around them. They could amuse themselves for hours with little rituals like spinning pot lids on the floor, but they were panicked by the smallest changes in their environments, such as a chair or favorite toy being moved from its usual place without their knowledge. Some of these children were unable to speak, while others only repeated things they heard said around them or spoke of themselves detachedly in the third person. Claiming that their condition differed “markedly and uniquely” from anything previously reported in the clinical literature, Kanner named their condition autism—from the Greek word for self, autos—because they seemed happiest in isolation.

Then a year later, in an apparent synchronicity, a Viennese clinician named Hans Asperger discovered four young patients of his own who seemed strangely out of touch with other people, including their own parents. Unlike Kanner’s young patients in Baltimore, these children spoke in elaborate flowery sentences while displaying precocious abilities in science and math. Asperger affectionately dubbed them his “little professors.” He also called their condition autism, though it’s still a matter of dispute if what he saw in his clinic was the same syndrome that Kanner described.

For decades, estimates of the prevalence of autism had remained stable at just four or five children in ten thousand. But that number had started to snowball in the 1980s and 1990s, raising the frightening possibility that a generation of children was in the grips of an epidemic of unknown origin. After telling my editor about the frightening thing that the teacher in the café said about what was happening in Silicon Valley—the heart of Wired’s tech-savvy readership—I got permission to pursue this intriguing lead.

My research was facilitated by the fact that our apartment in San Francisco is located just down the hill from the University of California, which boasts one of the best medical libraries in the country. I became a regular browser in the stacks, poring through articles on epidemiology, pediatrics, psychology, genetics, toxicology, and other relevant subjects. Meanwhile, my shelves at home filled up with books like Clara Claiborne Park’s The Siege, Oliver Sacks’s An Anthropologist on Mars, and Temple Grandin’s Thinking in Pictures. Each offered a view of the diverse world of autism from a unique vantage point.

The Siege, published in 1967, was the first book-length account of raising an autistic child by a loving and devoted parent. In a dark age when psychiatrists falsely blamed “refrigerator mothers” for causing their children’s autism by providing them with inadequate nurturing, Park offered a candid portrait of life with her young daughter Jessy (called Elly in the book), who would sit by herself for hours, sifting sand through her fingers. With the meticulous eye of an explorer mapping uncharted territory, Park chronicled each small thing that Jessy learned to do in her first...