IStock Photo 9368051 © Kim Gunkel

“When you take a drug to treat high blood pressure or diabetes, you have an objective test to measure blood pressure and the amount of sugar in the blood. It is straightforward,” writes Temple Grandin. “With autism, you are looking for changes in behavior.”

Though she’s a PhD and a professor at Colorado State, Grandin is not a professional autism expert—in fact, her field is animal behavior. Rather, she’s described by the Autism Society as “inarguably the most accomplished and well-known adult with autism in the world,” and she’s the author of books on both animal behavior— Animals Make Us Human, Animals in Translation—and autism: Thinking in Pictures: and Other Reports from My Life With Autism.

As Grandin has recognized, diagnosing and even defining autism is challenging. Though parents whose children are severely autistic never argue about the syndrome, others say that the spectrum of the disorder is so wide that proper diagnosis is a puzzle with too many pieces. No wonder the prevalence, causes, and treatments of autism are so hotly debated.

A look at the numbers shows how variable the diagnosis can be. The odds that a person between 3 and 22 years old has autism are 1 in 273.9, but the odds someone the same age has an Autism Spectrum Disorder (ASD) are higher—one in 90.9 for all children, and one in 57.8 for boys. Additionally, white and Hispanic children are about twice as likely to have an ASD as black and multiracial children. Autism Spectrum Disorders cover many different medical and social problems, from Asperger syndrome to severe developmental disorders. The criteria for ASD have recently been expanded, and now the diagnosis includes many highly functioning individuals who might not have been identified as autistic in the past. The most recent statistical information came from a phone survey of parents, leading some researchers to question if the high numbers came from worry or fact.

While prevalence means the number of people who have autism, incidence counts the number of people who are diagnosed each year. The odds a 3-to-22-year-old was diagnosed with autism in 2003 are 1 in 503. In 2002, the odds were 1 in 585.9, and in 2000 they were 1 in 848.3. While these numbers represent an increase, researchers don’t know if the rise is caused by broader definitions for autism, better ways to diagnose the disorder, or a real escalation in cases of autism.

At some point, the issues around diagnosis will likely be resolved, and in the process, treatment options may become clearer, too. Other mysterious illnesses have had similar issues around diagnosis—for example, Chronic Fatigue Syndrome was often misunderstood until a recent discovery made the case that a rare retrovirus was the cause. Until the biological mechanisms of the disorder are fully understood, it will be puzzles upon puzzles for people with autism.