The vast majority of autistic adults have never been formally diagnosed. A 2021 review from King’s College London estimated that 89 to 97 percent of autistic adults over age 40 in the UK remain undiagnosed. While no equivalent figure exists for the United States, the CDC estimates that 2.21% of American adults have autism, a number that translates to millions of people, most of whom grew up in an era when the condition was far more narrowly defined.
The Scale of the Gap
That 2.21% CDC estimate, drawn from 2017 data, puts the total number of autistic adults in the U.S. well into the millions. California alone accounts for an estimated 701,669, followed by Texas (449,631), New York (342,280), and Florida (329,131). These figures include everyone on the spectrum, whether diagnosed or not. When you pair them with the King’s College London finding that roughly nine out of ten autistic adults over 40 have never received a diagnosis, the undiagnosed population is enormous.
The King’s College researchers re-analyzed UK healthcare records from 2018 and found that 89% of autistic people aged 40 to 59 and 97% of those 60 and older had no formal diagnosis. Younger adults fare somewhat better because childhood screening has improved over the past two decades, but a significant gap persists across every age group.
Why So Many Adults Were Missed
Before 2013, the diagnostic manual used by clinicians in the U.S. split autism into several separate categories, including Asperger’s syndrome and a vague catch-all called “pervasive developmental disorder not otherwise specified.” Many people, particularly those who could speak fluently and attend mainstream schools, didn’t fit neatly into these older categories. The updated manual (DSM-5) consolidated everything under one umbrella, autism spectrum disorder, and critically, it now allows clinicians to diagnose someone based on both current and past functioning. That means adults who showed subtle signs in childhood but didn’t hit a clinical threshold until later in life can now qualify.
But changes on paper don’t automatically translate into diagnoses. The practical barriers are steep. A survey of U.S. autism diagnostic centers by the Centers for Medicare and Medicaid Services found that nearly two-thirds of centers had wait times longer than four months. Fourteen percent reported waits exceeding a full year, and some had stopped accepting new referrals entirely. Evaluations themselves range from a quick one to two hours at some centers to more than eight hours at others, and 84% of surveyed centers accept private, out-of-pocket payment, suggesting that insurance coverage remains inconsistent.
How Masking Hides Autism in Plain Sight
Many autistic adults have spent decades developing strategies to appear non-autistic in social settings, a phenomenon researchers call camouflaging or masking. This includes suppressing repetitive hand movements, forcing eye contact, rehearsing conversational scripts, and using memorized rules to interpret other people’s body language and facial expressions. These strategies are often learned through years of careful observation and imitation, sometimes consciously, sometimes not.
Masking can be effective enough to fool clinicians. A person who has spent 30 years learning to perform neurotypical social behavior may not look autistic during a brief clinical evaluation, even though the effort behind that performance is exhausting and unsustainable. This is one reason the standard screening tools used for adults have limited accuracy. A study examining three of the most common self-report questionnaires found that the best-performing one correctly identified autistic individuals only 73% of the time, and overall diagnostic accuracy across all three hovered between 61% and 68%.
Women and the Diagnosis Gap
Clinical records have long suggested that autism is roughly four times more common in males than females. Population-based screening studies, which look at entire communities rather than just people who show up at clinics, tell a different story: the true ratio is closer to 3.25 males for every one female. That gap between clinical records and population data represents a large number of women and girls who are autistic but never identified.
Several factors drive this disparity. Girls and women tend to mask more intensively and from a younger age, making their social difficulties less visible to teachers, parents, and clinicians. Research shows that cognitively capable autistic women are diagnosed significantly later than men, even when their parents raised concerns at the same age and sought the same number of professional referrals. In other words, the delay isn’t because women’s symptoms are milder. It’s because the diagnostic process, built largely around how autism presents in boys, systematically overlooks them.
What Living Undiagnosed Looks Like
For many adults, the absence of a diagnosis doesn’t mean the absence of difficulty. Undiagnosed autistic people frequently receive other diagnoses first: anxiety, depression, ADHD, personality disorders, or chronic fatigue. These conditions can genuinely co-occur with autism, but when the underlying autism goes unrecognized, treatments often miss the mark. Someone prescribed social skills therapy for generalized anxiety, for instance, may struggle because the root issue is sensory overload or the sheer cognitive effort of masking, not a lack of social motivation.
A late diagnosis, even in middle age or beyond, often brings relief rather than distress. It reframes a lifetime of feeling different into something concrete and understandable. It also opens doors to accommodations at work, targeted support strategies, and community with other autistic adults. The challenge is getting to that diagnosis in the first place, given long wait times, inconsistent insurance coverage, and screening tools that miss a substantial portion of autistic adults, particularly women and older individuals who have refined their masking over decades.