Why So Many Adults Are Diagnosed Neurodivergent Later in Life

Introduction

If you've recently received a diagnosis of autism, ADHD, or AuDHD as an adult, you might be wondering: how did this go unnoticed for so long? You're not alone in asking that question, and the answer isn't that you're unusual or exceptionally complex. The answer lies in the history of how neurodevelopmental conditions have been understood, researched, and assessed — and in how effective many neurodivergent people are at adapting to a world not designed for them.

This article explores the systemic, cultural, and neurological reasons why so many people don't receive a diagnosis until adulthood, and why that number continues to rise.

What the Research Says

Research consistently shows that autism and ADHD are significantly underdiagnosed in adults. A 2021 study estimated that around 70% of autistic adults in the UK may be undiagnosed. Similarly, a study published in JAMA Psychiatry in 2019 found that ADHD diagnosis rates in adults have increased substantially over the past decade, suggesting not an epidemic but a correction — a catching up on decades of missed diagnoses.

The gender gap in diagnosis is particularly well-documented. Research by Dworzynski and colleagues (2012) found that girls with autism are significantly less likely to receive a diagnosis in childhood than boys, even when presenting with equivalent traits. A 2022 systematic review in Autism Research confirmed that women are diagnosed with ADHD on average several years later than men, often not until adulthood.

Ethnicity also plays a documented role. Research from the United States and UK has found that Black and minority ethnic children and adults are less likely to be identified as autistic or to receive ADHD assessments, reflecting both structural inequalities in healthcare access and cultural biases in diagnostic practice.

Why This Happens

The diagnostic criteria for autism were first formalised in the 1940s, based almost entirely on observations of young boys. The DSM and ICD criteria went through many revisions, but the research base remained skewed for decades. This meant that the clinical picture most assessors were trained to recognise was often male, often white, and often presenting in very specific, overt ways.

Many autistic women and girls, for example, develop highly refined social scripts — learning to observe and imitate social behaviour so closely that their differences are not visible in casual interactions. This is sometimes called the "camouflage" or "masking" effect, and while it is certainly not exclusive to women, research suggests it is particularly pronounced in those who were socialised as girls.

For ADHD, similar biases exist. Hyperactive-impulsive presentations — more commonly associated with boys — were historically the face of the diagnosis. The inattentive presentation, which is more commonly seen in women, was often missed entirely or attributed to anxiety, personality, or character.

Environmental and social factors also play a significant role. Many adults grew up in environments where neurodivergence was not yet a concept in public discourse. Schools and healthcare systems were ill-equipped to identify it, and cultural narratives often attributed struggles to laziness, sensitivity, or moral failing.

How This Shows Up in Real Life

Many late-diagnosed adults describe spending decades feeling as though they were failing at something everyone else found easy — socialising, organising, regulating emotions, maintaining relationships, holding down jobs. They may have received a string of other diagnoses along the way: anxiety, depression, borderline personality disorder, chronic fatigue. These may have been genuine co-occurring conditions, but they were often also symptoms of an underlying neurodevelopmental difference that was never identified.

Some adults reach a tipping point — a burnout, a crisis, or a conversation with someone who suggests the possibility — and seek assessment for the first time. Others stumble onto the answer through reading, online communities, or a loved one's own diagnosis.

For many, the diagnostic process itself reveals how much they had come to doubt their own experiences. Years of being told they were fine, or that they were exaggerating, or that they simply needed to try harder, can make it genuinely difficult to trust your own perception of your struggles.

Practical Takeaways

If you are pursuing a diagnosis, know that advocating for yourself is often necessary, and that is not your fault — it is a reflection of the systems you're navigating.

Keeping a record of your experiences — specific examples of challenges across your life, in different contexts — can be genuinely useful in an assessment. You don't need to perform your neurodivergence or catastrophise; just describe your authentic experience.

If you have been dismissed before, consider seeking an assessor who specialises in adult neurodivergent presentations, particularly one with experience working with women, non-binary people, or people from minority communities.

Remember that a late diagnosis, however it comes, is never too late. Understanding yourself better at any point in life creates possibilities for change, for compassion, and for finding ways of living that genuinely work for you.