ADHD (attention deficit hyperactivity disorder) is a neurodevelopmental condition that affects how the brain regulates attention, impulse control and activity levels. It's present from childhood, though it's often diagnosed later, and it affects both children and adults.
The three presentations of ADHD
Clinically, ADHD is described in three presentations, depending on which symptoms are most prominent:
- Predominantly inattentive presentation — difficulty sustaining attention, organising tasks, following instructions or details, and a tendency to lose things or be easily distracted. Historically sometimes called "ADD."
- Predominantly hyperactive-impulsive presentation — restlessness, difficulty staying seated or still, excessive talking, interrupting others, and acting without thinking through consequences.
- Combined presentation — a mix of both inattentive and hyperactive-impulsive symptoms, which is the most commonly diagnosed presentation.
Presentation can also change over time — hyperactivity, for example, is often more visible in childhood and can become less overt (though not necessarily less present) in adulthood, sometimes shifting toward inner restlessness rather than visible fidgeting.
What causes ADHD
ADHD doesn't have one single cause. The current understanding, based on UK and international clinical research, points to a combination of factors:
- Genetics — ADHD has one of the strongest genetic links of any neurodevelopmental condition; it often runs in families.
- Brain development and function — differences in the structure and activity of brain regions involved in attention and self-regulation, and in neurotransmitters like dopamine and noradrenaline, are consistently found in ADHD research.
- Prenatal and early-life factors — certain factors during pregnancy and early childhood are associated with higher likelihood of ADHD, though none of these alone explain most cases.
Importantly, ADHD is not caused by parenting style, too much screen time, or diet alone, though these factors can affect how symptoms present day-to-day.
ADHD is a spectrum, not a fixed pattern
ADHD affects people differently. Two people with the same diagnosis can look very different day-to-day — one might struggle mostly with organisation and time management, another with impulsivity and restlessness. Severity also varies, from mild difficulties that are manageable with strategies alone, to symptoms significant enough to affect work, relationships and daily functioning without treatment.
How common is ADHD?
ADHD is one of the more commonly diagnosed neurodevelopmental conditions, affecting a meaningful proportion of children, with a substantial number continuing to have symptoms into adulthood. Exact prevalence figures vary between studies and diagnostic criteria used, so we don't quote a single fixed statistic here — but the broad clinical consensus is that ADHD is common enough that most schools, GP practices and workplaces will have encountered it, even where it isn't always recognised or diagnosed.
ADHD alongside other conditions
ADHD frequently occurs alongside other conditions, including anxiety, depression, learning differences such as dyslexia, and autism. This is one of the reasons a proper specialist assessment matters — it's not just about confirming ADHD itself, but understanding the full picture, since co-occurring conditions can affect symptoms, and sometimes treatment choices, in either direction.
Commonly confused terms
A few distinctions come up often enough to be worth clarifying directly:
- ADHD vs ADD — ADD was the earlier term for what's now recognised as ADHD's predominantly inattentive presentation. It isn't a separate condition; "ADHD" is the current clinical umbrella term for all three presentations.
- ADHD vs autism — the two are distinct conditions with different diagnostic criteria, though they can and do co-occur, and some traits (difficulty with transitions, sensory sensitivity, social differences) can overlap in how they present day to day.
- ADHD vs anxiety — anxiety can cause difficulty concentrating, restlessness and irritability that superficially resemble ADHD, and the two conditions frequently occur together, which is exactly why a structured assessment — not a symptom checklist alone — is needed to tell them apart.
- "Everyone's a bit ADHD" — occasional forgetfulness or distraction is a normal, universal human experience, not ADHD. What separates a clinical diagnosis is the persistence, severity and cross-situational impact of symptoms, assessed formally rather than self-identified.
Getting a formal understanding of your own symptoms
Reading about ADHD online is a reasonable starting point, but it isn't a diagnosis. If what you've read here sounds familiar, the next steps are covered in our guides on ADHD symptoms in more detail, and on how UK ADHD diagnosis actually works.