Adult ADHD: Causes, Symptoms, Presentations and Types

What is ADHD?

Attention Deficit Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder characterised by three core symptoms:

Inattention: Difficulty sustaining focus or following through on tasks.
Hyperactivity: Restlessness or excessive movement that may seem inappropriate in certain settings.
Impulsivity: Acting quickly without considering consequences or struggling with self-control.

Once thought to be a disorder that children “outgrew”, we now understand that ADHD is a chronic condition that often persists into adulthood, affecting various aspects of a person’s life—including academic and professional performance, interpersonal relationships, and everyday functioning.

For adults with ADHD, everyday tasks others take for granted—staying on top of emails, remembering appointments, or completing chores—can require significant effort. Yet many also experience bursts of creativity, intuitive and lateral thinking, and periods of intense hyperfocus that, when channelled effectively, can lead to exceptional outcomes.

At Kantoko, we aim to provide an accessible and clinically grounded understanding of adult ADHD— to offer clarity, reduce stigma and support better outcomes. Whether you’ve been recently diagnosed, are exploring the possibility that you might have ADHD, or are supporting someone in your life who does, this guide provides evidence-based insight into the complexities while honouring the diverse experiences of adults living with ADHD.

This comprehensive guide explores the science of adult ADHD, organised into four key areas:

Neurobiological basis of ADHD
Causes and Risk Factors
Types of ADHD
Symptoms and Presentations

Neurobiological Factors: Differences in Brain Development

Neuroimaging has advanced our understanding of ADHD by revealing differences in both brain structure and function—particularly in areas involved in attention, self-regulation, and executive function. These studies consistently show that individuals with ADHD have distinct patterns of brain development and activation compared to neurotypical individuals.

1. Brain Volume and Regional Differences

Some people with ADHD tend to have slightly smaller total brain volumes, along with structural differences in regions critical for behaviour regulation. Affected areas include:

Prefrontal cortex (attention, emotional regulation, planning, decision-making)
Basal ganglia, especially the caudate nucleus (motor control and reward)
Cerebellum (coordination, timing and higher cognitive processes)

2. Delayed Cortical Maturation

A key discovery in ADHD neurobiology is that cortical maturation—the process of the brain’s outer layer reaching its full thickness— often lags several years behind that of neurotypical peers, occurring several years later in children with ADHD.

This delay is most pronounced in the prefrontal cortex, which supports attention, self-control, and working memory. While most children reach peak cortical development by age 7–8, children with ADHD may reach this stage significantly later.

Crucially, this reflects a delay—not a deviation—in brain development. In some individuals, as the brain matures, ADHD symptoms may reduce or change in presentation. However, for many, symptoms continue into adolescence and adulthood.

3. Neurochemical Differences

These structural and developmental patterns are closely tied to how the brain regulates two key neurotransmitters: dopamine and norepinephrine.

Dopamine helps with motivation, attention, reward processing, and learning.
Norepinephrine supports alertness and the ability to sustain focus.

In ADHD, these systems often function less efficiently. Dysregulation in dopamine and norepinephrine systems contributes to difficulty with attention, impulse control, and motivation—especially in low-stimulation environments.

Together, these findings support the view that ADHD has a clear biological basis rooted in brain development and chemistry. While brain development is dynamic and individual, and no brain scan can diagnose ADHD on its own, these group-level patterns help explain many of the condition’s hallmark symptoms and reinforce that ADHD is not a result of laziness, personality or willpower.

Causes and Risk Factors:

ADHD doesn't have a singular cause. Rather, it arises from a complex interplay of genetic, environmental, and psychosocial factors. Importantly, research consistently suggests ADHD as a highly heritable condition with a significant environmental influence—meaning that both biology and life experiences contribute to its development and expression.

Genetic Factors: It Runs in Families

ADHD tends to run in families. This is evidenced by family and twin studies illustrating heritability estimates around 70–80%. Adoption studies further bolster this link revealing that biological parents of children with ADHD were more likely to have the condition than adoptive parents.

But there’s no single "ADHD gene". Instead, many different genes each contribute a small increase in risk, making ADHD a polygenic (multi-gene) condition. Some identified gene variants are associated with dopamine regulation and brain development, but their effects are modest. Rare genetic variants (such as certain chromosomal micro-deletions or copy number variants), absent of other genetic variants, can also play a role in a minority of cases.

Still, genes aren’t the whole story. The fact that identical twins don’t always both have ADHD shows that environment matters too. While genetic vulnerability plays a key role, it’s expression is influenced by gene-environment interactions and possible epigenetic mechanisms—biological processes that influence how genes are expressed.

Environmental Factors: Influences on Brain Development

While ADHD is highly heritable, environmental factors—especially those that affect brain development early in life—also contribute to risk. These include influences before birth (prenatal), around the time of birth (perinatal), and during early childhood (postnatal).

1. Prenatal Factors

Conditions during pregnancy can impact fetal brain development and increase the likelihood of ADHD. For example:

Prenatal alcohol exposure is linked to structural brain changes, especially in the cerebellum, and is associated with hyperactivity, impulsivity, and increased risk of psychiatric issues.
Maternal smoking during pregnancy has been linked to a higher risk of ADHD, with some studies showing up to a 2.7-fold increase. This may be due to nicotine’s effects on receptors involved in dopamine regulation—a system strongly implicated in ADHD.

2. Perinatal Factors

Complications around the time of birth also play a role. Research shows:

Premature birth and very low birth weight are associated with roughly double the risk of developing ADHD.
Other birth complications, such as poor oxygen supply during delivery, may also affect brain development

3. Postnatal Factors

Early-life exposures and conditions can further influence risk:

Toxic lead exposure in early childhood is a well-established risk factor for ADHD, with studies consistently linking it to attention problems and impaired brain development.
Severe brain injuries, such as trauma or stroke affecting areas responsible for self-control, can result in ADHD-like symptoms.
Nutritional deficiencies, including iron deficiency, omega-3 fatty acid imbalance and malnutrition, have been proposed as contributing factors, though more evidence is needed.
Early social deprivation or neglect may also impact the development of brain circuits involved in emotional regulation and attention.

Despite popular belief, research does not support that sugar intake, and screen time cause ADHD. However, children and adults with ADHD may be more likely to engage in these behaviours as they provide immediate stimulation and reward. While these may exacerbate symptoms in some individuals who already have ADHD, they do not cause ADHD.

Psychosocial Factors: Shaping the Expression of ADHD

Psychosocial factors—the influences of one’s social and family environment do not cause ADHD, but can influence symptom severity and how ADHD manifests. In essence, adverse psychosocial environments in childhood may “turn up the volume” on ADHD symptoms in those genetically vulnerable.

Children who grow up with chronic stress or significant early-life adversity may experience more intense ADHD symptoms. For example, factors like:

Maternal depression
Family conflict or disorganisation
Inconsistent, neglectful or negative parenting
Extreme poverty or low parental education

have been associated with worsened inattention, impulsivity, and emotional regulation in children with ADHD. However, these experiences are NOT known to directly cause ADHD, and current research suggests that in many cases, difficult social relationships—such as peer rejection or parent-child conflict—may be a consequence of ADHD symptoms, not the root cause.

Moreover, psychosocial stressors may shape the overall impact of ADHD by increasing the likelihood of co-occurring conditions like anxiety, depression, or oppositional behaviours. They can also influence how much ADHD interferes with daily life—especially in the absence of support or intervention.

On the flip side, supportive environments can make a significant difference. Predictable routines, consistent caregiving, emotional support, and access to resources like therapy or academic help can reduce the functional impact of ADHD and improve quality of life.

Types of ADHD: Different Presentations, Same Condition

Clinically, ADHD is categorised by presentation types, based on which symptoms are most prominent. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) defines three primary ADHD presentations:

1. Predominantly Inattentive Presentation

This type is characterised by inattention-related symptoms such as disorganisation, forgetfulness, distractibility, and difficulty following through. It involves minimal hyperactive or impulsive behaviour and is sometimes informally referred to as “ADD,” though that term is now outdated.

People with this presentation may appear calm or daydreamy rather than disruptive, which means it often goes unnoticed—especially in girls and women. Diagnosis tends to happen later in life, as the symptoms are less likely to draw external attention but still cause significant functional impairments.

2. Predominantly Hyperactive-Impulsive Presentation

This type involves major issues with hyperactivity and impulsivity, with fewer signs of inattention. Individuals may be fidgety, talk excessively, interrupt frequently, or feel constantly on the go—like they’re "driven by a motor."

This presentation is often identified early in childhood, especially in boys, because the behaviours are more outwardly visible. However, it's less common in adults, as some hyperactive traits may diminish over time or shift into internal restlessness, while inattentive symptoms may become more prominent.

3. Combined Presentation

This is the most common form, where an individual has significant symptoms of both inattention and hyperactivity/impulsivity. To qualify for the combined presentation, adults must meet the symptom threshold in both categories (at least five of each).

In adults, combined-type ADHD often manifests as someone who is might be disorganised, easily distracted, impulsive, and restless all at once. Many adults with longstanding ADHD fall in this combined category, even if one set of symptoms (say inattention) is slightly more dominant.

Diagnostic Criteria Updates: Evolving Understanding

DSM-5 made a few important updates to better capture ADHD in adults. First, the age by which symptoms must have started was raised from before 7 years (DSM-IV) to before 12 years in DSM-5. This change was based on evidence that many people with genuine ADHD weren't identified that early, yet still clearly had childhood-onset attention problems by early adolescence.

Second, DSM-5 allows a lower symptom threshold for older adolescents and adults: if you're 17 or older, you need only 5 symptoms from either domain to qualify, instead of the 6 required for younger children. This adjustment reflects how some symptoms diminish with age and was meant to reduce under-diagnosis of adult ADHD.

Additionally, DSM-5 clarified that ADHD can co-occur with autism spectrum disorder (it removed ASD as an exclusion), and it introduced specifiers for severity and partial remission. These refinements have made it easier for adults who genuinely have ADHD to be diagnosed, even if their presentation is somewhat different from the classic hyperactive child presentation.

Adult-onset ADHD?

An interesting and controversial question is whether ADHD can first appear in adulthood. By definition, ADHD is developmental and should start early childhood. However, some studies in recent years reported cases of people who seemed to acquire ADHD-like symptoms for the first time in adulthood (sometimes called "late-onset" or "adult-onset" ADHD).

This sparked debate, because it challenges the requirement of childhood onset. Upon closer examination, many clinicians suspect that most of these adult-onset cases were actually missed ADHD in childhood (symptoms were mild or masked), or are misdiagnoses – for example, concentration problems due to anxiety, depression, substance abuse, or other factors being mistaken for ADHD.

A comprehensive 2021 analysis found no solid evidence for new-onset ADHD in adulthood independent of a prior history or other psychiatric issues. In practice, current diagnostic guidelines still require that some symptoms were present by age 12. So while adults can certainly be diagnosed later in life, this is usually recognising a longstanding condition rather than a truly new disorder.

Symptoms and Presentations in Adulthood: How ADHD Actually Shows Up

The core symptoms of ADHD fall into two broad domains: inattention (difficulty focusing, disorganisation, forgetfulness) and hyperactivity/impulsivity (excessive restlessness, fidgeting, acting without thinking). Adults with ADHD typically exhibit patterns of these symptoms that have persisted since childhood – but the way symptoms appear can change with age. Below we describe common adult ADHD symptoms and how they differ from childhood presentations.

Inattention in Adults: When Focus Is Elusive

Many adults with ADHD struggle with sustaining attention, especially for mundane or lengthy tasks. They may have trouble concentrating on work duties, following through on complex projects, or sticking with tasks that aren't immediately interesting.

Disorganisation is a hallmark – for example, misplacing important items, forgetting appointments or deadlines, and feeling overwhelmed by planning responsibilities are frequent complaints. An adult with ADHD might procrastinate or jump between unfinished tasks, finding it hard to prioritise and complete projects.

These issues echo the inattentive symptoms seen in children (such as careless mistakes, not listening, or losing homework), but in adulthood the impacts are seen in contexts like job performance and home management rather than school.

One key point is that inattention in ADHD is not due to lack of intelligence or care; it's an inherent neurobiological difficulty in maintaining focus and executive control. Adults often report that their mind feels "scattered" or prone to drifting off, even when they want to concentrate. They may also experience "hyperfocus" at times – becoming intensely absorbed in something fascinating – but struggle to focus on routine tasks, which can be confusing to both the individual and others.

Hyperactivity/Impulsivity in Adults: The Internal Motor

Hyperactivity in ADHD often becomes less outwardly visible with age. While young children might run, climb, or constantly move, adults typically experience this as an internal sense of restlessness or constantly feeling “on the go”. Long meetings or quiet downtime can feel unbearable. Many adults redirect this energy in subtle, socially acceptable ways—tapping their foot, shifting in their seat, or staying constantly busy.

Impulsivity, too, takes on new forms. Instead of blurting out answers or grabbing toys from peers, adults might:

Interrupt conversations or speak without thinking
Make impulsive purchases or financial decisions
Drive recklessly or take unnecessary risks
Rush through tasks, increasing the chance of errors

In adulthood, these behaviours can have real-world consequences, from workplace friction and strained relationships to accidents or increased vulnerability to substance use—often as a way to self-regulate impulsive urges.

Executive Dysfunction and Other Adult Challenges: The Hidden Symptoms

Adults with ADHD may often experience significant impairment in executive functions – the brain's ability to regulate behaviour, direct our ability to plan and achieve goals which can be devastating to daily functioning:

Task Initiation: The paralysing difficulty of starting tasks despite understanding their importance and consequence
Time Perception: beyond challenges with time management, adult with ADHD have a fundamentally different experience of time, leading to consistent underestimation of task duration
Cognitive Flexibility: Difficulty shifting between activities, often resulting in either hyperfocus (inability to stop engaging with preferred tasks) or scattered attention (inability to maintain focus on necessary tasks)
Working Memory: The temporary storage system for encoding, retaining, maintaining a and manipulating of information needed for complex cognitive tasks is frequently impaired, affecting everything from following conversations to remembering important details.
Organisation and Prioritisation: Struggles with breaking down complex projects into manageable steps, structuring tasks logically and prioritising tasks by importance

These struggles with executive function are often invisible to others. Many adults describe a constant mental load, a sense of underachievement, and frustration at being unable to execute on things they care about.

Impairments in Daily Life: The Real-World Impact

When ADHD goes unrecognised or untreated in adulthood, the effects can accumulate across many areas of life. Common challenges include:

Lower occupational attainment and workplace satisfaction
Frequent job changes, employment stagnation or terminations
Trouble managing finances, paying bills, or budgeting
Strain in personal relationships due to forgetfulness, impulsivity, or emotional reactivity
Increased risk of accidents or injuries (e.g., more traffic violations)
Higher rates of co-occurring mental health conditions, including anxiety, depression, and substance use disorders

While about one-third of children with ADHD experience full remission by adulthood, most continue to have some degree of symptom persistence. Even those who no longer meet the full criteria may still struggle with residual attentional or executive function difficulties that affect daily life.

Recognising ADHD in adulthood is critical. With the right treatment, support, and strategies, many adults can significantly improve their functioning, reduce emotional distress, and build more fulfilling lives—both at work and at home.

The Paradox of High-Functioning ADHD

The heterogeneity of ADHD means it can express in many different ways—and not all of them are immediately obvious. While some individuals experience more visible hyperactive symptoms, others may appear outwardly composed, productive, or even high-achieving. This is sometimes referred to as “high-functioning ADHD.”

But traditional markers of success don’t necessarily reflect internal wellbeing—and they certainly don’t negate the fact that many people with ADHD are quietly struggling. Their ability to “hold it together” often comes at the cost of constant self-monitoring, mental exhaustion, and a deep sense of effort just to keep up. On the outside, they may look like they’re coping. On the inside, they may be:

Mentally exhausted and chronically anxious from constant self-regulation
Overcompensating and overwhelmed by executive function impairments—particularly in task paralysis, working memory, and planning
Experiencing persistent feelings of underperformance accompanied by self-doubt and self-criticism

The internal cost of high-functioning ADHD is often invisible—and can be deeply invalidating. Friends, partners, or even clinicians may dismiss their distress with phrases like “but you’re doing fine” or “you’re just stressed.” This kind of minimisation can delay diagnosis, discourage people from seeking help, and reinforce harmful self-narratives about laziness or inadequacy.

Over time, without appropriate recognition and support, this pattern can lead to burnout, emotional dysregulation, or co-occurring conditions like anxiety or depression—as adults with ADHD overextend themselves to meet expectations that weren't designed with their brain in mind. Whether due to lack of diagnosis, limited access to resources, or reluctance to seek help, the eventual breakdown of their coping mechanisms can seem like irrefutable evidence of the personal inadequacy they've long feared in themselves.

Moving Forward

ADHD in adulthood can be exhausting, frustrating, and, at times, deeply isolating—especially when it goes unrecognised or is masked by years of overcompensating. But diagnosis is not a label of limitation—it’s a framework for understanding. It’s the beginning of self-compassion and the opportunity to work with your brain, not against it.

Whether you're navigating a new diagnosis, exploring whether ADHD might be part of your story, or supporting someone you care about, know this: ADHD is real, it is manageable—and with the right tools and strategies, it's possible to thrive.

At Kantoko, we’re here to walk with you—offering clarity, guidance and care that honours both the challenges and strengths that come with ADHD. Ready to take the first step? Get started with us today.