Types and Causes of ADHD in Adults

19th May 2025

Understanding the different types of ADHD is essential to receiving an accurate diagnosis and the most effective treatment. ADHD presents in multiple forms, each with its own set of challenges. In adults, recognising your specific ADHD subtype can lead to more tailored, successful care.

What Are the Main Types of ADHD?

ADHD (Attention-Deficit/Hyperactivity Disorder) is commonly divided into three distinct subtypes. Each type affects attention, behaviour, and daily functioning differently:

1. Predominantly Inattentive Type (ADHD-PI)

Often referred to as ADD (Attention-Deficit Disorder), this subtype accounts for roughly one-third of adult ADHD cases. It is characterised by:

Frequent daydreaming and mental “drifting off”
Difficulty focusing on tasks, especially repetitive or routine ones
Being easily distracted by irrelevant sights, sounds, or thoughts
Trouble with organisation, memory, and follow-through

Adults with inattentive ADHD may appear quiet or passive, often going undiagnosed until later in life.

2. Predominantly Hyperactive-Impulsive Type

This less common form affects around 7% of adults with ADHD. The core symptoms include:

Restlessness or constant fidgeting
Talking excessively or interrupting others
Difficulty waiting or staying seated for long periods
Acting on impulse without considering consequences

While attention issues may be present, they are typically less dominant in this subtype.

3. Combined Type ADHD

The most prevalent form, combined ADHD, represents approximately 60% of adult ADHD cases. Individuals with this subtype experience significant symptoms of both inattention and hyperactivity/impulsivity, such as:

Disorganisation and forgetfulness
Difficulty staying focused
Impulsive behaviour and mood swings
Physical restlessness and low frustration tolerance

Combined ADHD often leads to challenges across multiple areas of life, including work, relationships, and self-esteem.

What Causes ADHD in Adults?

While the exact causes of ADHD are not fully understood, research suggests a mix of genetic, neurological, and environmental factors contribute to its development.

Common contributing factors include:

Genetics: ADHD tends to run in families, suggesting a strong hereditary component.
Brain structure and function: Differences in brain activity, particularly in areas controlling attention and impulse regulation.
Prenatal influences: Exposure to toxins, alcohol, or smoking during pregnancy may increase risk.
Early childhood experiences: Premature birth, low birth weight, or early exposure to stress may also play a role.

It’s important to note that ADHD is not caused by poor parenting, diet, or laziness, although these factors may influence how symptoms are expressed or managed.

Getting the Right Diagnosis and Treatment

If you identify with any of these ADHD subtypes or symptoms, a professional assessment is the first step. An accurate diagnosis can help you access evidence-based treatment options tailored to your specific needs.

Need support? Schedule a confidential ADHD assessment with 101 Autism & ADHD Assessments today.

This article is intended for informational purposes only and should not be considered a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with any questions you may have regarding a medical condition.

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