Attention-deficit hyperactivity disorder (ADHD) – as the name suggests, people with ADHD have weaker control over their attention and impulses than similarly aged peers. Although the cause of ADHD remains unclear, ADHD is believed to be linked with structural (e.g. pre-frontal cortex responsible for executive functioning) and functional (e.g. dopamine deficiency) impairments of the brain. According to the definition of the American Psychiatric Association’s Diagnostic and Statistical Manual, fifth edition (DSM-5), ADHD can be categorized into 3 major subtypes: 

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1. Predominantly Inattentive:
   Mainly presented symptoms of inattentiveness, fewer symptoms of hyperactivity​
    

2. Predominantly Hyperactive-Impulsive:
   Mainly presented symptoms of hyperactivity and impulsivity, fewer symptoms of inattentiveness​
    

3. Combined:
   Both inattentive and hyperactive presentation criteria are met