Coming soon: AboutKidsHealth is getting a new look! Learn more Watch a video tour

Attention deficit hyperactivity disorder: Overview


What is attention deficit hyperactivity disorder?

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental condition (a condition that causes subtle differences in the brain at or around birth). It mainly involves difficulties with:

  • controlling attention
  • regulating behaviour and activity levels (impulsivity/hyperactivity).

When left untreated, ADHD can lead to difficulties with school performance, self-control, memory, emotions, friendships, relationships with family and other areas of life.

​What causes ADHD?

The exact causes of ADHD are not known. However, two factors associated with the disorder are genetics and environmental factors.


ADHD runs in families. A child with ADHD is likely to have a sibling or parent with ADHD. When a doctor diagnoses ADHD in a child, it is not uncommon for parents to suspect that a parent or another child may also have the disorder. That being said, there is currently no genetic test to confirm or rule out a diagnosis of ADHD.

Genetic causes differ for each child. Researchers are now studying:

  • how genes work together
  • how genes interact with environmental factors to influence behaviour
  • how genes influence responses to treatment.

Environmental factors

Environmental factors that play a role in ADHD include:

  • smoking or alcohol use in pregnancy
  • prematurity or low birth weight
  • exposure to lead or other toxins.

Other environmental factors can worsen symptoms of ADHD. These include family stressors such as:

  • financial difficulties
  • exposure to violence
  • family breakup
  • harsh or inconsistent parenting
  • inconsistent supervision or support, for example if a caregiver cannot supervise a child properly because of their own physical or mental health challenges.

How common is ADHD?

On average, ADHD affects 5 per cent of school-aged children around the world, or about one in every 20 children. ADHD also affects about 4 per cent of adults, or about one adult in 25.

ADHD is about three times more common in boys than in girls. It is often easier to recognize and diagnose ADHD in boys because they tend to be more active. Girls with ADHD may be more likely to be overlooked because their symptoms are less obvious even though they may have as much difficulty as boys with academic and social skills.

When to see a doctor about ADHD

See your child’s doctor if you suspect your child may have ADHD. You can find out more in the page about ADHD signs and symptoms.

If your child is diagnosed with ADHD, your doctor can review parenting approaches and, depending on your child’s age, discuss medication treatments.

Does ADHD occur with other conditions?

ADHD can occur with other conditions such as:

  • autism spectrum disorder
  • speech and language difficulties
  • specific learning difficulties
  • Tourette’s syndrome and other tic disorders (conditions that cause short, sudden and repeated movements or sounds)
  • epilepsy
  • developmental co-ordination disorder (DCD) (suspected when a child’s co-ordination is poorer than expected for their age and intelligence)
  • acquired brain injury.

Children with ADHD may sometimes also have mental health conditions such as anxiety, mood disorders (such as depression or bipolar disorder) or behaviour disorders. These can affect school performance and lead to problems in the teen years.

ADHD has also been linked with physical conditions such as asthma, sleep difficulties and hearing and vision problems.

About two thirds of children with ADHD have at least one other condition. It is important to understand if a child has another condition alongside ADHD, as it may affect diagnosis and treatment.

Key points

  • ADHD is a neurodevelopmental condition that causes difficulties with controlling attention to a task at hand and regulating behaviour and activity levels.
  • ADHD can affect a child’s learning ability, school performance and relationships with peers and family, among other areas.
  • ADHD is more common when a parent or sibling has the condition. Symptoms can worsen if a child experiences stresses such as divorce, family financial difficulties, harsh or inconsistent parenting or family conflict.
  • ADHD is linked with a number of neurological, mental health and physical conditions.

Further information

For more information on ADHD, please see the following pages:

ADHD: Signs and symptoms

ADHD: How to help your child at home

ADHD: Communicating with your child's school​

ADHD: Treatment with medications​

​​Alice Charach, MD, MSc, FRCPC
Rosemary Tannock, PhD


The following resources offer support and additional information about ADHD.  


Learning Disabilities Association of Ontario

Centre for ADHD Awareness, Canada - CADAC​​

Journal articles

Heinonen, K. et al (2010). "Behavioural symptoms of attention deficit/hyperactivity disorder in preterm and term children born small and appropriate for gestational age: A longitudinal study." BMC Pediatrics. Dec 15 2010 10:91. doi: 10.1186/1471-2431-10-91.

Sucksdorff, M. et al (2015). "Preterm Birth and Poor Fetal Growth as Risk Factors of Attention-Deficit/ Hyperactivity Disorder." ​​Pediatrics ​Sept 2015 136 (3) e599-e608. doi:10.1542/peds.2015-1043