The exact causes and development of ADHD are still not fully understood. Today, researchers believe that many different genetic factors, together with environmental factors, can affect brain development. These changes can lead to differences in how certain brain circuits work, which are responsible for ADHD.
One of these changes involves the brain’s messenger system (called neurotransmitters), which helps nerve cells communicate with each other. In people with ADHD, differences in this system—especially involving the messengers dopamine and noradrenaline—have been found.
ADHD doesn’t always develop in the same way. This means that not every person with ADHD has the same brain changes or difficulties. Also, the changes found in people with ADHD are not unique to this disorder—they can also occur in other conditions.
Because of these brain metabolism and function problems, people with ADHD often have difficulty focusing on one thing for a long time and struggle with self-control. They may also have trouble using their skills and knowledge effectively, which makes it harder for them to plan and act in a thoughtful, goal-oriented way.
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Genetics and ADHD
Inherited from our parents, genes are the “blueprints” for who we are. Many studies indicate that genetic factors play a significant role in the development of ADHD. Strong evidence comes from family, twin, and adoption studies. Twin studies show that about 80% of identical twins and nearly 30% of fraternal twins display similar symptoms.
Molecular genetic studies have also identified specific regions in human DNA that show typical changes in individuals with ADHD. These changes are especially found in the genes responsible for producing and regulating the neurotransmitter dopamine.
Results from several international twin studies show that ADHD often runs in families. Children with ADHD have thinner brain tissue in the brain areas (prefrontal cortex) associated with attention. Those children carry a version of a certain gene which seems to be the underlying cause for developing ADHD.
Environmental Factors versus Genetics in ADHD
The genetic changes identified so far can only explain a small part of why ADHD develops. The interaction between multiple genes—as well as the combination of genetic and environmental factors—is believed to be particularly important.
Current research suggests that many small genetic changes work together, and these genetic factors are also influenced by other elements such as complications during pregnancy or birth, as well as environmental conditions.
Science-based recommendations from experts can help parents take practical measures towards minimizing the risk. One of such measures is limiting the amount of television watched and computer games and putting down mobiles. Studies have shown that infants and toddlers who watch television or play with mobiles more than two hours a day are more likely to develop ADHD.
On the other hand, parents should encourage reflection and patience-promoting activities such as reading, social interactions, having family meals, reading together, playing board games, going outside and playing sports. They should engage their children in slow-paced, step-by-step activities by avoiding multitasking.
Brain Abnormalities in ADHD
Brain studies suggest that ADHD is linked to problems in specific brain circuits. These circuits include areas like the striatum (part of the basal ganglia), the frontal lobe, and also the cerebellum. These brain areas work together to manage motivation, emotions, thinking, and movement. If these areas don’t function properly, it can lead to too much or too little of certain neurotransmitters in specific parts of the brain.
Imaging studies using MRI and PET-scan on the brain of people with ADHD also showed reduction in the volume of the prefrontal cortex.
A review of published studies involving neuroimaging, neuropsychological genetics, and neurochemistry suggest that four connected front striatal regions (the lateral prefrontal cortex, dorsal anterior cingulate cortex, caudate, and putamen) play a role in the pathophysiology causing ADHD.
Complications and Stress During Pregnancy and Birth
Smoking, alcohol, or drug use during pregnancy—as well as a lack of oxygen at birth—may increase the risk that a child will later develop ADHD. Infections affecting the central nervous system during pregnancy, head injuries, and complications during pregnancy or birth have also been linked to later hyperactivity-related issues. However, most children and adolescents with ADHD have not experienced such complications. Moreover, these complications do not always lead to ADHD.
Expectant mothers should take care to avoid alcohol, cigarettes, illicit drugs, or mistreatment of themselves. A poor health care during pregnancy can increase the risk of developing ADHD. Thus, a proper medical care during delivery is particularly important.
Early Childhood Influence on ADHD
Researchers believe that disturbed relationships between the baby and the caregiver is one of the root causes for ADHD. Emotional detachment of the caregiver has a profound effect on child’s attentional and self-regulatory abilities. Behaviour typical for ADHD has been found in children who have suffered violence and emotional abuse.
The development and course of ADHD can be affected by family and school environments. While conditions at home, in kindergarten, or at school are not the sole cause of the disorder, they can significantly influence the severity of the symptoms and how the condition progresses over time.
If the parents of a child with ADHD also have mental health issues (for example, ADHD themselves), or if there is frequent conflict or financial stress within the family, the child’s ADHD symptoms may become more severe. It’s important to understand that these environmental factors interact with the child’s genetic makeup and any complications during pregnancy or birth.
Some common psychosocial risk factors include:
- Growing up in a single-parent household or without parents
- Mental illness in a parent, especially antisocial personality disorder in the father
- Family instability and constant conflict between parents
- Low family income and overcrowded living conditions
- Inconsistent parenting and lack of clear rules
- Frequent criticism and punishment
Evolutionary Theories and ADHD
The hunter vs. farmer theory suggests that hyperactivity may be an adaptive behaviour in pre-modern humans. The individuals with ADHD retain some of the older “hunter” characteristics associated with early pre-agricultural human society. According to this theory, individuals with ADHD may be more adept at searching and seeking. On the other hand, they are less adept at staying and managing complex tasks over time.
Hyperactivity as a leading symptom of today’s ADHD may be explained trough the adaptive evolution of ancient societies. In these societies, those with hyperactivity might have been more proficient in tasks involving risk or competition (i.e. hunting, mating rituals, etc.).
Researchers found a genetic variant associated with ADHD (DRD4 48bp VNTR 7R allele), is more frequent in more nomadic populations. Consistent with this, another group of researchers observed that the health status of nomadic men was higher if they had the ADHD associated genetic variant (7R alleles).