Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder characterized by inattention, hyperactivity and impulsivity that begins in childhood and can persist into adulthood. ADHD is considered a heritable disorder, meaning that genetics play a significant role. However, environmental factors also contribute to the development of ADHD.
What is ADHD?
ADHD is one of the most commonly diagnosed mental disorders in children. The main features of ADHD include:
- Difficulty paying attention or staying focused on tasks
- Excessive activity or restlessness
- Acting impulsively without thinking
These symptoms typically begin in early childhood, before the age of 12, and affect functioning in school, at home and in social settings. Although hyperactivity tends to decrease with age, attention difficulties and impulsivity often persist into adulthood.
ADHD is categorized into three subtypes depending on the predominant symptoms:
- Predominantly inattentive type – difficulty sustaining attention, easily distracted, forgetful.
- Predominantly hyperactive/impulsive type – fidgety behavior, excessive talking, difficulty waiting their turn.
- Combined type – displays both inattentive and hyperactive/impulsive symptoms.
It is estimated that ADHD affects around 5% of children and 2.5% of adults worldwide. However, many cases of ADHD are undiagnosed and untreated.
Is ADHD Genetic?
Research strongly indicates that genetics play a major role in the development of ADHD. ADHD has been shown to run in families and heritability estimates range from 60-90%. This means thatinherited DNA differences account for 60-90% of the risk for developing ADHD.
Specific genes that have been linked to ADHD include:
- DRD4 – involved in the functioning of dopamine
- DAT1 – regulates dopamine and norepinephrine signaling
- 5-HTT – regulates serotonin
- SNAP25 – implicated in neurotransmitter release
Individuals with ADHD are more likely to have these genetic variants compared to people without ADHD. Possessing certain combinations of these risk genes likely increases susceptibility.
The high heritability also indicates that a person with ADHD is more likely to have a parent or sibling with ADHD compared to the general population.
Familial Risk
Many studies show that ADHD runs in families:
- Around 25% of close relatives of a child with ADHD also have ADHD
- Siblings of an individual with ADHD have a 30-40% chance of also having ADHD
- Identical twins share 95-99% of risk factors for ADHD while non-identical twins share 40-60%
- Children of parents with ADHD have a 57% chance of also developing the disorder
This increased risk among family members provides strong evidence that genetics significantly contribute to ADHD.
Heritability Estimates
Heritability measures the proportion of variation in a trait due to inherited genetic factors. Studies using twin designs have estimated ADHD heritability to be around 76%.
This indicates that 76% of the differences in ADHD symptoms in a population can be explained by genetic variation between individuals. The remaining 24% is attributed to environmental influences.
Heritability also increases with age, suggesting that genetics may play an even larger role in persistent ADHD compared to childhood ADHD.
Environmental Factors
Although genetics are a major contributor to ADHD risk, they do not tell the whole story. Environmental influences also play an important role in the development of ADHD. Potential environmental risk factors include:
- Exposure to toxins, such as lead or cigarette smoke, during pregnancy
- Low birth weight and premature birth
- Brain injury from trauma or infection
- Alcohol use during pregnancy
- Poor nutrition during pregnancy and early childhood
The interplay between genes and environment is complex. But it appears environmental influences may act by ‘switching on’ or amplifying certain genetic risks. For example, pregnancy and birth complications likely cause ADHD symptoms only in children who are already genetically predisposed.
Gene-Environment Interactions
Gene-environment interactions refer to genetic factors moderating environmental risk factors, and vice versa. There are likely complex interactions between genetic vulnerabilities and environmental exposures that ultimately lead to the development of ADHD.
For instance, children with high genetic loading for ADHD appear more sensitive to the effects of maternal smoking during pregnancy compared to children with lower genetic risk.
On the other hand, a stable family environment may help reduce ADHD symptoms in genetically susceptible children. This highlights the combined influence of nature and nurture in the development of ADHD.
Other Causes
Aside from genetics and environment, other possible contributors to ADHD include:
Brain Structure
Structural differences in specific brain regions have been noted in ADHD, including the frontal cortex, cerebellum, and basal ganglia. These areas are involved in inhibition control and attention pathways. However, it is unclear whether these anatomical differences are a cause or a result of ADHD.
Neurotransmitters
Neurotransmitters implicated in ADHD include dopamine, noradrenaline and serotonin. Altered levels or activity of these chemical messengers likely disrupts normal signaling between brain cells. Stimulant medications for ADHD target dopamine and noradrenaline systems.
Executive Function Deficits
Impairments in executive functions such as working memory, planning and inhibition control are common in those with ADHD. However, it is debated whether these deficits are symptomatic of underlying neurological differences.
Overall, research supports a complex interaction between multiple genetic and environmental influences resulting in the onset of ADHD. While heritability estimations clearly demonstrate the disorder has a strong genetic component, many questions remain regarding the precise biological pathways involved. Ongoing study of the genetics underpinning ADHD will further unravel the intricate causes of this multifactorial disorder.
ADHD and Genetics FAQ
Here are answers to some frequently asked questions about the link between ADHD and genetics:
Is ADHD caused by only genetics?
No, ADHD development involves a combination of genetic and environmental risk factors. While genetics play a major role, environmental exposures can also contribute to ADHD onset, especially in those with genetic vulnerabilities.
Can you have ADHD without genetic risk?
It is possible but less common to develop ADHD with minimal genetic risk factors. Environmental factors like brain injury early in life can potentially lead to ADHD without strong genetic loading. But for most cases, genetics and environment work together to bring about ADHD.
Does ADHD run in families?
Yes, ADHD tends to run in families and first-degree relatives of someone with ADHD have around a 30% chance of also having it. The high familial association indicates ADHD has a significant genetic component.
Can you inherit ADHD from your parents?
ADHD is not strictly a single-gene inherited disorder. However, children of parents with ADHD are at increased risk due to the hereditary nature. Around 57% of children with one parent with ADHD will develop the condition themselves.
How is ADHD diagnosed genetically?
There are currently no genetic tests available to diagnose ADHD. The diagnosis is clinically made based on a medical evaluation of symptoms, history and ruling out other causes. However, identifying genetic markers may help with early identification in the future.
Can ADHD go away with age or be outgrown?
In most cases, ADHD is a lifelong disorder though some symptoms like hyperactivity tend to diminish with age. The genetic component likely explains why ADHD persists across the lifespan for the majority of individuals.
What percentage of ADHD is inherited?
ADHD has a heritability rate of around 70-80%. This means genetics account for roughly 70-80% of the differences seen in ADHD traits in the population. The remaining 20-30% is attributed to environmental factors.
Do adopted children have a higher rate of ADHD?
Some studies show adopted children have a slightly elevated rate of ADHD compared to biological children. This suggests both genetic and environmental factors in the child’s early life before adoption play a role in ADHD development.
Conclusion
In summary, there is compelling evidence from family, twin and genetic studies that ADHD has a strong biological and hereditary component. While exact causes are still being investigated, it is clear genetics make a substantial contribution by increasing vulnerability. However, environmental influences before and after birth also interact with genetic susceptibilities to bring about emergence of ADHD symptoms. Ongoing research will provide greater insight into the complex web of factors underlying this common and impairing neurodevelopmental disorder.