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 (1). ADHD has a strong genetic component and children with a parent who has ADHD are at increased risk of also developing the disorder. In this article, we will review the latest research on the heritability of ADHD and quantify how much more likely a child is to develop ADHD if one or both parents have it. We will also discuss other risk factors that can interact with genetics to influence a child’s chance of developing ADHD.
Overview of ADHD
ADHD is considered a neurodevelopmental disorder because it arises during childhood as the brain is developing (1). It is estimated to affect around 5% of children and 2.5% of adults worldwide (2). The core symptoms of ADHD are (1,3):
– Inattention – Difficulty focusing, easy distraction, lack of persistence on tasks, disorganization, failure to pay close attention to details, difficulty listening.
– Hyperactivity – Excessive movement, fidgeting, inability to sit still, excessive talking, restless energy.
– Impulsivity – Hasty actions without forethought, poor planning, social intrusiveness, difficulty waiting turn, frequent interruptions.
ADHD symptoms must be present before age 12, persist for at least 6 months and negatively impact school, work or relationships to meet diagnostic criteria (1).
There are three presentations of ADHD (1):
– Predominantly inattentive – 6 or more inattention symptoms present.
– Predominantly hyperactive/impulsive – 6 or more hyperactive/impulsive symptoms.
– Combined presentation – 6 or more symptoms of both inattention and hyperactivity/impulsivity.
ADHD often co-occurs with other disorders like oppositional defiant disorder, conduct disorder, anxiety and depression (1). It is associated with impairments in academic, occupational, and social functioning. Effective treatments include medications like stimulants and atomoxetine as well as behavioral therapies and classroom accommodations (1).
ADHD is Highly Heritable
Twin, family and adoption studies have demonstrated that genetics play a major role in the development of ADHD (4-7). ADHD is considered one of the most heritable psychiatric disorders (8). Heritability measures the proportion of variation in a trait that can be attributed to inherited genetic factors versus environmental factors in a population (9). The heritability of ADHD is estimated to be around 70-80% based on meta-analyses of twin studies (4,10). This means that 70-80% of the differences between people in ADHD symptoms can be explained by differences in their genes. The high heritability underscores that ADHD runs strongly in families.
Evidence from Twin Studies
Twin studies compare the similarity of a trait between identical (monozygotic) twins who share 100% of their DNA and fraternal (dizygotic) twins who share around 50% of their segregating genes. A meta-analysis of 20 twin studies with over 20,000 twin pairs found the average heritability of ADHD to be 77% based on a best fitting model (4). The concordance rate for ADHD was significantly higher for identical twins (0.77) compared to fraternal twins (0.32) (4). This indicates a strong genetic component because identifying twins share more genes in common and more closely resemble each other for a highly heritable trait like ADHD compared to fraternal twins.
Evidence from Family Studies
Numerous family studies show that ADHD runs in families and first-degree relatives of individuals with ADHD have a higher prevalence of the disorder than the general population (5). For example, the parents and siblings of children with ADHD have been found to have ADHD at rates of 15-20% and 25-35% respectively compared to around 5% prevalence in the general population (5). Twin and family studies provide quantification of how heritable ADHD is and show conclusively it clusters in families due to inherited genetic factors.
Evidence from Adoption Studies
Adoption studies also support the genetic basis of ADHD by showing that biological relatives of adoptees with ADHD have higher rates of ADHD than adoptive relatives, who share only an environment (6,7). For instance, biological parents and siblings of adoptees with ADHD had respective prevalence rates of 15.8% and 24.4%, compared adoptive parents (5.9%) and adoptive siblings (6.8%) (7). The higher rates in biological vs. adoptive relatives isolates the influence of shared genetics from shared environment on the familial transmission of ADHD.
Likelihood of Child Developing ADHD if Parent Has It
Given the high heritability of ADHD, it is common for the disorder to affect multiple members of a family. Children with a parent who has ADHD are at substantially increased risk of developing the disorder themselves compared to the general population. Quantifying the exact increase in risk has been examined through family studies tracking the rates of ADHD in children of affected parents:
One Parent with ADHD
– Children have a 22-50% chance of developing ADHD if one parent has it (11-13).
– This is a 4-9 fold increase over the estimated 5% prevalence in the general population (2).
– Sons appear somewhat more likely to develop ADHD from an affected parent compared to daughters (13).
Both Parents with ADHD
– Children have a 50-80% chance of developing ADHD if both parents have it (11,14).
– This is a 10-16 fold increase over the general population prevalence.
– Exceptionally high concordance rate suggesting ADHD inheritance from both parents places a child at greatly elevated risk.
Summary of Risk
| Parental ADHD Status | Child’s Risk of ADHD |
|---|---|
| No parents with ADHD | 5% |
| One parent with ADHD | 22-50% |
| Both parents with ADHD | 50-80% |
These familial risk figures demonstrate having a parent with ADHD dramatically increases a child’s likelihood of developing the disorder but does not make it a certainty. The wide ranges in risk estimates highlight that other factors besides genetics also contribute. Next we’ll discuss additional risk factors that can combine with parental ADHD to increase or decrease a child’s risk.
Other Factors Influencing Risk
While parental ADHD status is a major determinant of risk, researchers have identified other factors that can also affect the chances of a child developing ADHD either independently or by interacting with genetics (15):
Parental ADHD Severity
– More severe forms of parental ADHD further increase risk in offspring compared to milder presentations (15).
– Having a parent with childhood onset ADHD predicts greater risk than late onset ADHD (16).
– The more inattentive and hyperactive/impulsive symptoms a parent exhibited as a child, the higher their child’s risk (17).
Family Environment
– Family adversity factors like low socioeconomic status, parental mental health problems, disrupted family structure and hostile parenting can contribute to child ADHD risk independent of parental ADHD (18-20).
– Chaotic home environments may interact with genetics to exacerbate ADHD severity (21).
Preterm Birth
– Being born significantly preterm (under 37 weeks gestation) increases ADHD risk, likely due to potential for abnormal brain development (22).
– 8-12% of preterm children develop ADHD symptoms (22).
Low Birth Weight
– Weighing less than 2500 grams (5.5 lbs) at birth is associated with a 5-fold increase in risk of ADHD, even after controlling for prematurity (23).
– May reflect fetal growth restriction impacting the developing brain.
Smoking During Pregnancy
– Nicotine exposure in utero can affect fetal brain development and raise ADHD risk 2-4 fold (24,25).
– Maternal smoking combines with genetics as child is more likely to develop ADHD if mother smoked and has ADHD herself (26).
Alcohol Use During Pregnancy
– Drinking alcohol during pregnancy is linked to ADHD symptoms through effects of prenatal alcohol exposure on the brain (27).
– Risk is dose-dependent with frequent heavy drinking conferring greater risk (27).
Environmental Toxin Exposures
– Developmental exposures to environmental toxins like organophosphate pesticides and lead have been associated with increased ADHD risk, potentially via impacts on dopamine pathways (28,29).
– May interact with genetic predisposition.
Child Temperament
– Difficult temperament traits in infancy like high reactivity and low self-regulation ability may represent very early emerging ADHD risk (30).
– Likely interacts with genetics due to partial heritability of temperament itself.
While ADHD has strong genetic underpinnings, there are clearly multiple pathways that can lead a child to develop symptoms. Understanding risk factors that make a child more vulnerable to expression of ADHD genetics allows for better prediction, prevention and early intervention efforts.
Conclusion
In summary, parental ADHD status dramatically increases the odds that a child will develop ADHD due to the high heritability of the disorder. Children face a 22-50% chance with one affected parent and 50-80% chance if both parents have ADHD. While this represents a substantial risk magnitude, it is not deterministic as other factors like family environment, prenatal exposures and child temperament also contribute. Quantifying the increased likelihood conferred by parental ADHD along with other risks provides insight into the complex etiology of ADHD arising from gene-environment interplay. Moving forward, a precision prevention approach should be pursued by targeting interventions towards the children at highest genetic and environmental risk for developing ADHD.
