By Sophia Auld
In the mid-1800s, English scientist Francis Galton coined the term “nature versus nurture” to describe the relative influences of environment and heredity on human development. When it comes to ADHD in families, the question about whether genes (nature) or environmental factors (nurture) play the biggest part has produced significant research — along with some false and unhelpful claims.
Despite abundant evidence to the contrary, myths that ADHD is caused by poor parenting, excess sugar consumption or too much screen time persist. And if genetics are the cause, what are the implications for families? Current research indicates both genetic and environmental influences can have a role in the development of ADHD symptoms. Importantly, whatever the cause, we do know how to treat ADHD effectively.
All in the genes?
There is no doubt ADHD is what scientists call a “familial” condition. A 2019 article in the journal Molecular Psychiatry notes that studies involving families, twins and adopted children show ADHD runs in families.
Sanam Hafeez, a neuropsychologist and the director of Comprehend the Mind psychological consultancy in New York, says most children with ADHD “usually have a sibling or parent who either has an ADHD diagnosis or very similar features.”
“Parents often come back to me after we complete the assessment for their children to have themselves evaluated,” she adds.
One way scientists measure how much of a role genetics versus environmental factors play in a given trait is to study twins. Hafeez explains studies have found ADHD has a heritability rate of 77% to 87% in identical twins (who share the same genetic code), which is similar to that of schizophrenia and bipolar disorder.
Importantly, heritability rates are not a measure of the percentage of ADHD cases caused by genes. Nor do they measure the percentage of an individual’s symptoms that can be attributed to genetic factors. Rather, heritability rates indicate how much of the variation in a trait can be attributed to genetic variations within a population.
The Molecular Psychiatry paper noted studies involving adopted children suggest the familial nature of ADHD is related to genetic factors rather than a shared environment.
According to the National Human Genome Research Institute, scientists believe at least two genes are involved in ADHD. However, the genetics of ADHD is far from being fully understood. Ronald Brown, PhD, professor and dean on the Department of Integrated Health Sciences at the University of Las Vegas, Nevada, says that while scientists have published some interesting research , it’s not at the level “we can hang our hats on.”
“We know ADHD is probably constitutional, or something people are born with. But it’s not like colon or breast cancer that we’ve identified specific genes for,” Brown says. “In terms of the genetics of ADHD, we don’t have all that data.”
Environmental factors and ADHD
Along with genes, the environment can affect ADHD. In fact, it seems environment and ADHD have a two-way relationship. On the one hand, exposure to certain things during development may lead to ADHD symptoms.
“Childhood factors, such as exposure to toxins and lead, can cause ADHD, as can maternal drug use,” says Hafeez. “Premature birth, childhood trauma and brain injury may also trigger the onset of ADHD.”
In a 2020 paper in The Lancet Psychiatry, researchers analyzed results from 35 studies exploring environmental risk factors for ADHD. They found a strong association between ADHD in children and things that affected their mothers, including:
- high blood pressure
- smoking during pregnancy
- acetaminophen use during pregnancy
- pre-pregnancy overweight and obesity
- low vitamin D levels.
There was also a strong link between ADHD and childhood asthma and eczema.
Another 2020 study looked at risk factors for ADHD by comparing 297 children with ADHD aged 5 to 12 years to 297 children in the same age group without ADHD. Researchers found that preterm birth, disease during infancy, high birth weight, a lower level of mother’s education and fewer offspring were associated with a higher chance of a child having ADHD.
It’s important to note these findings do not prove that the environmental factors identified cause ADHD — just that they are linked.
Brown makes the point that ADHD symptoms are also dependent upon the environment.
“It can lessen symptoms of the disorder or make them worse,” he says. “For example, say you have a child with ADHD in a four-person classroom in rural Iowa. They may never display any symptoms. But take that same child and put them in a classroom in New York City with 40 children, and they begin to evidence a lot of symptoms.”
He explains this is why behavior management works so well for children with ADHD. “What you do is change their environment so that you mitigate symptoms,” Brown says.
Hafeez agrees environmental factors can exacerbate or reduce the impact of ADHD. “For instance, several studies show a high correlation between video game and cell phone usage and ADHD,” she says. “Video games can continually stimulate the frontal lobe part of the brain that is under-stimulated in ADHD and create an addiction — worsening attention to other more important things.”
The other side of the ADHD/environment relationship is evident when a child’s symptoms alter what happens in their life circumstances. Brown explains people may respond to children with ADHD in a certain way. “They may get more controlling or authoritative,” he says.
“There was a study many years ago involving school classrooms,” he says. “The teachers got more controlling in response to having kids with ADHD in the classroom. They talked more and gave more commands. So this disorder can change some things in the environment, and it also responds to the structure the environment provides.”
What about parenting?
Clearly, the way a child is raised is a key environmental factor. But does parenting style influence ADHD development?
Brown emphasizes that “bad” parenting does not cause ADHD. He notes many people don’t parent all that well, and their kids grow up fine. “And there are some parents that parent very well and some of their kids have ADHD,” he says.
What about studies that find a link between ADHD and parenting? For example, a 2022 cross-sectional study in The Malaysian Journal of Medical Sciences used questionnaires to assess the parenting style of 55 parents of students aged 9 to 12. The authors concluded there is a significant relationship between parenting style and ADHD risk in children.
Brown explains studies like these are correlational, which means they only show an association rather than proving causality. In other words, the study found parenting style and ADHD are linked, but not that one causes the other. It’s possible, for example, that another factor could cause both a certain parenting style and the ADHD.
“What we know, however, is that in children who have ADHD, parents can respond a certain way,” Brown says. “They may be more controlling or more authoritative. Their parents may be more irritated, volatile or controlling.”
“The other thing is that many parents of children with ADHD also have ADHD themselves,” Brown says. “Parenting can be really difficult for people who have ADHD because they may be impulsive or have difficulty organising, managing or controlling their kids.”
Hafeez says researchers have long supported the benefits of the “authoritative parenting” style (in which parents are responsive and nurturing but set firm boundaries), especially for children with ADHD. She says “authoritarian parenting” (where parents are extremely strict) can worsen aggression in children with ADHD, while a “permissive parenting” style (where parents are warm but fail to set firm limits) may worsen impulsivity and self-direction.
Furthermore, Hafeez explains parenting may alter body chemistry in a way that affects ADHD symptoms. She says “lack of parental attention or neglectful parenting” can increase levels of dopamine (a neurotransmitter, or chemical messenger, in the brain) and norepinephrine (a substance involved in the body’s stress response), “which can lead to hyperactivity and worsening symptoms of ADHD.”
The bottom line: Both nature and nurture can play a part
Researchers continue to unravel the complex interplay between nature and nurture in ADHD — and seem to agree both are involved. Findings reported in the 2021 international consensus statement from the World Federation of ADHD suggest it is “rarely caused by a single genetic or environmental risk factor.” Rather, “most cases of ADHD are caused by the combined effects of many genetic and environmental risks — each having a very small effect,” they write.
And while we may not fully understand the cause of ADHD, the good news is we know how to manage it. Says Brown: “We’ve spent the last 50 years figuring out how to treat ADHD with medication and behaviour management and we know both work.”
He suggests parents read material that is evidence-based (based on findings from reputable research), work with their child’s school, comply with treatment plans and get whatever assistance they need.
He says: “There’s a lot of help and support systems for families and they should take advantage of that.”