Is ADHD Hereditary? Yes and No
What causes ADHD? Is it a genetic condition? Influenced by culture and environment? What about diet? We don’t know for certain what causes attention deficit, but medical research points to genetic and neurological explanations for symptoms exacerbated by some external factors.
Attention deficit hyperactivity disorder (ADHD) is a brain disorder with a number of likely causes — though the medical community still can’t pinpoint exactly which one explains your child’s (or your own) symptoms. Researchers suspect that a gene involved in the creation of dopamine, a chemical that controls the brain’s ability to maintain regular and consistent attention may be to blame. But conclusive studies are far outnumbered by myths and misperceptions today.
What we do know is that ADHD is not caused by bad parenting, too much sugar, or too many video games. It is a brain-based, biological disorder. Brain imaging studies and other research show many differences in the brains of individuals with ADHD. Other studies reveal that a child with ADHD is four times as likely to have had a relative also diagnosed with the condition.
The ADHD Brain
ADHD brains exhibit structural and functional differences.
The image above shows differences between an adult with ADHD (right) and a non-ADHD brain (left). (The purple halo surrounding the brain image is an image artifact and not part of the brain.)
Scans and other neuro-imaging research have shown that the brains of children with ADHD mature more slowly than do those of children without the disorder. In addition, recent studies with fMRI imaging show variations in areas of the brain that control many ADHD symptoms. In children with ADHD, several brain regions and structures (pre-frontal cortex, striatum, basal ganglia, and cerebellum) tend to be smaller by roughly 5%. While this average difference is observed consistently, it is too small to be useful in making the diagnosis of ADHD in a particular individual.
These findings may one day lead to using brain imaging to diagnose ADHD, however this remains a rare and somewhat controversial practice today.
The Chemical Connection
ADHD brains also exhibit chemical differences.
ADHD was the first disorder scientists found to result from the deficiency of a specific neurotransmitter — in this case, dopamine — and the first disorder found to respond to medications designed to correct this underlying deficiency. Children and adults with ADHD seem to have low levels of dopamine.
ADHD appears to impair neurotransmitter activity in four functional regions of the brain:
- Frontal cortex. This region orchestrates our high-level functioning: maintaining attention, organization, and executive function. A deficiency of dopamine within this brain region might cause inattention, problems with organization, and/or impaired executive functioning.
- Limbic system. This region, located deeper in the brain, regulates our emotions. A dopamine deficiency in this region might result in restlessness, inattention, or emotional volatility.
- Basal ganglia. These neural circuits regulate communication within the brain. Information from all regions of the brain enters the basal ganglia, and is then relayed to the correct sites in the brain. A dopamine deficiency in the basal ganglia can cause information to “short-circuit,” resulting in inattention or impulsivity.
- Reticular activating system. This is the major relay system among the many pathways that enter and leave the brain. A dopamine deficiency in the RAS can cause inattention, impulsivity, or hyperactivity.
These four regions interact with one another, so a deficiency in one region may cause a problem in one or more of the others. ADHD results from problems in one or more of these regions.
Available evidence suggests that ADHD is genetic — passed down from parent to child. It seems to “run in families” — at least in some families.
- A child with ADHD is four times more likely to have a relative with ADHD.
- At least one-third of all fathers who had ADHD in their youth have children with the condition.
- The majority of identical twins share the ADHD trait.
Researchers in the U.S. and Europe are working now to determine which genes, specifically, make an individual susceptible to ADHD. Scientists are investigating many different genes that may play a role in developing ADHD, especially genes linked to the neurotransmitter dopamine. They believe it likely involves at least two genes, since ADHD is such a complex disorder.
There’s still a lot of work to do on the genetic aspect of ADHD. The discovery of genes that contribute to ADHD could make diagnosing the condition easier. It might also make it possible to find better treatments for people with varying symptoms of ADHD.
Toxins and Pollution
Scientific research suggests that exposure to toxic chemicals — everyday toxins found in foods, carpeting and flooring, cleaning and lawn products, and personal-care products, like toothpastes — may contribute substantially to disorders such as ADHD, autism, and learning disabilities. Infants and children are especially vulnerable to toxic chemical exposure because their biological systems are still developing. During fetal development, exposure to even minuscule amounts of toxins at critical junctures can have a lifelong impact on the child’s brain and physical health. When toxins disrupt brain development, disabilities like ADHD can occur.
In 2010, the Learning and Developmental Disabilities Initiative (LDDI) released the first-ever report identifying toxic chemical pollution in people from the learning and developmental disability community, called “Mind, Disrupted: How Toxic Chemicals May Affect How We Think and Who We Are.” It concluded that you don’t have to live next to a toxic waste site to be exposed to brain-damaging chemicals. Examples of household chemicals include:
- Perfluorinated compounds (PFCs) are used to prevent food and other substances from sticking to carpets, drapes, and cooking pans. Teflon and Scotchgard are examples.
- Polybrominated diphenyl ethers (PBDEs), used as fire retardants, are found in clothing and furniture, as well as bedding.
- Triclosan is an antibacterial agent found in soaps, toothpastes, and many other personal-care products.
- Bisphenol A (BPA) is an epoxy resin used to line food cans and other containers. It is also used to make plastic containers, like baby bottles, and certain paper products.
- Phthalates make rubber-based materials soft and pliable. They are found in vinyl, plastic bottles, toys, shower curtains, and raincoats. They are also used to make personal-care products, air fresheners, and shampoos.
Every participant in the Learning and Developmental Disabilities Initiative tested positive for at least 26 of the 89 chemicals studied.
A 2015 study, completed by the University of Calgary, linked the chemicals used in making plastic (BPA and BPS) to hyperactivity in zebrafish, which are often used to study embryonic brain development because they share 80 percent of the genes found in humans, and have similar developmental processes. They called the results of their study, “a smoking gun” that linked negative changes in brain development to BPA and BPS exposure.
Lead exposure may also cause ADHD, according to a study published in Psychological Science in 2015. The study’s researchers emphasized that lead exposure is not the only cause of ADHD symptoms; rather, it’s one environmental factor that may lead to a formal ADHD diagnosis. Similarly, lead exposure doesn’t guarantee an ADHD diagnosis, but it may provide doctors with further clues about the root of a child’s symptoms.
At one time, doctors believed that refined sugar and food additives made children hyperactive and inattentive. As a result, parents were encouraged to stop serving children foods containing artificial flavorings, preservatives, and sugars.
However, after studying the data, researchers from the National Institutes of Health (NIH), the federal agency responsible for biomedical research, held a major scientific conference to discuss the issue in 1982. These scientists concluded that the restricted diet only seemed to help about 5 percent of children with ADHD, mostly either young children or children with food allergies.
Despite a lack of conclusive research, many families still swear by the 40-year-old Feingold Diet, a nutrition plan that claims to reduce symptoms of ADHD in children by eliminating their consumption of food additives and coloring, artificial sweeteners, and certain preservatives.
A newer, more controversial theory contends that ADHD is a byproduct of our fast-paced, stressed-out, consumer-driven lifestyle. Michael Ruff, M.D., a clinical associate professor of pediatrics at Indiana University, believes that this cultural influence explains parts about ADHD that genetics can’t.
In an article in Clinical Pediatrics, Dr. Ruff called ADHD an ‘epidemic of modernity.’ “I’m talking about the cultural environment that prevails today — the modern way of life and its impact on the developing brain,” says Ruff. “Today’s children are immersed in a world of instant messaging and rapid-fire video games and TV shows. When kids get accustomed to such a rapid tempo, it’s hard for them to adjust to the comparatively slow pace of the classroom. They transfer the sense of urgency they’ve seen at home to their academic endeavors.”
Dr. Ruff suggests we ask, “Have we failed to acknowledge the extent to which environmental factors influence these processes?”
There’s no disputing that ADHD is a complex disorder and likely has many different causes and factors — all currently under investigation.