The National Institute of Mental Health notes that scientists aren't entirely sure what causes attention deficit hyperactivity disorder. Anything from genes and brain injuries to low birth weight and exposure to environmental toxins at a young age may play a role.
However, of these variables, experts tend to agree that genetics is the most likely link. But this isn't to say the other factors go entirely by the wayside. For example, one study, published in Human Genetics, mentions that although ADHD is "highly heritable," it's a "multifactorial disorder, in which many genes, all with a small effect, are thought to cause the disorder in the presence of unfavorable environmental conditions."
The Centers for Disease Control and Prevention reinforces this notion, outlining that in addition to the "important role" genetics plays in ADHD, other possible risk factors and causes may include brain injury, exposure to lead during pregnancy or at a young age and premature delivery.
According to James M. Swanson, professor of pediatrics at the University of California, Irvine, whether genetics is the main reason a person develops this disorder remains "a tricky question." He explains that while " ADHD does seem to run in families" and that the "statistical estimate of heritability is very high," he stresses that this does not necessarily mean that all ADHD cases have a genetic basis. "Interpretation of estimates of 'heritability' is complicated."
"Simply put, the etiology of ADHD is complex and can involve multiple causes," says Russell A. Barkley, clinical professor of psychiatry, Medical University of South Carolina. "To date, all of the major ones fall in the realm of neurology and genetics -- biological causation -- with no evidence that social factors alone can account for the condition." He explains that head trauma or other neurological injuries, alcohol use during pregnancy, significant premature birth and biohazard exposure "might interact with genetic liability to the disorder to exacerbate it."
Barkley explains that the closer someone's genetic relationship is to a child with ADHD, the more likely it is that the relative also shares the disorder. For example, he notes that 25 to 35 percent of parents of ADHD children are adults with ADHD, 25 to 50 percent of siblings of children with ADHD have the disorder and 70 to 92 percent of identical twins of a child with ADHD have ADHD.
Thomas E. Brown, director of the Brown Clinic for Attention and Related Disorders in Manhattan Beach, California, agrees that genetics play a role in ADHD. In his book, "Outside the Box: Rethinking ADD/ADHD in Children and Adults: A Practical Guide," he writes that "like eye color and height," ADHD starts with genes. "It runs in families." Yet he too, makes it clear that there are several variables, including the fact that some people's genes are less stable than others, sometimes not becoming "active until many years after the person is born."
Here are some other ways genetics can influence ADHD development -- and it's not always strictly about inheritance.
In some cases, new, or de novo, mutations occur in a child's genes despite the fact that they're not present in the parent's genome, Barkley says. "We think this may account for at least 10 percent of ADHD, especially if they are new cases arising in a family that has no increased risk among the relatives." Such mutations can occur in egg and sperm-producing cells, he explains, and arise simply by exposure to the mutation-causing agents that everyone experiences during their life, such as environmental toxins and the sun's radiation. Although the mutations are not present in the parent's DNA, the gene mutations may still be passed to a child.
"Some genes turn on and off depending on other things going on in the body or in the environment," Brown writes.
Barkley explains that such gene-by-environment interactions can give rise to a child's ADHD development. "A child inherits genes for ADHD that cause a susceptibility to the disorder and the expression of these genes then interacts with some other agent in the environment to magnify the risk for ADHD beyond the genes alone," he says. "For instance, maternal alcohol or tobacco use during pregnancy increases the risk for ADHD about 2.5 times the population risk. But should a child have one or two of the risk genes for ADHD, the occurrence may go up eight times that of the population risk."
Exposure to infections, chronic elevated parental stress during pregnancy and malnutrition earlier on in childhood also fall into this category, he says.
Swanson adds that "severe deprivation in early life may cause ADHD," referencing a large-scale study beginning in the 1990s involving Romanian orphans who "suffered different degrees of early environmental deprivation and then were adopted into families in the U.K." In addition to getting hardly any personal care or cognitive stimulation prior to adoption, these children also received insufficient food. According to The Lancet, those living in Romanian institutions for more than six months as children had higher rates of social problems -- which lasted into adulthood -- including inattention and overactivity. "These studies may be relevant to the question about additional plausible environmental causes of ADHD," Swanson says.
Another way genetics can influence ADHD is through epigenetics, which Barkley says refers to "small chemical 'flags' (usually methylated tags) that get inserted or attached on to a gene during or after its transmission to an offspring." The flag affects timing of gene activation as well as whether the gene is even activated in the first place. He says these epigenetic effects have been discovered in some disorders such as autism, adding that "some evidence is just beginning to accrue that it might occur in ADHD as well."
The Big Picture
"All in all," Barkley says, "about 65 to 75 percent of all ADHD cases arise from these genetic factors, chiefly inheritance of ADHD gene variants."
Despite the genetic links associated with ADHD, remember that every person is different and that obtaining a diagnosis is a detailed process. "Assessment for ADHD requires collection of information about how the person functions in a wide variety of complex daily tasks at many different times of day in many different settings," Brown notes.
Jennifer Lea Reynolds is a Health freelancer at U.S. News. She draws on her life and career experiences, including losing 70 pounds and writing copy at health-centric advertising agencies. Her articles have been published online in Smithsonian, Reader's Digest, Woman's Day and The Huffington Post. She's also the owner of FlabbyRoad.com, where she writes about weight loss, fitness, nutrition and body image. You can follow her on Twitter @JenSunshine.