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How Did This Happen?
When a doctor diagnoses a child with attention deficit hyperactivity disorder (ADHD), the first question parents ask is almost always “What caused this?" After the initial shock, the next reaction is often guilt. Is it something I did or didn’t do when I was pregnant? Are we not raising our child correctly? Are we bad parents? But you should understand that ADHD most likely results from a combination of factors, and bad parenting is not one of them.
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ADHD Runs in the Family
Although scientists are not sure what causes ADHD in children, evidence points to genes playing a role, probably a large one. According to the National Human Genome Research Institute, children with ADHD also usually have a close blood relative who is affected. Having a parent or sibling with ADHD increases the odds by 30 percent. If both parents have ADHD, their children are 50 percent more likely to contract the disorder compared to a person who has no close relatives with ADHD.
Twin studies also strengthen the case for a genetic cause. The heritability index measures hereditary influence on the development of a condition. It quantifies the extent to which variation between individuals on a trait is due to variation in genes.
Because identical twins share nearly 100 percent of their genes, any variability between them is likely due to environmental factors. The closer the heritability index is to 1.0, the stronger the genetic influence on the development and the weaker the influence of environmental factors. In twin studies the heritability index for ADHD is 0.77.
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A child’s environment is also a subject for researchers to determine the cause of ADHD. Children of mothers who smoke or drink during pregnancy are at greater risk for ADHD, as they are for other medical conditions. Low birth weight may also be a contributor, as well as lead exposure, diet or food additives. The interaction of genetic and environmental components is important to consider. Environmental factors may increase the risk of children developing ADHD, but may not be the underlying cause.
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Some researchers claim that at least some cases of ADHD result from societal factors – our fast-paced lives, television, computers, video games, and multitasking. Dr. Michael Ruff, a clinical associate professor of pediatrics at Indiana University believes parents should consider a “lifestyle imbalance" as a possible cause of ADHD, rather than only a “chemical imbalance." He cites evidence from his group practice in Indiana, which cares for over 800 Amish families: doctors have not diagnosed a single child with ADHD, but they have diagnosed ADHD in children of formerly Amish families who have chosen a modern lifestyle.
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Injury at a Young Age
Traumatic brain injuries at a young age are associated with development of ADHD in some cases, but they are no more likely than other traumatic injuries to dispose a child to ADHD. In 2008, Keenan et al. carried out a study of children of less than 2 years old with either “medically attended" brain or burn injuries. The study found that children with head injuries were twice as likely as a population-based control group to develop ADHD. But so were the children with burn injuries. Researchers concluded the brain injury itself “did not seem to be causal" in the development of ADHD. Rather, any “medically attended injury" may be a marker for a later diagnosis of ADHD.
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Brain trauma during birth and prematurity, “especially when minor brain hemorrhaging is involved," are also risk factors for developing ADHD, according to Dr. Russell Barkley, professor at Upstate Medical University in New York and ADHD expert. Tumors and strokes, or anything that affects the brain’s prefrontal cortex may contribute to the development of ADHD. The prefrontal cortex, or the front part of the brain, controls executive functioning, and is responsible for time management, planning and organizing, and paying attention and remembering details.
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The search for what causes ADHD in children is complex and time-consuming. Researchers have made continuing progress since they began to seriously study ADHD in the 1930s. To date, most evidence indicates a mainly genetic cause, though environmental factors may play a role as well.
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Centers for Disease Control and Prevention, http://www.cdc.gov/nchs/fastats/adhd.htm
National Institute of Mental Health, http://www.nimh.nih.gov/health/publications/attention-deficit-hyperactivity-disorder/what-causes-adhd.shtml
National Human Genome Research Institute, http://www.genome.gov/10004300
Thapar, Anita et al., “An Overview on the Genetics of ADHD," http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2854824/
Sherman, Carl, “Culture Vs. Biology: What Causes ADHD?" http://www.additudemag.com/adhd/article/1546.html
Keenan, Heather et al., “Early Head Injury and Attention Deficit Disorder: Retrospective Cohort Study,"
Barkley, Russell A. Attention Deficit Hyperactivity Disorder in Adults. Jones and Bartlett, 2010.