One of the first questions you may ask after you've been diagnosed with adult ADHD is, “Did I do something wrong to cause it?” Over the years, medical science has come up with many theories regarding what might cause ADHD. Today, most researchers agree that the disorder is not only highly genetic, but that it also has a neurobiological cause.

Here are five current theories as to what may cause ADHD.

1. ADHD is caused by structural abnormalities in the brain. Research using magnetic resonance imagers (MRIs) has shown that four brain regions in children with ADHD are smaller than those in children without ADHD.

2. ADHD is caused by an insufficient supply of the neurotransmitter dopamine in the brain. This theory would explain why stimulant medications that increase dopamine in the brain are effective in controlling ADHD symptoms. Researchers speculate that the lack of dopamine may affect how it interacts with two other neurotransmitters, norepinephrine and serotonin.

3. ADHD is really a sleep disorder in disguise. Some researchers believe the disorder may be caused by a sleep-deprived brain, and the hyperactivity people with ADHD exhibit may be an effort to compensate for drowsiness. Many people with ADHD have sleep disorders, while others sleep so soundly it's hard to wake them up.

4. ADHD is a hereditary condition. While researchers don't fully understand why and how ADHD is passed from one generation to the next, they agree there is a strong genetic component. Children with ADHD are extremely likely to have at least one close relative with the disorder.

5. Environmental agents such as cigarette and alcohol use during pregnancy may increase the risk of ADHD in children. High levels of lead may also cause ADHD.

10 Theories That Have Been Debunked

Although most experts have largely discounted earlier theories about the causes of ADHD, some of them continue to persist.

1. Food additives and sugar cause ADHD. A study conducted in 1982 by the NIH found that restricting sugar in the diets of children with ADHD was beneficial in just 5 percent of cases, mostly in children who already had food allergies. There is no research indicating that excess sugar consumption causes ADHD, although it may cause hyperactivity in some people.

2. The dramatic rise of ADHD in recent years is caused by increased toxins in the environment. While it's true that both the incidence of ADHD in children and adults and the amount of toxins in the environment has increased, there are no studies indicating a link between the two. In fact, experts largely attribute the increase in ADHD to advances in diagnostic tools.

3. Exposure to lead causes ADHD. The accumulation of lead in the brain was once believed to cause ADHD. While research has shown that some people with ADHD may not tolerate lead as well as people who don't have the disorder, there is no definite research linking lead to ADHD.

4. ADHD is caused by brain damage. This early theory originated around the time of the 1918 flu epidemic, when affected children came down with symptoms that resembled ADHD, including hyperactivity, inattentiveness, and impulsivity. This theory was later refuted, but it nevertheless paved the way for the state-of-the-art research that is being conducted today.

5. ADHD is caused by traumatic brain injury resulting from a lack of oxygen during birth or from a head injury in early childhood. That brain injuries have many symptoms that overlap or mimic the symptoms of ADHD led researchers to suspect a correlation between the two. No studies have confirmed this theory.

6. Allergies and food sensitivities trigger ADHD. While these conditions have symptoms that may mimic or overlap with the symptoms of ADHD, there is no research showing a connection between the two. Most people see a dramatic reduction or elimination of their food allergy symptoms when they begin medication for the condition.

7. A poor diet causes ADHD. While malnutrition has not been linked with ADHD, some studies show that a lack of omega-3 fatty acids in the diet may exacerbate ADHD symptoms. These fats are important for brain development and function, and having insufficient amounts may contribute to, if not actually cause, developmental disorders in children. Fish oil supplements appear to decrease the symptoms of ADHD in some children and adults and may even help them improve their performance at school and at work.

8. ADHD is the result of “moral defectiveness.” This early theory claimed that people with ADHD were morally defective by nature. While many people with ADHD suffer from behavioral issues that lead to problems later in life, there is no evidence the disorder is caused by an inherent moral defectiveness.

9. ADHD is a willful behavior caused by defiance. This theory puts the blame on parents, claiming that children who fail to pay attention could be “cured” if parents taught them not to misbehave or daydream. It assumes that children with ADHD were being inattentive, impulsive, and defiant by choice. Unfortunately, many people today continue to believe that ADHD is a fictional disease used to excuse everything from lax parenting to misbehavior.

10. ADHD is caused by a poor upbringing. This early theory remains one of the leading misconceptions about ADHD today. While improper parenting can exacerbate the symptoms of ADHD, research shows that being overly strict with children and teenagers who suffer from the disorder nearly always backfires. Studies show that while ADHD symptoms can't be scolded or disciplined away, a variety of treatment modalities, including medication, psychotherapy, and biofeedback, can help decrease or even eliminate them.