ADHD: Its Roots and Causes

Parents and teachers have long known how children experience ADHD: as difficulty in sustaining attention, controlling impulses, or following through with tasks. More recently, scientists have begun to define what ADHD actually is, describing it in neurological terms as variations in the way certain parts of the human brain function and communicate. Even more recently, they have succeeded in moving beyond a description or definition to an understanding of where ADHD comes from, its roots and its causes. Thus far, researchers have identified multiple factors that appear to increase a child’s risk of developing ADHD symptoms. Generally, they fall into three categories: environmental, hereditary, and evolutionary.

Environmental Factors
Studies conducted over the past two decades have shown that the likelihood of a child having ADHD increases with exposure to certain environmental conditions that are present before or shortly after birth. In many cases, it is associated with low birth weight, which can result from premature birth and from health risks experienced by the mother, including heart disease, high blood pressure, poor nutrition, and insufficient prenatal care. The ADHD risk is also elevated if the pregnant mother abuses alcohol or drugs, but the substance that presents the highest Smoking risk is nicotine. Compared to nonsmokers, mothers who do smoke during pregnancy are more than 3 times more likely to have children with ADHD symptoms, a rate that is somewhat lower when the children are girls but higher among boys. Despite anecdotal reports, there is little research support for claims that ADHD is associated with several other “substances,” including sugar, yeast, gluten, and vitamins.

A second set of environmental risk factors are early childhood injuries. Some result from brain damage through accidents or oxygen deprivation that occur before, during, or after birth. Unfortunately, a far greater number of children have developed ADHD through injuries of a much different and more profound kind: early childhood traumas inflicted on them by their parents or other adults. As most child welfare specialists will attest, an unusually large proportion of children who have experienced physical abuse, sexual abuse, abandonment, profound neglect, domestic violence, highly erratic parenting, extended periods in orphanages, and multiple foster placements are unusually likely to develop a pattern of behaviors that may involve not only inattention and impulsivity but also social problems, defiance, depression, aggression, and withdrawal.

Hereditary Factors
Among the most interesting discoveries scientists have made about ADHD is that it tends to run in families. For example, a study of identical twins has shown that if one has the symptoms, there is a 90% likelihood that the other will, too. A study of families found that among people diagnosed with ADHD, approximately 80% had first-degree relatives (mother, father, child, or sibling) who also exhibited symptoms. Yet another research project consolidated the data from multiple studies, determining that when one family member had ADHD, the probability that another also had it ranged from 55% to 90% with an average (again) of 80%. To put all this another way, researchers have established with a reasonable degree of certainty that roughly 4 of every 5 ADHD cases are hereditary and therefore genetic in origin.

But they’ve gone even further, identifying at least 2 and possibly 4 genes that are very likely associated with ADHD behaviors. In one way or another, all of them seem involved in determining how the brain manages the neurotransmitter dopamine, a chemical that (among other things) regulates how well people can control their current attention and impulses based on their anticipation of future rewards or punishments. In each individual this ability appears to fall somewhere along a continuum. Those at one end can effectively control their behaviors now in order to receive or avoid consequences later; these people experience few if any ADHD symptoms. Those at the other end are far less able to control their attention and impulses based on expected benefits or risks; they exhibit ADHD behaviors far more often and more intensely than people elsewhere on the continuum.

This idea of a continuum is an important one, because it suggests that ADHD is not an abnormality. Instead, it is a normal human variation, something that all of us share to some extent. As Russell Barkley puts it in his book, Taking Charge of ADHD: The Complete and Authoritative Guide for Parents: “This means that ADHD should not be considered some grossly abnormal pathological condition. In fact, it is a condition not qualitatively or categorically different from normal at all, but likely to be the extreme lower end of a normal trait; thus the difference [between people with and without ADHD] is really just a matter of degree and not a truly qualitative difference from normal” (76).

Because scientists are by nature curious people, they have not been satisfied to find that most ADHD cases are rooted in genetics; they have also tried to find out why. For some, the search has led to looking at things in terms of the ways in which humans have evolved over time. Evolutionary theory says that each species adapts to survive in its environment. Long ago, most humans lived as tribes of hunters and gatherers in places where food was scarce, life was dangerous, and two types of behavior helped ensure the tribe’s continued existence. The gatherers needed to be individuals who could successfully forage the woods and fields for food by keeping their behavior contained and their attention narrowly concentrated on the task at hand. They were problem-focused and plan-makers. Those who could effectively hunt food and protect the tribe against predators or other tribes needed to be vigilant at all times, easily distracted by anomalies in their surroundings. They were response-ready and action-takers.

In ancient times, these two sets of attributes were equally useful but, as civilization developed, the set associated with problem solving became more adaptive than the one associated with quick reacting. Ultimately this became particularly visible in a school environment, where the children who were adept at solving problems and planning tasks became labeled with terms such as “good students” while the distractible, quick responders became referred to as “kids with ADHD.” These ideas appear to be more than speculation. They have been supported by recent studies on a group of Australian aborigine children who continue to live in primitive environments but attend contemporary schools. The studies reported that while 5-6% of American children exhibited signs of ADHD, the number among the aborigine children was closer to 50%.

Some Conclusions
When I put all this information together, I find myself coming to three conclusions about children and ADHD. First, if the evolutionary perspective is accurate, then the 80% of ADHD cases that are inherited have resulted from genetic adaptations that developed over tens of thousands of years. On the other hand, homework, math tests, and similar tasks requiring sustained attention have existed for less than five thousand. This suggests that ADHD is less a “disorder” than an artifact, one that has occurred because human culture has evolved more rapidly than the human genome. Second, most ADHD cases associated with environmental factors – especially those that result from trauma – also represent adaptations, but not of the species. These are the adaptations of individual children seeking ways to grow up in dangerous or neglectful homes. And, third, contrary to what many teachers and parents may think in our lesser moments, ADHD does not represent a failure of willpower. Rather, it is a neurological predisposition that is “wired in” to certain children. It is a way of being in the world that reflects the influence of their genes and their environments – one that testifies to the power of human adaptability and the determination to thrive in the face of adversity.