1.4 Risk factors for ADHD
There has been extensive research into the causes and risk factors for ADHD over the last 50 years. Although the picture is still far from clear, some risk factors are now relatively well established.
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What is a risk factor?
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A risk factor is something that shows a strong positive association with a condition, such that people who are exposed to the risk factor have a greater risk, or probability, of developing the condition than people who are not exposed to the risk factor.
Family studies have found that the risk of ADHD among children who have siblings with a diagnosis of ADHD is nine times greater than that of children who have no siblings with a diagnosis of ADHD (Chen et al., 2008).
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What might underlie this increased familial risk?
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It could be conferred by genetic factors or factors associated with the shared family environment.
Several studies have been conducted that control for a range of environmental factors that a family would typically share, including intactness of family, and socioeconomic status, and these have still found higher levels in children with other family members with ADHD (Biederman et al., 1990; 1992).
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What do the results of these studies suggest?
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If environment is controlled for and there is still an increased risk, this suggests a role for genetic factors.
One way to pick these two factors apart further is to use adoption studies, examining families where a child has been adopted. Specifically, the following information must be available:
- Details of the health or diagnosis of specific conditions in the biological parents, obtained from the adoption agency or medical records.
- Details of the health or diagnosis of specific conditions in the child who has been adopted, normally collected by an interview with the child.
- Details about the adoptive family, both in terms of the family environment and any diagnoses of health conditions, normally collected by an interview with a family member.
Using this information, it is possible to compare the similarity between those with a genetic association (biological parent and child) and an environmental association (adoptive parent and child). If there is greater similarity, in terms of expression of ADHD, in biological associates, this indicates a key role for genetics. By contrast, if there is greater similarity in environmental associates, this indicates that the shared environment is critical.
Research has shown that the biological relatives of children with hyperactivity or ADHD are more likely to have hyperactivity or ADHD than adoptive relatives (Cantwell, 1975; Sprich et al., 2000), suggesting that genetic factors are likely to underpin the increased ADHD seen in family members of children with ADHD (Faraone and Larsson, 2019).
As well as family and adoption studies, ADHD has been the subject of several twin studies, where the prevalence of the condition is examined in monozygotic (identical) and dizygotic (non-identical) twins.
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What is the difference between these two types of twins?
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Monozygotic twins are identical in their genetic make-up whereas dizygotic twins are just like other siblings in that they share 50% of their genes.
By looking at the expression of ADHD in twins it is possible to calculate their concordance. Concordance is the probability that a pair of individuals will both have the condition, given that one of them has it.
The extent to which identical twins are more concordant for ADHD than non-identical twins can be used to calculate the heritability of the condition. Heritability is the degree to which variability in a health condition or trait in the population can be accounted for by genetics.
A review of 37 twin studies from across the world found the heritability of ADHD to range from 54% to 98% with the average (or mean) heritability reported as 74%. These figures suggest ADHD is one of the most heritable mental health conditions (Faraone and Larsson, 2019). Unsurprisingly, this high heritability has led to significant research on the genes that may play a role in ADHD, which will be explored in the next section.