COMMENTARY

The Genetics of Attention-Deficit/Hyperactivity Disorder: Current Status and Clinical Implications

Stephen V. Faraone, PhD

Disclosures

November 02, 2006

In This Article

Do Genes Influence the Onset of ADHD?

The observation that a malady "runs in families" usually provides the first signal that genes may be involved in the disorder. Several studies have reported an elevated prevalence of ADHD among the biological relatives of patients with ADHD (we use the term ADHD to refer to current and previous definitions of the syndrome). For a review of this literature, see Faraone and Biederman.[1]

Because disorders can run in families for environmental reasons such as shared exposure to toxins, cultural transmission, and social learning, we must turn to adoption and twin studies to figure out whether genes account for the familial transmission of ADHD. The logic of adoption studies is straightforward. If genes contribute significantly to ADHD risk, biological relatives of ADHD children should be at greater risk for ADHD than adoptive relatives of adopted ADHD children. Although few adoption studies have been published, each of these supports a strong role for genes in the familial transmission of ADHD.[2,3,4]

In all, 20 twin studies of ADHD have been conducted. The twin method compares monozygotic ("identical") twins, who share 100% of their genes, with dizygotic ("fraternal") twins, who share 50% of their genes. The extent to which identical twins are more concordant for ADHD than fraternal twins can be used to compute heritability, which is the degree to which variability in ADHD in the population is caused by genes. The mean heritability across these studies is 76%.[5] This shows that ADHD is among the most of heritable of psychiatric disorders.

Although the focus of this review is on the genetic susceptibility to ADHD, we must keep in mind that environmental risk factors also play a role in the etiology of the disorder.[6] Some of these environmental risk factors are psychosocial, such as exposure to parental psychopathology and family conflict. Others have a more direct biological impact, such as fetal exposure to maternal smoking or alcohol use, and pregnancy and delivery complications. The mechanisms whereby genes and environment work together to cause ADHD have not been worked out but it is believed that, for some of these risk factors, interactions occur such that some environmental risk factors lead to ADHD only in genetically susceptible people.[7]

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