ADHD is somewhat heritable, study finds

A study of a massive number of twins revealed that behaviors indicative of attention deficit-hyperactivity disorder (ADHD) exhibit a notable degree of heritability. This suggests that the resemblance in ADHD behaviors between parents and children can be partially attributed to shared genetic factors. The influence of parental ADHD behaviors on their children was found to be relatively minor, as was the impact of gene-environment interactions. The paper was published in Psychological Medicine.

ADHD is a neurodevelopmental disorder characterized by a persistent pattern of inattention, hyperactivity, and impulsivity that interferes with functioning or development. Individuals with ADHD struggle with maintaining attention on tasks or activities, behave in ways that are excessively active and often act without thinking about the consequences. This disorder is typically identified in childhood, but can continue into adulthood. It often adversely affects academic, occupational, and social functioning.

Studies show that children with ADHD-like traits often have parents with similar traits. However, it so far remained unknown whether this transmission of ADHD traits is due to children learning to copy behaviors of parents (i.e., environmental transmission), because of their shared genes (genetic transmission), due to some interaction between genes and the environment, or due to conditions in the environment that affect both children and their parents, making both develop ADHD-like behaviors. Some previous studies have proposed that ADHD is highly heritable (70%-80% heritability), but these conclusions were contentious.

Study author Thomas H. Kleppesto and his colleagues wanted to investigate whether the similarities between parents and their children in ADHD-like behaviors are caused by genetic or environmental factors. The key issue they wanted to disentangle is whether parents transmit these behaviors to children through their interactions with children or through genetics.

They analyzed data from the Norwegian Mother, Father, and Child Cohort Study, a large population-based study conducted by the Norwegian Institute of Public Health. Participants of this study were recruited from all over Norway between 1999 and 2008.

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For this particular study, the researchers included extended family units composed of two nuclear families where one parent in each nuclear family is a sibling to one parent in the other family. In total, they included 22,276 parents and 11,566 children in these analyses. Of these, in the parental generation, there were 69 pairs of monozygotic twins, 95 pairs of dizygotic twins, 9,442 pairs of full siblings, and 744 pairs of half siblings.

Measures of maternal and paternal ADHD behaviors were obtained using a short screening assessment for ADHD (the Adult ADHD Self-Report Scale). Mothers reported on their own ADHD behaviors when children were 3 years old, and fathers did so during pregnancy. Measures of children’s ADHD were obtained from others (the Rating Scale for Disruptive Behavior Disorders) when the children were approximately 8 years old. At that time, mothers had a mean age of 39, and fathers were 41 years old on average.

The findings revealed a weak association between children’s ADHD behaviors and those of their parents, with a stronger correlation to maternal behaviors. The genetic analysis indicated that 11% of the variation in children’s ADHD behaviors could be attributed to genetic factors present in both generations, while 46% of the variation was due to genetics specifically affecting the children.

“Our study indicates that passive transmission of genetic factors explains the parent–child resemblance in ADHD behaviors,” the study authors conclude. “Children tend to resemble their parents in levels of impulsiveness, hyperactivity and (in)attention because of shared genetic influences, not because of shared home environments or effects of parental ADHD on child ADHD. Non-genetic factors play an important role in the development of ADHD, but these factors are largely unique to each child.”

The study sheds light on the genetic influences on ADHD behavior. However, it should be noted that the correlations between ADHD behaviors of a child and his or her relatives were weak. In spite of the fact that much of that similarity is due to genetic factors, the main cause of ADHD is likely an interplay of environmental factors unique for each child.

The paper, ”Intergenerational transmission of ADHD behaviors: genetic and environmental pathways”, was authored by Thomas H. Kleppesto, Espen Moen Eilertsen, Elsje van Bergen, Hans Fredrik Sunde, Brendan Zietsch, Magnus Nordmo, Nikolai Haahjem Eftedal, Alexandra Havdahl, Eivind Ystrom, and Fartein Ask Torvik.