Intergenerational transmission of ADHD behaviors: More evidence for heritability than life history theory

Background: Parents and children resemble each other in ADHD behaviors. A key theoretical postulate from the evolutionary life history theory is that children use information from their environment (e.g., predictability and resource availability) and develop traits that are optimized for future success in that environment. Life history theory therefore expects that variation in children’s ADHD behaviors reflects evolved calibration to the developmental context that parents provide. Alternatively, children may resemble their parents not because of the provided environments, but because of their transmitted genes.Methods: We employed a large children-of-twins-and-siblings sample (N=22,350 parents & 11,566 8-year-old children) of the Norwegian Mother, Father and Child Cohort Study (MoBa). This enables disentangling intergenerational influences via parental genes and parental behavior (i.e., genetic and environmental transmission, respectively). Fathers reported on their symptoms (or absence thereof) and mothers on their own and their child’s symptoms.Results: Child ADHD behaviors correlated 0.24 with ADHD behaviors in mothers and .10 with those in fathers. These correlations were largely due to genetic transmission; genetic transmission was five times larger than environmental transmission. Variation in children’s ADHD behaviors was largely explained by heritability (57%), with small effects of parental ADHD behaviors (2% environmental transmission), and gene-environment correlation (3%). The remainder was due to unique environmental influences and noise.Conclusion: The intergenerational transmission of ADHD behaviors is primarily due to genetic transmission, with little evidence for parental ADHD behaviors causing children’s ADHD behaviors. This contradicts the life history theory. The child-specific variability in ADHD behaviors is due, in equal amounts, to genetic factors that influence children’s but not parents’ ADHD behaviors, and to environmental influences and noise that make siblings unique from one another. We conclude that ADHD is not the outcome of adaptive behavior to the (family) environment, but rather a neurodevelopmental disorder with a strong genetic basis.