📄 psychiatry and clinical psychology

Familial factors inflate the estimated effects of genetic liability to ADHD on behavioural outcomes

This study demonstrates that conventional population-based analyses overestimate the direct causal impact of ADHD genetic liability on adverse health and behavioral outcomes, revealing that much of this burden actually stems from shared familial and environmental factors rather than the individual's condition itself.

Original authors: Crick, D. C. P., Davies, N. M., Medland, S., Evans, D.

Published 2026-10-27

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CC BY 4.0

Original authors: Crick, D. C. P., Davies, N. M., Medland, S., Evans, D.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

For decades, scientists have known that attention-deficit hyperactivity disorder, or ADHD, runs in families. It is a condition marked by difficulty focusing, impulsive behavior, and high energy, and it appears to be strongly influenced by a person's genetic makeup. When researchers look at large groups of people, they see a clear pattern: those with ADHD often face more challenges in life, from struggling in school to dealing with depression or smoking more cigarettes. The natural assumption has been that the disorder itself causes these difficulties. If the condition is the root of the trouble, then treating the individual should fix the problem. However, this view overlooks a crucial complication. Because families share both genes and environments, it has been impossible to tell if a person's struggles come from their own brain chemistry or from the difficult circumstances they grew up in. It is like trying to hear a single instrument in a loud orchestra; without a way to isolate the sound, you cannot be sure which player is making the noise.

A new study has found a way to separate the individual from the family, and the results challenge the long-held belief that ADHD directly causes many of the negative outcomes associated with it. Researchers used a method that compares siblings to each other. Since brothers and sisters share the same parents and usually grow up in the same home, they share the same family environment and a large portion of their genetic background. By looking at the small genetic differences between siblings, the scientists could see what happens when one sibling has a higher genetic risk for ADHD than the other, while everything else about their upbringing remains the same. This approach allowed them to strip away the influence of the family environment and focus strictly on the effect of a person's own genetic liability.

The team analyzed data from nearly 40,000 people with ADHD and over 180,000 controls to identify specific genetic markers linked to the condition. They then applied these markers to a massive group of more than 164,000 siblings to see how these genetic risks played out in real life. When they looked at the data using standard methods that treat everyone as unrelated individuals, the results matched what has been seen for years. The analysis suggested that a higher genetic risk for ADHD was linked to smoking more, drinking more alcohol, experiencing more depressive symptoms, achieving less education, and having children at a younger age. These findings reinforced the idea that the disorder drives these life outcomes.

However, when the researchers switched to the sibling-based approach, the picture changed dramatically. The strong links that appeared in the general population largely vanished. For example, the study found that while the standard method suggested a genetic risk for ADHD increased the number of depressive symptoms a person experienced, the sibling analysis showed almost no effect. Similarly, the apparent link between ADHD risk and having a child at a younger age disappeared when the family context was removed. The effect on educational attainment also weakened significantly. In short, the negative outcomes that seemed to be caused by the individual's own ADHD were actually much more likely to be the result of the family environment in which the ADHD developed.

The study suggests that much of the burden attributed to ADHD is not a direct result of the individual's condition, but rather a reflection of the family and social context surrounding it. Factors such as parental stress, household dynamics, or shared genetic traits that affect the whole family may be the true drivers of these difficulties. This does not mean that ADHD is harmless or that treatment is useless, but it does imply that interventions focused solely on the individual might not reduce these specific life outcomes as much as previously thought. The researchers emphasize that their findings highlight the need to look beyond the individual when trying to understand the full impact of the disorder. While the study confirms that ADHD is real and has genetic roots, it reveals that the story of how it affects a person's life is far more complex than a simple cause-and-effect relationship. The true picture requires understanding the family as a whole, not just the person with the diagnosis.

Problem Statement
Attention-deficit hyperactivity disorder (ADHD) is strongly associated with adverse health, behavioral, and social outcomes, including addiction, mental health disorders, lower educational attainment, and early mortality. While these associations are widely interpreted as evidence that ADHD causally drives these harms—justifying individual-level interventions—conventional observational studies cannot disentangle the effect of an individual's own ADHD from the familial and environmental context in which it arises. Previous Mendelian randomization (MR) studies, which use genetic variants as instrumental variables to infer causality, have largely relied on samples of unrelated individuals. These population-based estimates are susceptible to confounding by population stratification, cross-trait assortative mating, and dynastic effects (genetic nurture), where parental genotypes influence offspring outcomes via the environment rather than direct genetic transmission. Consequently, it remains unclear whether the observed burdens are caused by the individual's genetic liability to ADHD or by the shared family environment.

Methodology
This study employed a comparative design using two-sample Mendelian randomization to distinguish between direct individual effects and familial confounding.

Genetic Instruments: The authors utilized 22 genome-wide significant single-nucleotide polymorphisms (SNPs) associated with ADHD, derived from a large European sex-combined meta-analysis (38,691 cases, 186,843 controls).
Study Designs:
1. Population-Based MR: Used summary statistics from unrelated individuals to estimate the effect of ADHD genetic liability on 16 health and behavioral outcomes.
2. Within-Family MR: Used sibling-pair summary data from the Within Family Consortium (up to 164,220 siblings across 18 European cohorts). This design exploits random Mendelian segregation within families, controlling for population stratification, assortative mating, and dynastic effects by comparing siblings who differ in their inherited ADHD liability but share the same familial background.
Analysis: The primary analysis used the inverse-variance weighted (IVW) estimator. Sensitivity analyses included weighted median, weighted mode, MR-PRESSO (to detect pleiotropy), and heterogeneity tests (Cochran's Q). The authors also tested for heterogeneity between the population-based and family-based estimates using a Wald test.
Validation: The study involved 10 young people with lived experience of ADHD to interpret findings and frame clinical implications. Sensitivity analyses were conducted to assess robustness against potential inflation of SNP-exposure estimates.

Key Results

Population-Based Estimates: Replicated previous literature, showing that greater genetic liability to ADHD was associated with increased smoking, alcohol use, depressive symptoms, and reduced educational attainment, age at first child, and cognitive performance.
Within-Family Estimates: In contrast, the family-based analyses showed significant attenuation of these effects. For several key outcomes, the effects reduced to the null:
- Depressive Symptoms: The effect size dropped from 0.059 SD (95% CI 0.039, 0.080) in population-based analysis to 0.002 SD (95% CI –0.052, 0.055) in family-based analysis.
- Age at First Child: The effect shifted from –0.045 SD (95% CI –0.058, –0.033) to 0.003 SD (95% CI –0.040, 0.047).
- Significant Heterogeneity: There were statistically significant differences between the two designs for depressive symptoms ( $p=0.048$ ) and age at first child ( $p=0.034$ ).
Other Outcomes: Effects on educational attainment, smoking, and cognitive performance also attenuated in family-based models, though the differences were not always statistically significant at the corrected level. Health measures like BMI and waist-to-hip ratio showed less pronounced attenuation, with estimates remaining consistent with both the null and population-based values.
Sensitivity: Results were robust across different MR methods and sensitivity analyses, including tests for horizontal pleiotropy and heterogeneity in SNP-exposure estimates.

Significance and Claims
The paper claims that much of the excess health and behavioral burden attributed to ADHD in population-based studies likely reflects the family and environmental context in which ADHD arises, rather than a direct causal effect of the individual's own genetic liability. Specifically, the study demonstrates that for outcomes such as depressive symptoms and age at first child, the apparent causal effects observed in population-based designs are inflated by familial factors (e.g., dynastic effects or assortative mating).

The authors conclude that interventions targeting the individual (such as pharmacological treatment) may not reduce these specific outcomes as effectively as current observational and population-based genetic estimates imply. They emphasize that distinguishing direct individual effects from familial confounding requires family-based genetic designs. While population-based MR remains valuable, the study underscores the necessity of larger family-based GWAS samples to rigorously test causal claims regarding the individual-level effects of ADHD liability. The findings suggest that burden-of-disease models attributing these outcomes solely to ADHD may overstate the fraction modifiable by treating the individual.

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