Visual Abstract

The relationship between ADHD and key cognitive phenotypes is not mediated by shared familial effects with IQ

IQ and ADHD have separate genetics

A C Wood , F Rijsdijk, K A Johnson, P Andreou, B Albrecht, A Arias-Vasquez, J K Buitelaar, G McLoughlin, N N J Rommelse, J A Sergeant, E J S Sonuga-Barke, H Uebel, J J van der Meere, T Banaschewski, M Gill, I Manor, A Miranda, F Mulas, R D Oades, H Roeyers, A Rothenberger, H C Steinhausen, S V Faraone, P Asherson, J Kuntsi

Summarized by:

May 8, 2023

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What they studied

Researchers wanted to see whether ADHD and IQ share genetic and environmental links.

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What they found

While genetics and environment strongly influence both ADHD and IQ, the factors behind them are mostly independent of each other.

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What this means

Having ADHD doesn't necessarily mean that someone has a lower IQ, and vice versa. While both contribute to academic performance, they are caused by separate factors.

The relationship between ADHD and key cognitive phenotypes is not mediated by shared familial effects with IQ

Objectives

The relationship between ADHD and IQ is complicated. On average, children with ADHD tend to perform worse on IQ tests, though many highly intelligent children have ADHD.

There is also evidence that genetics may play a role in IQ and ADHD.

This study wanted to understand how genetics influences the relationship between IQ and ADHD, for example, whether the genes that cause ADHD also cause lower IQ scores.

Study Quote

Wood et al. 2011

Twin and sibling studies have identified specific cognitive phenotypes that may mediate the association between genes and the clinical symptoms of attention deficit hyperactivity disorder (ADHD). ADHD is also associated with lower IQ scores. We aimed to investigate whether the familial a...

Methods

Researchers ran a study comparing siblings with ADHD and similar siblings without ADHD.

This allowed researchers to estimate how much ADHD risk and IQ is caused by genetics and environment.

They performed a series of cognitive tests on the participants, measuring factors such as reaction time and accuracy. These cognitive tests correlate with known cognitive deficits in ADHD.

Study Quote

Wood et al. 2011

Multivariate familial models were run on data from 1265 individuals aged 6-18 years, comprising 920 participants from ADHD sibling pairs and 345 control participants. Cognitive assessments included a four-choice reaction time (RT) task, a go/no-go task, a choice-delay task and an IQ asse...

Results

The results showed that, generally, the cognitive deficits in ADHD were mostly independent of IQ. This means that many of the “dysfunctions” we associate with ADHD, including difficulty with focusing, aren’t related to or a reflection of IQ.

The only factor somewhat related to IQ was choice impulsivity, which had a 50% overlap with IQ.

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The familial sharing between ADHD and CI was lower (with a familial correlation of x0.14) than that found for the other cognitive variables. The percentage of the covariation with ADHD that was independent of shared etiological influences with IQ was also low- er, at 61%, indicating that CI and IQ are more closely related constructs at the etiological level.

Wood et al. 2011

Study Quote

Wood et al. 2011

Significant familial association (rF) was confirmed between cognitive performance and both ADHD (rF=0.41-0.71) and IQ (rF=-0.25 to -0.49). The association between ADHD and cognitive performance was largely independent (80-87%) of any contribution from etiological factors shared with IQ. ...

Conclusions

Based on these results, the authors concluded that many genetic and environmental factors behind ADHD are unrelated to IQ. While IQ and ADHD run strongly within families, the factors behind them aren’t closely related.

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The aetiological factors that give rise to lower IQ in ADHD seem to be largely distinct from those that give rise to the association of ADHD with RT variables, CE and OE. Lower IQ does not seem to be a general explanation for the impairments in these specific cognitive domains.

Wood et al. 2011

Other studies have found evidence that ADHD symptoms can affect IQ test performance. For example, lower scores due to difficulty with focus may be more reflective of ADHD than of raw intelligence.

Study Quote

Wood et al. 2011

The aetiological factors underlying lower IQ in ADHD seem to be distinct from those between ADHD and RT/error measures. This suggests that lower IQ does not account for the key cognitive impairments observed in ADHD. The results have implications for molecular genetic studies designed to...