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Old 06-19-12, 05:29 AM
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ADHD and General Intelligence - the Biological Perspective

I have noticed that the question of whether or not ADHD covaries with higher or lower intelligence interests many posters on this forum.

The issue periodically reappears in new threads and unfortunately tends to get bogged down sooner or later in the technical minutiae of IQ tests and their validity and measures of human intelligence.

In what follows I'd like to avoid this by going back to basics and looking at the nature of intelligence from the biological/neuropsychological perspective, and then seeing what the science says about the implications for ADHD.


Most basic definitions of of intelligence say something to the effect that: intelligence is the ability to reason, to understand , and to profit from experience.

For over one hundred years to date scientific research in neuropsychology and related disciplines has proposed that these abilities are located in the human prefrontal cortex, whose overall role is the coordination of high -level cognitive "Executive Functions" like working memory, planning and problem-solving.

British researcher, John Duncan and his team sought the location of general intelligence (the so-called "g" factor) in the brains of average people. They used tasks that required reasoning and were known to correlate with general intelligence more than with any specific ability.

Duncan and co. found that although verbal and spatial tasks had different patterns of activation, prefrontal activation was common to both tasks; he therefore concluded that general intelligence was probably located there.

cf: (1) Duncan, J. et al (2000) A Neural Basis for General Intelligence. Science. 289, 457-460.

There is support for this view in the fact that frontal damage impairs general intelligence more than performance on traditional IQ tests which emphasise crystallised intelligence (i.e; skills and information learned earlier); more posterior lesions do not have this effect.

cf: (2) Duncan, J. et al, (1995) Fluid Intelligence after Frontal lobe Lesions, Neuropsychologia, 33, 261-268.

(3) Gray and Thompson (20004), Neurobiology of Intelligence: Science and Ethics. Nature Neuroscience, 5, 471-482.

Because frontal areas are involved in working memory and executive control of problem-solving, it is not surprising that they would contribute to general intelligence.

With regard to ADHD, there is increasing evidence that ADHD arrises from deficiencies in the development, structure and function of the prefrontal cortex and its networks with other brain regions, especially the striatum and cerebellum. Given this, it seems probable that general intelligence will be impaired in persons with ADHD. Such a likelihood makes sense, for instance , of the fact that research studies for the past three decades to date have consistently found significantly reduced IQ scores in individuals with ADHD across the lifespan.

BRAIN SIZE and INTELLIGENCE - ADHD is associated with reduced overall Brain Volume

Within a species brain size has been found to correlate with intelligence. That is, on average more intelligent brains are larger brains.

MRI neuroimaging studies have found correlations between brain size and measures of intelligence; a compilation of 37 samples involving 1,530 people found a modest correlation of 0.33. (cf: McDaniel, 2005, Big-Brained people are smarter. A Meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence, 33, 337-346). Squaring the correlation coefficient tells us that about 11% of the difference among people in intelligence is related to brain size.

Neuroimaging studies indicate that ADHD is associated with about a 5% reduction in total brain volume and a 10-12% reduction in the size of key brain regions (like the prefrontal cortex) involved in the higher-order control of behaviour.

NERVE CONDUCTION VELOCITY and MYELINATION- their relation to general intelligence and ADHD.

IQ scores are also correlated with nerve conduction velocity. (cf McGarry-Roberts, Stelmack and Campbell, 1992 , Intelligence, Reaction Time, and event-related potentials. Intelligence. 16, 289-313. Vernon and Mori (1992) Intelligence, Reaction Times, and Peripheral Nerve Conduction Velocity. Intelligence. 16. 273-288.)

Faster nerve conduction probably makes its contribution through improved central processing efficiency and enhanced myelination of the brain's neurones ( cf: A.R. Jenson (1998). The "g" Factor. Westport, CT:Praeger.)

Myelin both improves conduction speed and and insulates neurones from each other; reducing cross-talk that would otherwise interfere with accurate processing.

Myelination, speed of information processing and intelligence all follow a curvilinear path, increasing from childhood to maturity, then declining in old age.

ADHD patients have been reported to have reduced white matter (myelination). (cf: Castellanos et al (2002) Developmental Trajectories of Brain Volume Abnormalities in children and Adolescents with ADHD. Journal of the American Medical Association. 288. 1740-1748.)
In addition, Elizabeth Sowell found decreased volume in prefrontal and lateral temporal cortex and denser gray matter in the posterior temporal and inferior parietal cortex, probably due to missing myelination (i.e., white matter was reduced overall) (cf; Sowell, E et al, 2003, Cortical Abnormalities in children and Adolescents with ADHD. Lancet, 362, 1699-1707.)

WORKING Memory, IQ and ADHD.

Many theorists believe that short-term memory is the ultimate limitation on human reasoning and problem-solving ability. It is a fact that short-term memory is a good predictor of IQ ( Miller and Vernon, 1992, The general factor in Short-Term Memory, Intelligence and Reaction Time. Intelligence. 16, 5-29.). Increased nerve conduction velocity may particularly enhance the efficiency of Working Memory, (which is a manifestation of Short-Term memory). Working Memory is also correlated with both gray and white matter volume, just as is general intelligence. (cf; Posthuma et al 2002, The Association between Brain Volume and Intelligence is of Genetic Origin. Nature Neuroscience. 5. 83-84.)

With regards ADHD, there is now good evidence that Working Memory is, in fact, impaired in the disorder.

(Cf (1) Martinussen R et al, 2005, A Meta-Analysis of Working Memory Impairments in Children with ADHD. Journal of the American Acadamy of Child and Adolescent psychiatry, 44, 377-384. and (2) Willicutt, E.G. et al , 2005, Validity of the Executive Function Theory of ADHD. A Meta-Analytic Review. Boilogical Psychiatry, 57 (11) , 1336-1346.).

This suggests that ADHD might b well expected to covary with reduced general intelligence and lower IQ; and again, the data we have to date from many studies of IQ in persons with ADHD we have to date suggests that this is in fact the case.

In sumary, from the biological perspective, ADHD is is known to be associated with reduced brain volume both overall ,and in particular in critical regions like the prefrontal cortex, reduced myelination, and impaired general functioning of the prefrontal cortex and the Executive Functions, in particular, Working Memory and problem -solving that it mediates. As all of these are factors linked to general intelligence, it seems likely that individuals with the disorder have lower 'G' factors than persons without the disorder.

Last edited by JOHNCG; 06-19-12 at 05:47 AM..
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