2.10.5 Intelligence and learning disability

General intelligence

Intelligence supports reasoning, learning, problem solving and adaptation. Attainment also reflects education, language, opportunity and health.

• Positive manifold is the finding that scores on different cognitive tests correlate positively.

• Spearman's two-factor theory used factor analysis of these correlated scores to give a general factor (g) common to all tests plus a factor specific to each test (s).

• The g factor describes shared variance in scores, not a single brain structure.

• Heritability of intelligence generally rises with age while shared family effects decline.

• Heritability describes variance in a population, not causation in an individual or differences between groups.

Thurstone's primary mental abilities

Thurstone described seven correlated primary abilities. Labels from Cattell and Horn, the Wechsler indices or Galton are not on his list.

Fluid and crystallised intelligence

Cattell and Horn separated two broad abilities.

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The Cattell-Horn-Carroll model

The Cattell-Horn-Carroll (CHC) model is a three-stratum hierarchy, and current Wechsler scales are structured on it.

• Quantitative reasoning is a narrow ability grouped under fluid reasoning, which itself loads on g.

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Gardner and Sternberg

Gardner's intelligences reach beyond reasoning to music, movement and self-knowledge, whereas Sternberg keeps to analytical, creative and practical thinking.

Gardner

• Multiple intelligences include musical, bodily-kinaesthetic, interpersonal and intrapersonal domains.

• Intrapersonal intelligence is knowing and regulating one's own emotions, motives and behaviour.

• Emotional intelligence is a separate model from Salovey and Mayer, popularised by Goleman.

• The main scientific objection is that the intelligences overlap and are not empirically separable, since tests of them correlate with one another and with general ability.

Sternberg

Sternberg's triarchic theory of successful intelligence balances three abilities in a sociocultural context.

• Conventional intelligence quotient (IQ) tests and academic examinations capture mainly analytical intelligence and miss creative and practical intelligence.

Measuring IQ

• Adult IQ is a deviation score from an age-normed instrument such as the Wechsler Adult Intelligence Scale (WAIS), not the obsolete ratio formula.

• IQ is normed to a symmetrical normal curve with mean 100 and standard deviation (SD) 15, so mean and median coincide and about half the population scores above 100.

• About two-thirds of people score 85 to 115 and about 2% score below 70.

• Reliability is good for full-scale and index scores but poor for single subtests, so report a confidence interval near thresholds.

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The Flynn effect

• The Flynn effect is the secular rise in mean IQ across generations of whole populations, so old norms become obsolete and tests need re-norming.

• An ageing test flatters current ability: a lower IQ on a newer edition with unchanged ability means outdated norms inflated the earlier score.

The Wechsler scales

• The adult scale starts at 16, not 18.

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• Diagnosing intellectual disability needs the full age-normed scale (WISC-V for 6 to 16, WAIS-IV from 16), not a screening estimate such as the WASI-II.

WAIS-IV indices

Earlier editions split ability into a verbal IQ and a performance IQ that combined into a full-scale IQ. The WAIS-IV replaced them with four indices that combine into a Full Scale IQ.

• The General Ability Index is built from Verbal Comprehension and Perceptual Reasoning only, so it is the better reasoning estimate when working memory and processing speed are disproportionately low.

• A Full Scale IQ below the General Ability Index points to a lowered Working Memory or Processing Speed Index.

• The Processing Speed Index is disproportionately lowered by the psychomotor slowing of depression.

Hold and don't-hold subtests

• Hold subtests draw on old, overlearned knowledge and survive brain damage and ageing.

• Don't-hold subtests, such as Digit Symbol, need speed, working memory and new learning, and decline early.

• A current IQ within one SD of the mean (85 to 115) can still represent decline when set against a reading-based premorbid estimate.

Intellectual disability

Intellectual disability needs three joint criteria:

1. Intellectual functioning is well below average, typically around two SDs below the mean.

2. The person has deficits in adaptive functioning.

3. Onset is in the developmental period.

• UK "learning disability" means intellectual disability, whereas "learning difficulty" often means a specific difficulty such as dyslexia.

• Prevalence is about 1 to 2%.

Severity

ICD-11 (International Classification of Diseases, 11th revision) grades a disorder of intellectual development using both intellectual functioning and adaptive behaviour.

• Borderline intellectual functioning (IQ about 70 to 84) lies above the disability range.

• Bands are told apart by the level of language and literacy and the support needed for self-care, work and independent living.

• Mild intellectual disability means largely independent daily living, and many people work.

• IQ alone does not define severity or support needs, and measurement uncertainty must be allowed for.

Adaptive functioning

• Adaptive functioning is assessed through informants and everyday performance, and vulnerability to exploitation is assessed directly.

Causes

Identifiable biological causes are commoner in severe and profound disability, while mild disability often reflects interacting genetic and environmental influences.

Mental health

• Psychiatric disorders are commoner, and epilepsy risk rises with severity.

• Diagnostic overshadowing is misattributing new symptoms to the disability itself.

• Depression may present as lost skills, withdrawal, irritability or self-injury.