Having examined the construct of disability, it is important to look at what
intelligence is seen to be. This section is summarised from a useful chapter by Carr, Linehan, O’Reilly, Walsh, and McEvoy (2016, p. 81-204).4
2.3.1.1. Evolutionary models.
Based on Darwin’s ideas, human cognitive abilities have been developing over time in order to adapt to the environment. Donald (1991, cited in Carr et al., 2016, p. 168)
developed a model which involves three transitions. The first is to mimetic culture which is a form of cognitively mediated representation which is pre-linguistic, “the ability to produce conscious, self-initiated, representational acts which are intentional but not linguistic”. He envisions it including body posture, facial expression, gesture, vocal tone and hand signals. It is understood collectively and is intentional and representational. This led to group
representational acts which in turn became cultural, including dance and ritual and which Donald hypothesises led to tool making and the use of fire, coordinated hunting in specific seasons, adaptation and a more complex social structure.
The second is that of mythic culture which involved symbolic representation and expression leading to language which in turn led to causative explanation about how the world worked along with prediction and control.
The third transition he describes is that to theoretic culture, a visuo-graphic system, i.e., written language. This also mediates an external memory system not dependent on a
4 The authors in the section are referenced as cited in Carr, Linehan, O’Reilly, Walsh, & McEvoy, J. (2016). I closely follow their very useful summary which they present in The
Handbook of Intellectual Disability and Clinical Psychology Practice (2016). For the most
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biological internal system. With this emerged the development of theoretical rather than mythical thinking, based on arguments, systematic classification, induction, deduction, verification and formal methods of measurement and evidence. Mithen (1996 cited in Carr et al., 2016) describes the evolutionary development of technological intelligence, natural history intelligence, social intelligence, linguistic intelligence and a fluid, flexible meta- representational ability.
2.3.1.2. Psychometric models.
Charles Spearman (1927 cited in Carr et al., 2016) can be considered a founding thinker in terms of psychometric models of intelligence. He observed that there seemed to be a general factor “g” which influenced a person’s performance of mental tasks in combination with other lower order factors which he referred to as “s” and proposed a psychological construct of general intelligence which differs from person to person which, together with a neurologically based mental energy, is employed in intellectual tasks.
Thurstone (1938 cited in Carr et al., 2016) favoured the separation of abilities into a number of primary mental abilities. This was further developed by Cattell (1943 cited in Carr et al., 2016) and Horn (1986 cited in Carr et al., 2016) positing that “g” is comprised of two distinct types of intelligence: fluid ability (Gf), which draws on biological neurological resources, and crystallised ability (Gc), drawing on knowledge accumulated and built within the persons unique and particular cultural context.
Drawing on findings from psychometric evidence, developmental psychology, neuropsychology, behavioural-genetics, scholastic and occupational achievement, studies of aging and neurological functioning, Horn and Blankstone (2005 cited in Carr et al., 2016) describe nine separate mental abilities within the Cattell-Horn model which they cluster into three categories. They consider this a work in progress.
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1. Vulnerable abilities: Fluid intelligence (Gf) Short term memory (Gsm) Processing speed (Gs)
Correct decision speed (CDS) 2. Expertise abilities: Crystallised intelligence (Gc)
Quantitative thinking
Fluency of retrieval from long-term memory (Glr) 3. Sensory Perceptual Abilities: Visual thinking (Gv)
Auditory Thinking (Ga) Carroll (1993 cited in Carr et al., 2016) reviewed 467 data sets published between 1927 and 1987 in order to develop an evidence based model of intelligence. He proposed three stratum of intelligence. The first strata comprised of 66 specific aspects of cognitive functioning, the second strata consisted of eight factors into which these specific aspects accumulated (Fluid Intelligence, Crystallised Intelligence, General Memory and Learning, Broad Visual Perception, Broad Auditory Perception, Broad Retrieval Ability, Broad Cognitive Speediness and Processing Speed). The final strata was the general intelligence factor “g”, an accumulation of the eight factors.
A consensus view has developed known as the Cattell-Horn-Carroll model (CHC) with a divergence as to whether “g” represents something real or is a theoretical distillate. The question remains if these are an accurate reflection of the cognitive processes of the brain or simply commonalities of the intelligence tests used, or reflect the statistical methods utilised to organise data in a particular manner.
2.3.1.3. Genetic inheritance.
Plomin et al. (2013 cited in Carr et al., 2016) look at the genetic heritability of intelligence and its interaction with the environment. Studies thus far have failed to identify
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specific points of an active gene that contribute to intelligence, but the direction of future research is Genome-wide Complex Trait analysis involving many thousands of single- nucleotide polymorphisms (Plomin & Deary, 2015 cited in Carr et al., 2016).
2.3.1.4. Neurobiology of intelligence.
The other area of research interest is the neurobiology of intelligence. Input is through the visual and auditory pathways fed through the parietal cortex to the frontal cortex with the whole process reliant on white matter to transmit information from sensory area to processing area to response selection and output (Jung & Haier, 2007 cited in Carr et al., 2016).
2.3.1.5. Intelligence vs. cognition.
Luckasson and Schalock (2013) differentiate between intelligence and cognition. They define intelligence as “a general mental capability that includes reasoning, planning, solving problems, thinking abstractly, comprehending complex ideas, learning quickly and learning from experience” (p. 96). They further describe intellectual functioning as an application of this intellectual capability to human functioning and living. Cognition, they argue, is not a synonym for intelligence. It involves acquiring, interpreting and appraising knowledge. Cognitive functioning is often a term used in relation to brain injury.
The concept of intelligence continues to be debated, with a growing understanding of the complexity and variety of processes involved.