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Chapter 5
Intellectual Developmental Disorder
Intelligence and Intellectual Disability
ID is a neurodevelopmental disorder; a group of conditions with onset in the developmental
period that produce impairments of social, personal, academic, or occupational functioning
ID is characterised by significant limitations in mental abilities that result in impairments in
adaptive functioning
3 essential elements define ID:
o Intellectual limitations
o Deficits in adaptive skills
o Early onset
The Eugenics Scare
Evolutionary degeneracy theory, a pervasive 19th C phenomenon, attributed the intellectual and
social problems of children with intellectual disability to regression to an earlier period in human
evolution
Mental deficiency experts in 19th C believed they had found the missing link between humans and
lower species
J. Langdon H. Down, best known for the description of Down Syndrome, interpreted the “strange
anomalies” as an evolutionary throwback to the Mongol race
o He believed that parents in one racial group might give birth to a child with intellectual
disability who was a “retrogression” to another group
Evolutionary degeneracy theory received growing support by the late-19th C
By 1910, the eugenics movement was gaining momentum
o Eugenics = “the science which deals with all influences that improve the inborn qualities
of a race”
o Emphasis shifted away from the needs of persons with intellectual disability toward a
consideration of the needs of society: society was to be protected from such persons
o Consequently, persons with intellectual disability were blamed for the social ills of the
time
o The characteristics of persons with intellectual disability were considered evidence of
their lack of moral fibre, a belief that led to the diagnostic term moral imbecile or moron
o This concept became a straightforward explanation for acts of deviance and justified
wide-ranging attempts to identify and control such individuals
Defining and Measuring Children’s Intelligence and Adaptive Behaviour
Around 1900, Alfred Binet and Theophile Simon led to some of the first major advancements in
the field of children’s intellectual functioning
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o They were asked to develop a method of identifying schoolchildren who might need
special help in school
o They approached this by developing the first intelligence tests to measure judgement and
reasoning
o The test later became the Stanford-Binet scale
General intellectual functioning is now defined by an intelligence quotient (IQ) which is based on
assessment with 1+ of the standardised, individually administered intelligence tests, such as the
WISC-V, the Stanford-Binet 5, and the Kaufman Assessment Battery for Children 2
o These tests assess various verbal and visual-spatial skills, and mathematical concepts,
which together are presumed to constitute the general construct known as intelligence
IQ scores are derived from a standardised table based on a person’s age and test score
o Approximately 95% of the population has scores within 2 standard deviations of the mean
o Sub average intellectual functioning is defined as an IQ of about 70 or below
Adaptive functioning refers to how effectively individuals cope with ordinary life demands, and
how capable they are of living independently and abiding by community standards
o Conceptual skills
Receptive and expressive language, reading and writing, money concepts, self-
directions
o Social skills
Interpersonal, responsibility, self-esteem, gullibility, naiveté, rule following
o Practical skills
Personal activities of daily activity such as eating, dressing
Instrumental activities of daily living such as preparing meals, taking medication,
etc.
o Occupational skills
Maintaining a safe environment
o Similar to IQ, these skills are typically assessed using standardised instruments such as
the Adaptive Behaviour Assessment System, or the Vineland Adaptive Behavioural Scales
The Controversial IQ
It is tempting to conclude that IQ is innate and fixed since it is relatively stable over time
o On the other hand, if intellectual and cognitive functioning is significantly shaped by
environment, perhaps cognitive growth can be stimulated at an early age and the level of
intellectual disability decreased
Because of its being measured in relation to cohorts, IQ generally is stable from childhood through
adulthood
o For typically developing children, IQ measured prior to the 1st birthday has virtually no
correlation with the IQ score achieved at age 12
HOWEVER, by the time they are 4 years old, the correlation with IQ 12 years
later is high (r = 0.77)
o The picture is dramatically different for infants and children with intellectual disability
At the lower IQ levels, even the youngest infants show IQ stability over time,
with correlations between infant and childhood test scores ranging from 0.50 to
0.97
Research has found a similar pattern of IQ stability from middle childhood to
young adulthood
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Even though the IQ of cognitively delayed infants and children is unlikely to change, proper env
circumstances will help them reach their fullest potential
o Despite its strong genetic component, mental ability is always modified by experience
o Infancy through early childhood offer the most significant opportunity because of the
rapid brain development and response to env stimulation during this time
The importance of genetic makeup for some individuals’ IQ can and does change by 10 – 20 points
between childhood and adolescence
Differences in outcome vary widely in relation to opportunities for each child to learn and develop
Are We Really Getting Smarter?
Scores have risen sharply since beginning of IQ testing – 5-25-point increase in a single generation
When James Flynn brought this phenomenon to attention of scientists in 1987, it became known as
the Flynn Effect
This gain has averaged about 3 points per decade, adding up to more than a full standard
deviation since the 1940s
Once a test is re-normed, the mean is reset to 100, resulting in a brief reversal of this gain in IQ
scores
In attempting to explain the Flynn Effect, considered the rising standards of living, better
nutrition, medical advances, more stimulating environments, and the influence of computer
games and complex toys
IQ tests have once again come under scrutiny, as have children’s exposure to problems similar to
those on the tests
o Yet consistent gains, accompanied by increases in brain mass are too large to be the result
simply of increased test familiarity
Experts on children’s intelligence suspect that the gains reflect a meaningful aspect of intellectual
growth and intelligence. A relatively permissive and child-focused parenting style has emerged
during recent decades, giving children greater facility with language and stronger overall
cognitive capacity
Moreover there are unprecedented cultural differences between successive generations
Possible downside is that test scores drop an average of 5.6 points among persons with borderline
and mild intellectual disability after a test is re-normed, which may have a significant impact on a
child’s eligibility for proper educational placement and other related services
Are IQ Tests Biased or Unfair?
The controversy in IQ score differences is fuelled by researchers who argue that IQ is 80%
heritable and therefore largely genetically determined
Other researchers argue that economic and social inequality are the simplest explanations for
existing group differences in test performance
When personal and family background characteristics are statistically controlled for, children
receive similar test scores
Poverty and inequality are linked to poor nutrition, inadequate prenatal care, fewer intellectual
resources, and similar realities that can have negative effects on children’s developing intelligence
Features of Intellectual Disabilities
IDs encompass perhaps the widest variation in cognitive and behavioural abilities of any
childhood disorder
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