7: Neurodevelopmental Disorders

Evolutionary degeneracy theory

A 19th-century belief that intellectual and social challenges in children with IDD reflected regression to an earlier stage of human evolution; they were seen as the “missing link” between humans and lower forms.

Eugenics movement (1910)

Movement promoting societal protection from those with IDD; labeled mildly impaired individuals as “morons.”

Neurodevelopmental disorders

Lifelong disorders that have an onset at birth affecting the development of the brain and nervous system, leading to difficulties with learning, behaviour and social skills.

Genetic contribution to intelligence

Genetic factors account for 40–60% of variance in generalized intelligence.

Brain size correlation

Correlation between brain size and generalized intelligence is approximately .45.

Brain efficiency and intelligence

Individuals with higher intelligence show more efficient brain functioning.

Intelligence conceptualization

Defined in terms of an Intelligence Quotient (IQ).

Adaptive functioning

How effectively an individual copes with ordinary life demands and lives independently according to community standards.

General intellectual functioning

Measured by IQ tests that are standardized and individually administered.

IQ stability

IQ is relatively stable after early childhood; mean = 100, SD = 15.

Environmental influence on IQ

Mental ability can be modified by environment; SES explains variance in IQ.

Flynn effect

Average IQ scores rise ~3 points per decade; tests must be updated to reflect increasing intelligence.

IQ concerns

Lower mean IQ scores reported for Black individuals; debate over 80% heritability; environment remains crucial.

Multiple intelligences (Gardner)

Framework proposing several types of intelligence; rejects traditional IQ testing.

Diagnostic criteria for IDD

Deficits in intellectual and adaptive functioning evident during childhood or adolescence. Concurrent (regardless of environment) and evident during childhood or adolescence.

Degrees of IDD severity (DSM-5)

Mild, Moderate, Severe, Profound.

Degrees of IDD support (AAIDD)

Intermittent, Limited, Extensive, Pervasive.

Mild IDD

85% of cases; difficulties in academics, abstract thinking, executive functioning; immature social interactions; age-appropriate self-care but needs help with complex tasks.

Moderate IDD

10% of cases; slow academic skills; basic language; needs ongoing support for communication, personal care, and work; often benefits from group homes.

Severe IDD

4% of cases; minimal understanding of written language/time/money; limited speech; needs help with hygiene and self-care; requires full-time support.

Profound IDD

2% of cases; aware only of physical world; minimal communication; dependent on others for all tasks; often co-occurring motor/sensory impairments.

Prenatal risk factors (biomedical)

Genetic/chromosomal disorders, metabolic issues, maternal illness, parental age.

Prenatal risk factors (social/behavioral)

Poverty, malnutrition, domestic violence, substance use, lack of care or parental maturity.

Perinatal risk factors

Biomedical: Prematurity, birth injury, neonatal disorders, Social: parental rejection, Educational: missed medical follow-up.

Postnatal risk factors

Biomedical: TBI, malnutrition, infections, Social: poverty, neglect, abuse, poor stimulation,Educational:M delayed diagnosis, poor education/support.

IDD causes

2/3 of moderate to profoudn have genetic or environmental cause, causes for mild only ¼ are known.

Organic cuases two group approach

Organic intellectual deficit and cultural-familial intellectual deficit

Organic intellectual deficit

IDD caused by chromosome abnormalities, single-gene conditions, or neurobiological factors. Associated with severe and profound.

• 1/3 to ½ of people with IDD

Cultural-familial intellectual deficit

No clear organic basis and associated with mild IDD; family history of intellectual deficit, economic deprivation, inadequate child care, poor nutrition and parental psychopathology.

Heritability of intelligence

Estimated at 50%; environment strongly influences outcomes, especially for disadvantaged children.

Genotype

Collection of genes influencing intelligence.

Phenotype

Expression of genotype shaped by environmental interactions.

Heritability (definition)

Proportion of trait variation in a population attributable to genetic factors.

Most common cause of moderate to severe IDD

Chromosome abnormalities

Down syndrome

Extra 21st chromosome; ~1 in 1,000 births; caused by nondisjunction (21st pair of mothers chromosome doesn’t separate during meiosis); risk increases with maternal age. Have moderate IDD, equivalent to 2-3 year odl. 

Fragile X syndrome

Most common inherited IDD from FMR-1 gene; 1 in 4,000 males / 1 in 8,000 females; mild to moderate IDD shyness, poor eye contact, repetitive behaviors.

• 1/3 to 1/2 of females who carry and transmit disorder are also affected with a variant of the syndrome.

• 20% of males with FMR1 transmit but not affected themselves.

Prader-Willi syndrome

Abnormal chromosome 15; intellectual deficits, short stature, constant hunger, low muscle tone. 5-10 out of 10 000 births; rare.

Phenylketonuria (PKU)

Single gene condition, Recessive gene disorder; lack of enzyme converting phenylalanine to tyrosine; buildup causes brain damage and IDD (errors of metabolism) 1 in 15000. Can be treated by environmental changes.

Angelman syndrome

Chromosome 15 disorder; moderate to severe IDD, jerky movements, hand flapping, seizures, absent speech.

Neurobiological causes

• Adverse biological conditions

• Infancy and childhood experiences

• Prenatal alcohol exposure

Fetal alcohol spectrum disorder (FASD)

Caused by prenatal alcohol exposure; includes CNS dysfunction and developmental deficits.

Fetal alcohol syndrome (FAS)

Most severe form of FASD; growth retardation, facial anomalies, IDD, ADHD-like symptoms; 6–9 per 1,000 school-aged children.

• Leading known cause of IDD

Social-psychological factors

Deprivation of care and stimulation; parental abuse or neglect; accounts for 15–20% of IDD cases.

Prevalence of IDD

1–3% of population; 1.5–2× more common in males; higher in lower SES and minority groups. 1.6 M to 1 F

Why are there SES differences?

Less access to quality education, home environment, economic deprivation

Developmental vs Difference Controversy

Do all children—regardless of intellectual impairments—progress through the same developmental milestones ina similar sequence, but at different rates? Or do children with intellectual developmental disorder develop in a different, less sequential, and less organized fashion?

Developmental position better aligns with…

familial intellectual developmental disorder as  they follow developmental stages in an invariant order, the same as children with typical intellectual abilities, with the possible exception of some children with co-occurring brain differences or autism 

Difference positon better aligns with 

organic developmental disorder

2 approaches of the developmental position

Similar sequence and similar structure

Similar sequence hypothesis

Children with IDD follow same developmental stages as peers but at slower rates or lower limits.

Similar structure hypothesis

Children with IDD use the same cognitive processes as typically developing peers at similar cognitive levels.

Difference position

Cognitive development differs qualitatively, not just quantitatively, from typically developing peers.

Language and social development in IDD

Weak expressive language (especially in Down syndrome); delayed self-recognition; fewer signals for attachment.

Emotional/behavioral problems in IDD

4× higher than general population; includes impulse control, anxiety, mood, ADHD symptoms, pica, and self-injury.

Anxiety and IDD

Higher anxiety rates; elevated separation anxiety at age 5; often co-occurs with externalizing behaviors.

Health in IDD

Higher chronic illness rates; life expectancy in Down syndrome ≈ 60 years; cognitive decline common after 40.

Prevention and education

Prenatal education, genetic screening, ultrasounds, and awareness about substances during pregnancy.

Psychosocial treatments

Child-focused early intervention (ideally preschool years); behavioural techniques, cognitive behavioural and family oriented strategies.

Carolina Abecedarian Project

Early intervention for low-income children; led to reduced IDD rates and higher academic achievement.

Behaviour therapy and CBT for IDD

Target antecedents, consequences, or skill acquisition; interventions tailored to individual and environmental needs.

Communication interventions

Use of simplified, structured adult language to enhance understanding.

Routines and choices

Structured schedules with opportunities for personal choice increase engagement and independence.