1. Intelligence: The Dynamics of Intelligence
Part six
Does intelligence remain stable over a lifetime or does it change?
Are individuals on opposite ends of the spectrum really that different?

2. Intelligence: The Dynamics
By age 4, children’s performance on intelligence tests begins to predict their adolescent & adult scores
High scoring adolescents tend to have been early readers
Intelligence scores become stable after about 7 years of age
In numerous studies, consistency of intelligence scores has been determined to increase with age

3. Intelligence: The Extremes of Intelligence
Part seven

4. Intelligence: The Extremes (Low)
Intellectual Disability/Developmentally Delayed (Mental Retardation)
A condition characterized by limited mental capacity
Indicated by a low test score (<70) and difficulty adapting to the normal demands of independent living; before the age of 18
1-3% of the population
An estimated 6.2 to 7.5 million Americans
Male to female ratio?

5. Intelligence: The Extremes (Low)
The three most common biological causes…
Down Syndrome
A condition of intellectual disability & associated physical disorders
47 chromosomes, as opposed to 46 (an extra 21st)
Fetal Alcohol Syndrome
Fragile X Syndrome
A condition in which a male has a defect in a gene on the X chromosome of the twenty- third pair
Leads to a deficiency in a protein needed for brain development

6. Intelligence: The Extremes (Low)
Other Causes?
Environmental Factors
Lead poisoning
Prenatal exposure to mercury
Poor nutrition
Lack of mental stimulation
Familial Intellectual Disabilities
A developmental delay related to living in poverty; generally produces relatively intellectual disability.

8. Intelligence: The Extremes (High)
Definitions of giftedness (>130) vary considerably
2% of the population is >130 (gifted)
½% of the population > (genius)
Misconceptions about the high extreme
“Early ripe, early rot…”
Socially awkward, physically weak, more likely to suffer from mental illness, etc.
The “mad scientists” or “evil geniuses”

9. Intelligence: The Extremes (High)
Lewis Terman (1921)
Terman’s “Termites”
Longitudinal Study of 1,528 children
IQs ranging from 130 – 200
Findings
Gifted individuals tend to be above average in…
Physical health
Emotional adjustment
Mental health
Social maturity

10. Intelligence: Heredity & Environment
Part eight

11. Intelligence: Heredity & Environment
Early pioneers of intelligence…
Maintained the nature view of intelligence
Today’s nature theorists…
Assert that a child’s intelligence CANNOT be changed; genetic destiny
Many argue that this downplays the value of special education programs for underprivileged groups

12. Intelligence: Heredity & Environment
Today’s nurture theorists…
Assert that traits with a strong genetic component are not necessarily unchangeable
Argue that even MORE funds be allocated for remedial programs, especially in lower- income areas

13. Evidence for Hereditary Influence

14. Intelligence: Heredity & Environment
Family Studies
Can determine only whether genetic influence on a trait is plausible
Twin Studies
The average correlation for identical twins is +0.86
The average correlation for fraternal twins is +0.60
Adoption Studies
Studies indicate that there is more than chance similarity between adopted children and their biological parents

15. Intelligence: Heredity & Environment
Heritability Ratio
An estimate of the proportion of trait variability in a population that is determined by variations in genetic inheritance
Can be estimated for any trait
90% for height
85% for weight
Intelligence?

16. Intelligence: Heredity & Environment
Heritability estimates for intelligence vary considerably
High Estimate: 80%
20% of the variation in intelligence is attributable to environmental factors
Low Estimate: 40%
60% of the variation in intelligence is attributable to environmental factors

17. Intelligence: Heredity & Environment
Limitations of heritability estimates
Heritability is a group statistic; CANNOT be meaningfully applied to individuals
A trait’s heritability may vary from one group to another
“There really is no single fixed value that represents any true, constant value for the heritability of IQ or anything else…”

18. Evidence for Environmental Influence

19. Intelligence: Heredity & Environment
Twin Studies
The average correlation for identical twins raised together is +0.86
The average correlation for identical twins raised apart is +0.72
Adoption Studies
There is some resemblance between adoptive parents and their children
Siblings reared together are more similar in IQ than siblings reared apart (0.48, as opposed to 0.23)

20. Intelligence: Heredity & Environment
Sandra Scarr (1991)
Reaction-Range Model
Heredity sets certain limits on intelligence and environmental factors determine where individuals fall within these limits
Genetic makeup places an upper limit on a person’s IQ
Cannot be exceeded; even when environment is ideal
Genetic makeup places a lower limit on a person’s IQ except in extreme cases