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  • Clearly, IQ is not the only psychological construct for which this debate applies, and instructors may wish to take a moment to name the other areas (i.e., personality, mental disorders, etc.) for which heritability is hotly debated.


  • 1. Nature vs. Nurture in IQ
    • Are differences between people due to environmental or genetic differences?
    • Misunderstanding the question
      • “ Is a person’s intelligence due more to genes or to environment?”
      • no genes = no intelligence
      • both genes & intelligence crucial for any trait
  • 2. Nature vs. Nurture in IQ
    • Rectangle analogy
      • is a rectangle’s area due more to its width or its length?
      • are differences in area among rectangles due more to differences in their length or in their width?
    Group A Group B Length Width Length Width
  • 3. Heritability & Environmentality
    • Heritability
      • degree to which variation in trait stems from genetic, rather than environmental, differences among individuals
    • Environmentality
      • degree to which variation is due to environmental rather than genetic differences
  • 4. The Heritability Coefficient
    • Single number, ranging from 0 to 1.0
    • Represents amount of trait due to genetic differences
    • 0 means no variance due to genetics
    • 1 means all variance due to genetics
    • .30 means 30% is due to genetic differences, 70% due to environmental
  • 5. Twin Studies & Family Influence
    • If trait genetic:
      • closely related more similar than less closely related
    • Many close relatives share environments too
    • Types of studies to separate effects
      • monozygotic twins reared together
      • monozygotic twins reared apart
      • siblings/dizygotic reared together
      • siblings/dizygotic reared apart
      • adoptive siblings reared together
  • 6. Family Influence on IQ
    • Transient influence of family in which you are raised
      • adoptive siblings as children and adults
      • identical vs. fraternal twins reared together
    Fraternal twins Identical twins
  • 7. IQ Differences among Racial/Cultural Groups
    • Find differences among racial & cultural groups on IQ
      • American Blacks score about 15 points lower than Whites
    • WHY?
      • genetics?
        • Witty & Jenkins (1935)
        • No support for genetic differences
      • environment?
  • 8. IQ Differences among Racial/Cultural Groups
    • Each wheat field planted from same package of genetically diverse seeds
    • One field is quite fertile, the other is not
    • Within each field, differences due to genetics
    • Between each field, differences due to environment (fertility)
  • 9. Why differences between Blacks & Whites?
    • Social designation influences
      • autonomous minorities
        • deliberately separate themselves
      • immigrant minorities
        • came to country to better lives
        • see selves as better than those left behind
      • involuntary minorities
        • did not choose minority status
        • routinely judged as inferior by dominant majority
        • cross-cultural findings on IQ scores
  • 10. Historical increase in IQ
    • Improved performance on IQ tests over the years
    • Big increases in tests assessing fluid intelligence, smallest in factual knowledge
    • Increased schooling only accounts for small portion of this increase
    • Due to c hanges in the culture?
    Year IQ Scores
  • 11. Summary
    • Nature vs. nurture debate
      • heritability vs. environmentality
      • studies of twins & family influence
    • Racial & cultural differences in IQ
      • why we can’t interpret differences as due to genetics
      • what we can attribute the differences to
    • Historical increase in IQ scores