Individual Differences

What is Intelligence?

Intelligence is a complex and multifaceted concept with various definitions:

  1. Adaptability: The ability to adapt to new environments (e.g., high school psychology text).

  2. Learning and Understanding: The ability to learn, understand, and deal with new or challenging situations (e.g., Merriam-Webster).

  3. Cognitive Abilities: The capacity to think abstractly, reason, plan, solve problems, and learn from experience (e.g., textbook definition).

  4. General Mental Capacity: A broader and deeper ability to comprehend surroundings, make sense of things, and figure out what to do (e.g., Linda Gottfredson, 1977).

  5. Operational Definition: Intelligence is often measured by IQ tests, with a mean score of 100 and a standard deviation of 15.

The Nature-Nurture Debate

The debate centers on the relative contributions of genetics (nature) and environment (nurture) to individual differences in intelligence and other traits.

Francis Galton and Eugenics

  • Galton's View: Argued that nature (genetics) prevails over nurture (environment) in determining intelligence and other traits.

  • Eugenics: Galton coined the term "eugenics" (meaning "well-born") and advocated for improving human qualities through selective breeding.

    • Positive Eugenics: Encouraging reproduction among individuals with desirable traits.

    • Negative Eugenics: Discouraging reproduction among individuals with undesirable traits.

  • Historical Context: Eugenics gained popularity in the early 20th century but declined after the 1930s due to ethical concerns and its association with discriminatory practices.

Radical Behaviorism

  • John B. Watson: Argued that environment (nurture) is the primary determinant of behavior and intelligence. Famously claimed he could train any child to become any type of specialist, regardless of genetics.

  • Criticism: Radical behaviorism failed to account for genetic influences, leading to a resurgence of interest in genetics in the 1970s.

Twin and Adoption Studies

These studies are used to disentangle the effects of genetics and environment on individual differences.

Types of Twins

  • Monozygotic (MZ) Twins (Identical): Result from a single fertilized egg splitting, sharing 100% of their genes.

  • Dizygotic (DZ) Twins (Fraternal): Result from two separate eggs fertilized by two sperm, sharing ~50% of their genes on average.

Key Findings

  1. Genetic Influence:

    • MZ twins are more similar than DZ twins for both physical traits (e.g., height) and psychological traits (e.g., IQ).

    • This suggests that genetics plays a significant role in individual differences.

  2. Environmental Influence:

    • MZ twins are not perfectly similar, indicating that environment also matters.

    • Adopted siblings (who share environments but not genes) show little similarity in traits like IQ, suggesting that shared rearing environments have limited impact.

  3. Twin Similarity:

    • MZ twins reared together are only slightly more similar than MZ twins reared apart, further emphasizing the importance of genetics over shared environment.

Minnesota Study of Twins Reared Apart (MISTRA)

  • Led by Thomas J. Bouchard Jr.: Studied MZ and DZ twins raised apart to explore genetic and environmental influences.

  • Famous Cases:

    • The Jim Twins: Jim Lewis and Jim Springer, who met at age 39, shared striking similarities in behavior, preferences, and life choices despite being raised apart.

    • The Firefighter Twins: Jerry Level and Mark Newman, who also exhibited remarkable similarities in personality and lifestyle.

  • Conclusions:

    • MZ twins reared apart are very similar, highlighting the importance of genetics.

    • MZ twins reared apart are not perfectly identical, indicating some environmental influence.

    • Shared rearing environments contribute little to twin similarity.

Key Takeaways

  1. Intelligence is a complex trait involving cognitive abilities, adaptability, and problem-solving skills.

  2. The nature-nurture debate explores the relative contributions of genetics and environment to individual differences.

  3. Twin and adoption studies demonstrate that:

    • Genetics plays a significant role in traits like intelligence.

    • Environment also matters, but shared rearing environments have limited impact.

  4. MISTRA provided compelling evidence for the importance of genetics while acknowledging the role of environment.