The NatureNurture Debate

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The Nature-Nurture Debate
Approaches to Psychology Lecture 26, The
Nature-Nurture Debate
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Lecture contents

• Group differences in IQ.
• Population variation in IQ Heritability and the
  environment.
• What can and cannot be inferred from heritablity
  indices.
• Culture and intelligence testing.
• Modifying IQ scores.
• My tentative conclusions.

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The U.S. Immigration Act of 1924

• Rigid quotas to restrict the influx of
  biologically weaker stock from southern and
  eastern Europe.
• The proof? Members of these groups had lower Army
  Intelligence Scores than obtained by Northern
  European-Americans.
• These scores turned out to correlate with length
  of time in the States, improving with English
  language proficiency and AUS cultural knowledge.
• After years of residency, European are of origin
  had no discernible effect.
• A seemingly hereditary difference was revealed as
  due to environmental factors related to genetic
  ones.

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Group differences

• Numerous studies have found that average IQ for
  U.S. blacks is 85-90, i.e., 10 to 15 points lower
  than for U.S. whites.
• The dispute revolves around why this is so and
  whether much can be done about it.
• The difference in average racial IQs decreases
  proportionate to the extent that environments are
  relevantly matched (e.g., on SES dimensions).
• Also, average IQ for U.S. blacks adopted into
  white middle-class households is 110 (25 points
  higher than average blacks, 10 points higher than
  average whites).
• Thus, differences in group means seem at least
  largely to be due to different environments.
• The rest of the lecture concerns variation in
  individual scores within given populations.

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A distinction and an interaction

• Genotypic traits are genetic potentialities.
• Phenotypic traits are manifest characteristics.
• All phenotypic traits are often claimed to be the
  result of an interaction between the genotype and
  the environment
• There can be no organism without a genotype, and
  this genotype cannot express itself independently
  of the environment (Gleitman et al., 1999, p.
  655)
• Organisms are dependent on the genotype, but are
  all traits so dependent once the organism exists?

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Heritability indices

• a heritability index shows the proportional
  contribution of genetic or heriditary factors to
  the total variance of a particular trait in a
  given population under specific conditions
• Anastasi Urbina (1997, p. 297)
• E.g., A heritability index of .70 means that 70
  of the variance is attributable to hereditary
  factors (with the remainder being attributable to
  the environment).

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How heritability indices are calculated

• Primarily, by examining resemblances among
  genetically related people. E.g.,
• Twin studies
• 1) Compare monozygotic and dizygotic twins.
• 2) Compare siblings (especially monozygotic twins
  reared together and apart.
• - Adoption studies
• Compare adopted children with both biological and
  adopted relatives.

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Correlations between IQ scores for family members
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Difficulties with heritability indices

• Unassessed contributions of environmental
  factors.
• Monozygotic (identical) twins may share more
  similar environments than do dyzygotic
  (fraternal) twins.
• Distal environments can be similar and close ones
  rather different.
• There may be genetic influences on environments.
• Issues of generalisability from twins to
  non-twins.
• E.g., Twins more likely to have prenatal trauma
  leading to severe mental retardation (and
  inclusion of such people raises heritability
  indices).

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Inheriting differences

• Heritability coefficients are estimates of genes
  on individual differences within a population.
  They are not estimates of the relative importance
  of genes and the environment.
• Westen (1998, p. 348)
• They indicate determinants of variances
  (differences) not means (similarities).

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Inheriting differences from differences

• Heritability estimates are highly dependent on
  the sample.
• Westen (1998, p. 348)
• Heritability indices are frequently obtained from
  relatively homogeneous environments but
  relatively genetically diverse people.
• Including a random sample of people from various
  environments would typically lower heritability
  indices.

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Inheriting differences from specific
gene-situation interactions

• Any change in either hereditary or environmental
  conditions would be liable to alter the
  heritability index.
• Anastasi Urbina (1997, p. 298)
• E.g., making either the gene pool or the
  environment more or less homogeneous in ways that
  affect the characteristic of interest (e.g.,
  intelligence).

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Average inheritance effects

• Almost irrespective of the size of a
  heritability index in the population, a
  particular individuals characteristics may be
  almost entirely determined by their genes, their
  environment, or any combination of the two.
• E.g., intelligence is largely hereditary, but any
  particular instance of extraordinarily high or
  low (or even average) intelligence may result
  from extraordinary genetic conditions,
  extraordinary environmental conditions, or both.

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Indicating actualities, not potentials

• Even if the heritability index of a trait in a
  given population is 100, it does not follow that
  the contribution of environment to that trait is
  unimportant.
• Anastasi Urbina (1997, p. 298)
• It simply follows that the environment is having
  no differential effect across individuals in that
  population.

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Cultural bias in IQ tests

• Linguistic familiarity.
• Vocabulary, dialect.
• Content familiarity.
• Information, concepts, tasks
• Medium familiarity.
• Paper and pencil, interview
• Test motivation.
• Every test tends to favour persons from the
  culture in which it was developed
• Anastasi Urbina (1997, p. 342)

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Modifying IQ scores

• IQ is not fixed it is amenable to modification.
• Educationally impoverished environments depress
  IQ.
• Interventions (and other factors) that richen
  such environments increase IQ.
• Worldwide improvements in IQ scores over time 3
  IQ points every 10 years (The Flynn effect).

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My (tentative) conclusions

• Substantially overlapping variation in IQ scores
  across groups makes differential treatment of
  members of those groups an example of prejudice.
• Whatever the difference in the means and the
  reasons for such difference.
• Evidence for genetic differences in average IQ
  across groups is poor.
• (I have encountered no relevant evidence about
  possible current differences in IQ heritability
  across groups.)
• Evidence for environmental differences in average
  IQ across groups is strong.
• Evidence for (current) genetic differences in IQ
  scores within groups is strong.
• Evidence for the ability to influence IQ scores
  environmentally is strong.
• All the above holds for intelligence as well as
  for IQ, only more so.
• Overall conclusion From social science and
  societal perspectives, it makes sense to look for
  both genetic and environmental determinants of
  intellectual and other important functioning.
• There is no compelling evidence that any such
  functioning is genetically determined.
• As a consequence of this, it is better to
  recognize inhibitors and enhancers, and then
  learn how to potentially overcome the former and
  exploit the latter.

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Points to ponder

• Environmental improvements can increase average
  IQ scores. Would you predict that they also
  affect heritability indices for IQ with the
  relevant populations. If not, why not and, if so,
  why (and how)?
• How convincing is Gleitmans argument (p. 662)
  that studies such as Quay (1971) demonstrate that
  cultural bias in IQ tests is evidently not
  responsible for average racial difference in IQ
  scores? Hint content (esp. information) bias.