Heredity and Environment in Intelligence

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Heredity and Environment in Intelligence

• Supplementary Lecture

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Evidence for Heredity 

• 1. Twin studies
• A review of twin studies by Bouchard and McGue
  (1981) finds these median correlations
• Correlations of twins IQs
• Identical twins raised in the same home .86
• Identical twins raised in different homes .72
• Fraternal twins raised in the same home .60

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• 2. Adoption studies
• When researchers obtain IQ scores for adopted
  children and for both their biological and
  adoptive parents, they typically find that the
  childrens IQ scores are more highly correlated
  with the scores of the biological parents who
  gave them up for adoption than with the scores of
  the adoptive parents who have raised them. In
  other words, in a group of people who place their
  infants up for adoption, those with the highest
  IQs tend to have children who, despite being
  raised by different parents, also have the
  highest IQs. Such evidence tells us once again
  that heredity is an important factor in
  intelligence (Clarke-Stewart, 1988 Plomin,
  Fulker, Corley, DeFries, 1997).

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Evidence for Environment

• 1. Twin studies
• Look again at the correlations for identical
  twins presented earlier
• Correlations of twins IQs
• Identical twins raised in the same home .86
• Identical twins raised in different homes .72
• Twins raised in separate homes are less similar
  than those raised in the same home. Home
  environment, then, does make a difference.

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• 2. Adoption studies
• Adoption studies show evidence of environmental
  influences as well (Capron Duyme, 1989 Devlin,
  Fienberg, Resnick, Roeder, 1995 Scarr
  Weinberg, 1976).
• Example In a study by Scarr and Weinberg
  (1976), some children of poor parents were
  adopted by parents with average IQs of 118-121.
  Others remained with their biological parents.
  Average IQs of the children in the two groups
  were as follows
• Average IQs
• Adopted children 105
• Nonadopted children 90
• Thus, environment accounted for a 15-point
  difference in intelligence test scores.

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• 3. Nutrition
• What and how regularly children are fed are
  aspects of the environment that have long-term
  effects on intellectual development.
• Example When pregnant women are malnourished,
  the brain cells of their developing fetuses are
  smaller in size and fewer in number (Berk, 1997
  Scott-Jones, 1984).
• Example Severe malnutrition, either prior to
  birth or during the early years of life, is
  associated with lower IQ scores, poorer attention
  and memory, and lower school achievement (Galler,
  1984 Lozoff, 1989 Ricciuti, 1993 Scott-Jones,
  1984).

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• 4. Home environment
• Developmentalists have identified several
  aspects of the home environment that are
  correlated with a childs intelligence.
  Following are some home variables consistently
  related to IQ scores
• The variety of stimulation and experiences to
  which children are exposed
• The extent to which parents interact and play
  with their children
• The amount and complexity of verbal
  communication between parents and children (e.g.,
  the extent to which parents use syntactically
  complex sentences, and the extent to which they
  use reasons in explaining things to their
  children)

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• The extent to which toys, puzzles, and reading
  materials are available and are appropriate for
  the age of the children
• The extent to which children are encouraged and
  expected to be independent and to develop new
  skills
• The educational level of parents
• (Bradley Caldwell, 1976a, 1976b, 1981, 1984
  Elardo, Bradley, Caldwell, 1975 Engel, Nechin,
  Arkin, 1975 Hess Holloway, 1984 Honzik,
  1967 McGowan Johnson, 1984.)

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• 5. Early intervention
• When researchers actually change the early
  environments of childrenwhen they conduct
  experimental studieswe find more convincing
  evidence for the effects of environment.
• Example Studies of Head Start and other
  short-term preschool programs indicate that such
  programs produce short-term gains in intelligence
  (Lazar Darlington, 1982 Seitz, Rosenbaum,
  Apfel, 1985 Zigler Finn-Stevenson, 1987).
• Example Studies involving long-term
  intervention indicate that permanently changing a
  childs environment from an impoverished one to
  an enriched, stimulating one can lead to
  long-lasting gains in intelligencegains of up to
  20 or 25 points in measured IQ score (Bloom,
  1964 Capron Duyme, 1989 Scarr Weinberg,
  1976 Skeels, 1966 Zigler Seitz, 1982).

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• 6. Effects of schooling
• A number of research findings converge to
  indicate that schooling influences IQ
• Example When students must start school later
  than they would otherwise for reasons beyond
  their families control, their IQs are about 5
  points lower for every year of delay (Ceci
  Williams, 1997).
• Example When other things are equal, students
  who drop out have lower IQ scores than students
  who remain in school. For every year of high
  school not completed, IQ drops an average of 1.8
  points (Ceci Williams, 1997).
• Example Students IQ scores decline during
  the summer months when they are out of school
  (Ceci Williams, 1997).