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Heredity and Environment in Intelligence. Supplementary Lecture. 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

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evidence for heredity
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

2. Adoption studies

When researchers obtain IQ scores for adopted children and for both their biological and adoptive parents, they typically find that the children’s 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).

evidence for environment
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.

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.

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).

4. Home environment

Developmentalists have identified several aspects of the home environment that are correlated with a child’s 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 theirchildren)

• 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.)

5. Early intervention

When researchers actually change the early environments of children—when they conduct experimental studies—we 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 child’s environment from an impoverished one to an enriched, stimulating one can lead to long-lasting gains in intelligence—gains of up to 20 or 25 points in measured IQ score (Bloom, 1964; Capron & Duyme, 1989; Scarr & Weinberg, 1976; Skeels, 1966; Zigler & Seitz, 1982).

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).