Chapter 74. Studying Intelligence in Twins and Adopted Children

Learning Objectives

correlation
a way of measuring the relationship between two variables
fraternal twins
dizygotic twins that developed from two separate ova and sperm cells, and thus are no more genetically similar than any two non-twin siblings
environment
external influences on development from the physical and social world
gene
segment of a DNA molecule that can synthesize a protein; located on chromosomes
heredity
internal genetic contribution to development, carried in the DNA within each body cell
identical twins
monozygotic twins that developed from a single ovum and sperm cell, and thus share the same genetic material
intelligence
mental capacity to learn from experience and adapt successfully to a particular environment
intelligence test
way of measuring mental abilities; typically produces a numerical score that allows comparison with other people
nature-nurture issue
controversy about the relative impact of heredity (genes) and environment (experience in the world) on a person’s development and behavior
prenatal development
referring to the period of development between conception and birth
Studying Intelligence in Twins and Adopted Children
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Learning Objectives:

Understand how studies of twins and adopted children provide evidence relevant to the nature vs. nurture debate.

Describe the general pattern of intelligence test results from studies of twins and adopted children.

Review

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1. Studies that compare the intelligence test scores of identical twins, fraternal twins, and adopted children with the scores of other family members have been very useful in exploring the influences of nature vs. nurture (heredity vs. environment) on the development of intelligence.

74.0.1 Review

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2. In general, twin and adoption studies support the idea that heredity makes a significant contribution to intelligence. The most genetically similar people have the most similar intelligence test scores. As the genetic similarity of pairs of individuals decreases, the correlation between their intelligence test scores also decreases.

74.0.2 Review

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3. But these studies also provide evidence for influences of the environment on intelligence. The intelligence test scores of fraternal twins reared together are more similar than scores of non-twin siblings reared together.

74.0.3 Review

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4. Although fraternal twins are no more genetically similar than any other siblings, the fact that they are the same age makes their environment (including the prenatal environment) more similar than for different-aged siblings.

74.0.4 Review

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5. Also, the intelligence test scores of identical twins raised apart and reunited in adulthood are less similar than those of identical twins raised together. Even with identical twins, environment plays a role in the development of intelligence.

Practice 1: Evidence for Genetic Factors from Twin Studies

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Practice 1: Evidence for Genetic Factors from Twin Studies

Instructions: Try to predict what the bar graph would look like for identical twins and fraternal twins. Then select the label below each bar to display the graph. What does this pattern of results tell us about genetic influences on intelligence? Once you've considered the question, select the button "What does this tell us?" for an explanation.

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Lower correlation for fraternal twins than identical twins shows genetic effects. The environments in which the two groups were reared were similar, so the difference in the correlation is probably due to the stronger genetic similarity between identical twins.

Practice 2: Evidence for Genetic Factors from Adoption Studies

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Practice 2: Evidence for Genetic Factors from Adoption Studies

Instructions: Try to predict the relative height of the bars on this graph. Then select the label below each bar to display the graph. What does this pattern of results tell us about genetic influences on intelligence? Once you've considered the question, select the button "What does this tell us?" for an explanation.

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Lower correlation for adopted siblings than biological siblings shows a genetic effect.

Lower correlation for adoptive parents than biological parents shows a genetic effect.

Practice 3: Evidence for Environmental Factors from Twin Studies

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Practice 3: Evidence for Environmental Factors from Twin Studies

Instructions: Try to predict the relative height of the bars on this graph. Then select the label below each bar to display the graph. What does this pattern of results tell us about environmental influences on intelligence? Once you've considered the question, select the button "What does this tell us?" for an explanation.

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Lower correlation for identical twins raised apart than identical twins reared together shows an environmental effect.

Lower correlation for non-twin siblings than fraternal twins shows an environmental effect. If genes completely control intelligence, we would expect that fraternal twins and siblings (who have an equal degree of genetic similarity) would show the same degree of similarity in intelligence test scores. But, in fact, fraternal twins (who have a more similar environment) are more similar in intelligence test scores than non-twin siblings.

Practice 4: Evidence for Environmental Factors from Adoption Studies

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Practice 4: Evidence for Environmental Factors from Adoption Studies

Instructions: Try to predict the relative height of the bars on this graph. Then select the label below each bar to display the graph. What does this pattern of results tell us about environmental influences on intelligence? Once you've considered the question, select the button "What does this tell us?" for an explanation.

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Since the degree of genetic similarity is equivalent for both groups, the difference in the strength of correlation is probably due to the more similar environments of adopted children raised in the same house.

Quiz 1

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Quiz 1

Drag each situation label to the gray area below the appropriate bar on the graph. When all the labels have been placed, select the CHECK ANSWER button.

Perhaps you should go back to review the general pattern of intelligence test results from studies of twins and adopted children.
Select the NEXT button and move to Quiz 2.
The image is a bar graph The X axis is missing the labels for the six bars displayed, which decrease in height from left to right. The options for these bars include the following: fraternal twins reared together, unrelated people reared apart, identical twins reared apart, adopted siblings reared together, identical twins reared together, and biological siblings reared together. The Y axis is labeled Intelligence test scores (correlation) and ranges from 0 to 1.00 in increments of 0.10. The bars sit at a 0 correlation before the interaction. The following table represents the data that displays in the graph. The table for graph consists of two columns Bar, Intelligence test scores (correlation). The first row in the first column first, in the second column 0.86, the second row contains second and 0.71, the third row contains third and 0.6, next row contains fourth and 0.46, next row contains fifth and 0.32, last row contains sixth and 0.02.
Fraternal
twins
reared
together
Unrelated
people
reared
apart
Identical
twins
reared
apart
Adopted
siblings
reared
together
Identical
twins
reared
together
Biological
siblings
reared
together

Quiz 2

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Select the NEXT button and move to the Conclusion.
Try to respond to the statements again.

Quiz 2

For each statement, select one of the buttons to indicate whether that statement provides evidence for genetic or environmental influences. When responses have been shown for all the statements, select the CHECK ANSWER button.

Shows genetic
influences		Shows environmental
influences

Fraternal twins reared together have a lower correlation than identical twins reared together.

Identical twins reared apart have a lower correlation than identical twins reared together.

Fraternal twins reared together have a higher correlation than biological non-twin siblings reared together.

Adopted children have a higher correlation with their biological parents than with their adoptive parents.

Conclusion

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Congratulations!
You have completed Studying Intelligence in Twins and Adopted Children