1. Genetic and Environmental Influences on Intelligence
Twin and Adoption Studies
Heritability
Environmental Influences
Group Differences in Intelligence Test Scores
The Question of Bias

2. Genetic and Environmental Influences on Intelligence
No other topic in psychology is so passionately followed as the one that asks the question, “Is intelligence due to genetics or environment?”
Preview Question 10: What does evidence reveal about hereditary and environmental influences on intelligence?
As always, the answer seems to be… a little bit of both. 

3. Genetic Influences on Intelligence
The “Nature” Side of the Argument

4. Genetic Influences
Arthur Jensen (1969) argued that cultural differences in average IQ are largely due to heredity. Studies of twins, family members, and adopted children together support the idea that there is a significant genetic contribution to intelligence (nature).

5. Figure 9.13: Studies of IQ similarity.
The graph shows the mean correlations of IQ scores for people of various types of relationships, as obtained in studies of IQ similarity. Higher correlations indicate greater similarity. The results show that greater genetic similarity is associated with greater similarity in IQ, suggesting that intelligence is partly inherited (compare, for example, the correlations for identical and fraternal twins). However, the results also show that living together is associated with greater IQ similarity, suggesting that intelligence is partly governed by environment (compare, for example, the scores of siblings reared together and reared apart). (Data from McGue et al., 1993; Plomin & Spinath, 2004)
Figure 9.13: Studies of IQ similarity.
The graph shows the mean correlations of IQ scores for people of various types of relationships, as obtained in studies of IQ similarity. Higher correlations indicate greater similarity. The results show that greater genetic similarity is associated with greater similarity in IQ, suggesting that intelligence is partly inherited (compare, for example, the correlations for identical and fraternal twins). However, the results also show that living together is associated with greater IQ similarity, suggesting that intelligence is partly governed by environment (compare, for example, the scores of siblings reared together and reared apart). (Data from McGue et al., 1993; Plomin & Spinath, 2004)

6. Adoption Studies
Adopted children show a marginal correlation in verbal ability to their adopted parents, but actually show more similarity to their birth parents as they get older (nature).

7. Heritability Figure 9.14: The concept of heritability.
A heritability ratio is an estimate of the portion of trait variation in a population determined by heredity—with the remainder presumably determined by environment—as these pie charts illustrate. Typical heritability estimates for intelligence range between a high of 80% and a low of 40%. In recent years, the consensus of the experts seems to hover around 50%. Bear in mind that heritability ratios are estimates and have certain limitations that are discussed in the text.

8. Environmental Influences on Intelligence
The “Nurture” Side of the Argument

9. Evidence for Environmental Influences – Adoption Studies
Adoption studies provide evidence that upbringing plays an important role in mental ability, as adopted children show some IQ resemblance to their foster parents and to their foster siblings.
Also, siblings reared together are more similar in IQ than siblings reared apart. In fact, entirely unrelated children who are reared together show resemblance in IQ.

10. Evidence for Environmental Influences – Deprived Environments
Early neglect from caregivers leads children to develop a lack of personal control over the environment, and it impoverishes their intelligence.
Studies of environmental deprivation (the cumulative deprivation hypothesis) show that children raised in substandard circumstances tend to exhibit a gradual decline in IQ as they grow older.
Conversely, studies of environmental enrichment show that deprived children who are moved to improved environments tend to exhibit increases in IQ.
Romanian orphans with minimal human interaction are delayed in their development.

11. Schooling Effects
Schooling is an experience that pays dividends, which is reflected in intelligence scores. Increased schooling correlates with higher intelligence scores.
To increase readiness for schoolwork,
projects like Head Start facilitate leaning.

12. Evidence for Environmental Influences – The Flynn Effect
The Flynn effect is the trend, all over the developed world, for IQ scores to increase from one generation to the next.
Today’s better prepared populations would outperform populations of the 1930s on intelligence tests.
Generational increases in measured IQ are perplexing, but they must be due to environmental changes.
Therefore, hypotheses for why this occurs focus on environmental variables, as evolution does not operate in a generation.

13. The Interaction of Heredity and Environment – Nature and Nurture at Work
The evidence clearly shows that intelligence is shaped by both heredity and environment and that these influences interact.
Clearly, heredity and environment both influence intelligence. Theorists use the term “reaction range” to refer to genetically determined limits on IQ. The environment determines whether a person will fall at the upper or lower end of their genetically determined range.

14. Figure 9.16: Reaction range.
The concept of reaction range posits that heredity sets limits on one’s intellectual potential (represented by the horizontal bars), while the quality of one’s environment influences where one scores within this range (represented by the dots on the bars). People raised in enriched environments should score near the top of their reaction range, whereas people raised in poor-quality environments should score near the bottom of their range. Genetic limits on IQ can be inferred only indirectly, so theorists aren’t sure whether reaction ranges are narrow (like Ted’s) or wide (like Chris’s). The concept of reaction range can explain how two people with similar genetic potential can be quite different in intelligence (compare Tom and Jack) and how two people reared in environments of similar quality can score quite differently (compare Alice and Jack).
Figure 9.16: Reaction range.
The concept of reaction range posits that heredity sets limits on one’s intellectual potential (represented by the horizontal bars), while the quality of one’s environment influences where one scores within this range (represented by the dots on the bars). People raised in enriched environments should score near the top of their reaction range, whereas people raised in poor-quality environments should score near the bottom of their range. Genetic limits on IQ can be inferred only indirectly, so theorists aren’t sure whether reaction ranges are narrow (like Ted’s) or wide (like Chris’s). The concept of reaction range can explain how two people with similar genetic potential can be quite different in intelligence (compare Tom and Jack) and how two people reared in environments of similar quality can score quite differently (compare Alice and Jack).
Fig. 9-16, p. 356

15. Group Differences In Intelligence Test Scores
Gender Similarities and Differences
Ethnic Similarities and Differences
Why do groups differ in intelligence? How can we make sense of these differences?

16. Gender Differences
Although gender similarities far outnumber gender differences, we find the differences in abilities more interesting. There are nine ways in which males and females differ in various abilities. According to different perspectives, these differences may be explained as evolutionarily adaptive for each gender or the result of social expectations and divergent opportunities.
1. Girls are better spellers
2. Girls are verbally fluent and have large vocabularies
3. Girls are better at remembering and locating objects
4. Girls are more sensitive to touch, taste, and color
5. Boys outnumber girls in counts of underachievement
6. Boys outperform girls at math problem solving, but under-perform at math computation
7. Males perform better at spatial ability tests (rotating objects)
8. Males’ mental ability scores vary more than females – more at both the low extreme and high extreme.
9. Women detect emotions more easily than men do

17. Ethnic Similarities and Differences
To discuss this issue we begin with two disturbing but agreed upon facts:
Racial groups differ in their average intelligence scores.
High-scoring people (and groups) are more likely to attain high levels of education and income.

18. Racial (Group) Differences
If we look at racial differences, for example, white Americans score higher in average intelligence than black Americans (Avery and others, 1994). However, white and black infants tend to score equally well on tests predicting future intelligence. So when and why does this change?
White-Americans
Black-Americans
Average IQ = 100
Average IQ = 85
Hispanic Americans

19. Evidence for Environmental Influences – Group Differences
Even if the heritability of IQ is high, group differences in IQ could be entirely environmental in origin
Some theorists argue that race is a social concept based on perceived differences in appearance not on disparities in genetic makeup
Socioeconomic disadvantage and cultural bias on tests (more on this later) may contribute to cultural differences in IQ

20. The Research of John Ogbu
Ogbu, a Nigerian anthropologist, claims that most minority children grow up believing that life will be restricted to a small and unrewarding set of options. They are convinced that it will be difficult if not impossible to advance in mainstream society.
As a result, Ogbu says, Black children turn their back on school as a possible avenue to a better future. In addition, teachers come to expect less of Black children and tacitly treat them in ways that make the expectation come true.
The poverty that inevitably comes with caste-like status also worsens the chances of educational success. These impoverished children experience stresses from which their more privileged classmates are insulated. When something goes wrong in the family, they are much more likely to carry their problems into their school life and it affects their performance.

21. The Research of John Ogbu
Perhaps most surprising is Ogbu’s study comparing two groups of Black high school students, one doing well in school and the other failing.
The group who fared poorly saw being studious as betraying their racial identity—by “acting White” in the students’ words.
“It’s not that the black children can’t do the work, but they don’t make the effort,” said Ogbu. “The underlying issue for them is one of racial identity. They see doing well and getting a high-status job as selling out. You see the same dynamic among Mexican-American children. They identify achievement with betraying their roots.”
This is the kind of power the environment can have on shaping intelligence scores .

22. Environmental Effects
Differences in intelligence among these groups are largely environmental, as if one environment is more fertile in developing these abilities than the other. Kamin’s cornfield analogy (representing socioeconomic disadvantage), depicts this issue.
Figure 9.17: Genetics and between-group differences on a trait.
Leon Kamin’s analogy (see text) shows how between-group differences on a trait (the average height of corn plants) could be due to environment, even if the trait is largely inherited. The same reasoning can be applied to ethnic group differences in average intelligence.

23. The Question of Bias Tests do discriminate.
But some argue that their sole purpose is to discriminate.
We have to look at the type of discrimination.

24. Intelligence is Culturally Defined
Remember earlier, when we talked about Liberia’s Kpelle tribesmen who “failed” a Western intelligence test, but they actually believed that the Western answer displayed stupidity?
Psychologists who view intelligence as the successful adaptation to the environment are skeptical about the prospects for a “culture-free” test of intelligence.
They maintain that tests designed for one culture are notoriously faulty when applied to another.
Standardized tests typically test for skills and knowledge appropriate to a White, middle‑class environment. According to Kaplan and Saccuzzo (2001), the practice of administering these tests to minorities “is analogous to testing a cat on a task designed to determine how well a rat is adapted to a rat’s environment” (p. 544).

25. See handouts for a deeper explanation of cultural bias in testing, and try out the Chitling Test!

26. “Stereotype Threat”
A self-confirming concern that one will be evaluated based on a negative stereotype. The phenomenon sometimes appears in intelligence testing among African-Americans and among women of all colors.
Claude Steele argues that derogatory stereotypes create feelings of vulnerability in the educational domain, undermining minority group members’ achievement and performance on tests.

27. This is why Barbie is dangerous!
Stereotype Threat – Social Psychology in Action
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Math Class is Tough!
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