Nature, Nurture, and Intelligence
that it means different things to different people. Not even psychologists agree on a definition. A person might seem to be smart at some things, but not smart at others. Some researchers say intelligence is a single general ability and others say it is a constellation of specific apti- tudes, talents, and skills. In this chapter, we will look at the various ways to define and under- stand intelligence, as well as how it changes over the lifespan.
9.1 Nature, Nurture, and Intelligence We know that intelligence is determined partly by genes that are present at birth. Then, dur- ing childhood, environmental influences build on that foundation to determine a wide range of intellectual pathways. Differences can develop over both the short term and extended peri- ods, since many circumstances affect the way inheritance unfolds (Schwartz & Elonen, 1975; Weinberg, Scarr, & Waldman, 1992). A simple example occurs when children who eat break- fast learn more at school than those who do not (Adolphus, Lawton, & Dye, 2013). In this way, genetics provides the foundation onto which environmental variables build.
Therefore, the multiple effects of heredity and the environment on intelligence offer an excel- lent example of how we study the confluence of nature and nurture. For many years, there was a focus among psychologists to assign percentages to the effects of nature and nurture; most researchers now agree that the range of each of these influences generally falls between 40% and 60% (Comer & Gould, 2012). Although we now give more thought to how nature and nurture interrelate rather than trying to determine which is the stronger factor, we still need to understand them both separately in order to understand how they interact.
Activity Review section 2.5 (the discussion about the experimental method), and then construct an operational definition for “intelligence.” As with other operational definitions, you must iden- tify specific variables that can be measured.
Genetic Influences on Intelligence In order to study the relative influence of nature and nurture, psychologists typically calculate a concordance rate—a statistical probability that traits will be shared. For instance, if you measured the left shoe size of a group of 100 people, the concordance rate for the right shoe is likely to be 1.0 (100%), since people buy pairs of shoes that are the same size. However, because left and right feet are not always the exact same length, the concordance rate for foot size will be something less than 1.0 (though very close). Further, a concordance rate of 0.0 would mean that there is no shared characteristic whatsoever between two variables. For intelligence, the more genetically alike two individuals are, the higher the concordance rates. Similarity, being raised in the same environment also predicts higher concordance rates, as we will see next.
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Section 9.1 Nature, Nurture, and Intelligence
Researchers commonly use studies of pairs of twins, siblings, and other pairs of individuals to explore the relative influences of nature and nurture on individuals. One study at the Univer- sity of Minnesota continues to follow over 8,000 pairs of twins. These studies report that there are many similarities between twins raised in the same household and twins who were separated at birth (usually through adoption) on psychological and physiological evaluations (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990).
In the case of intelligence, Bouchard and McGue (1981) reviewed research from 111 studies worldwide and found that the concordance rate between monozy- gotic (identical) twins reared together (in relatively the same environment) was about 0.85; for monozygotic twins reared separately (different environments), it was about 0.67. The higher concordance rate among monozygotic twins when compared to dizygotic (frater-
nal) twins indicates a genetic contribution of intelligence; however, all sets of twins who are reared together have a higher concordance rate than corresponding groups who are reared apart, demonstrating the contribution of nurture!
Bouchard and McGue (1981) also looked at group differences between monozygotic twins reared together and dizygotic twins reared together. These two sets of siblings are raised in relatively the same environment but differ genetically. Any group differences would thus point to a genetic effect. In this case, differences are again robust, as shown in Figure 9.1. This time differences suggest a strong biological component to intelligence. Moreover, the intel- ligence of adopted children is more similar to their biological parents than to their adoptive parents, further signifying a genetic link to intelligence (Petrill et al., 1998).
The Influence of Culture and Environment on Intelligence Although nature clearly plays a large role in the development of intelligence, the data on twins have shown that a more advantaged environment (nurture) can have a significant positive effect on intelligence (e.g., Duyme, Dumaret, & Tomkiewicz, 1999; Munsinger, 1975; Scarr & Weinberg, 1976; Schiff et al., 1978; van IJzendoorn, Juffer, & Poelhuis, 2005). Duyme et al. (1999) studied the files of 5,003 adopted children who had been abused or neglected during infancy and then adopted when they were between 4 and 6 years of age. Less than 10 years later, the children who had been adopted by families of high-SES families had significantly higher intelligence than those children who were adopted by low-SES families.
Furthermore, a study of adolescent criminal offenders found that young males who were born to single mothers had a higher risk of intellectual impairment than those who were born into two-parent families (Walsh, 1990). And in a well-known transracial study in Minnesota, the researchers concluded that “there is no question that adoption constitutes a massive [envi- ronmental] intervention” (Scarr & Weinberg, 1976). Their study found that black children who were adopted into higher-income white families scored significantly better on tests of intellectual ability, compared to children who were adopted by lower-income black parents (Scarr & Weinberg, 1976; Weinberg, Scarr, & Waldman, 1992). In this instance, the economic advantages of the white families were clearly the overriding factor; the “social variables [not race] accounted for a substantial portion” of the variance (Scarr & Weinberg, 1976).