Introduction to Abnormal Child and Adolescent Psychology. Robert Weis
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individuals, ranging from 1.0 (i.e., perfect similarity) to 0 (i.e., no similarity). The mean correlation between two biological siblings’ IQ scores is approximately .45, whereas the mean correlation between two unrelated children’s IQ scores is only about .27. These findings suggest that genetic factors play a role in children’s intelligence.
The primary limitation of family studies is that they do not adequately control for environmental effects. Although it is true that biological relatives share similar genes, they also usually live in similar environments. Most family members share the same house, live in the same neighborhood, and come from similar socioeconomic and cultural backgrounds. Therefore, when family studies indicate that closely related relatives are more likely to have a disorder than more distant relatives, we cannot determine whether this similarity is due to common genes or similar environments.
To tease apart the relative effects of genes and environment on behavior, behavioral geneticists conduct adoption studies. In an adoption study, researchers examine children who were separated from their biological families shortly after birth. If a behavioral attribute is influenced by genetics, we would expect children to show greater similarity to their biological relatives than to their adoptive relatives.
Figure 2.2 ■ The Effects of Genes and Environment on Intelligence
Note: Behavioral geneticists use family, adoption, and twin studies to estimate the heritability of intelligence. Based on Sattler (2019).
For example, the mean correlation between parents and their biological children’s IQ scores is approximately .40. In contrast, the mean correlation between parents and their adoptive children’s IQ scores is only .25. Because children show greater similarity to their biological parents than their adoptive parents, we can conclude that genetic factors play unique roles in the development of children’s intelligence.
The primary weakness of adoption studies is that parents who adopt children are often not typical of parents in the general population. Adoption agencies carefully screen prospective adoptive parents before placing a child in their custody. Consequently, adoptive parents are less likely to have mental health problems and are more likely to have higher income and educational backgrounds than other parents. Furthermore, parents who offer their children for adoption often have higher rates of mental illness and come from more disadvantaged backgrounds than parents in the general population. These differences between biological and adoptive families may partially account for the greater similarity between children and their biological parents compared to their adoptive parents.
A third way that behavioral geneticists identify the relative contributions of genes and environment on behavior is by conducting a twin study. In a twin study, researchers compare the concordance between monozygotic (MZ; identical) and dizygotic (DZ; fraternal) twins. MZ twins are the products of the same egg and sperm cell; consequently, they have a 100% genetic similarity. DZ twins are the products of different egg and sperm cells; consequently, they share only 50% of their genes, like other biological siblings. The correlation between IQ scores for MZ twins is .85, whereas the correlation for DZ twins is only .55. The higher concordance for MZ twins than DZ twins indicates that intelligence is at least partially genetically determined.
In some cases, twin and adoption studies are combined by examining twins who both live with their biological parents (e.g., the light bars in Figure 2.2) and twins separated at birth (e.g., the dark bars in Figure 2.2). For example, the mean correlation in IQ for MZ twins reared together is .85, whereas the mean correlation for MZ twins reared apart is also very high: .75. The high correlations for twins reared together or apart indicate that genetic factors play important roles in the development of intelligence. Even twins separated shortly after birth have remarkably similar IQs.
Behavioral geneticists often divide environmental influences into two types: shared environmental factors and nonshared environmental factors. Shared environmental factors are experiences common to siblings. For example, siblings usually are reared by the same parents, grow up in the same house, attend the same schools, and belong to the same church. Shared environmental experiences make siblings more alike. In contrast, nonshared environmental factors are experiences that differ among siblings. For example, siblings may have different friends, play different sports, or enjoy different subjects in school. Siblings may also have different types of relationships with their parents. These nonshared environmental factors often account for more of the variance in children’s behavior than do shared experiences. Nonshared environmental factors help to explain why siblings can be so different even though they grow up in the same home (Plomin, DeFries, Knopik, & Neiderhiser, 2017).
Molecular Genetics
Another way to study the effects of genes on behavior is to examine children’s genes at the molecular (rather than the behavioral) level. Recent advances in our knowledge of the human genome and in gene research technology have allowed scientists to search for specific genes that might be partially responsible for certain disorders (Kornilov & Grigorenko, 2016).
Recall that in neurotypical individuals, genes show natural variation, called alleles. Molecular genetics is the scientific field in which researchers attempt to link the presence of specific alleles with certain attributes, behaviors, or disorders. One way to identify which alleles might be responsible for specific disorders is to conduct a linkage study. In a linkage study, researchers search the entire genetic structure of individuals (i.e., perform a “genome scan”), looking for the presence of certain alleles and the existence of a specific disorder. If researchers find certain alleles in individuals with the disorder and do not find these alleles in people without the disorder, they hypothesize that the allele is partially responsible for the disorder (Schulze & McMahon, 2019).
Researchers tend to use linkage studies when they do not know exactly where to look for genes responsible for the disorder. Given the magnitude of the human genome, it is difficult to identify links between certain alleles and specific disorders. However, researchers have successfully used linkage studies to identify alleles responsible for disorders caused by single genes, such as Huntington’s disease. Linkage studies have been less successful in identifying the causes of disorders that depend on the presence or absence of multiple genes.
An alternative technique is to conduct an association study. In an association study, researchers select a specific gene that they believe might play a role in the emergence of a disorder. Then, they examine whether there is an association between a particular allele of this “candidate” gene and the disorder (Jaffee, 2016).
For example, researchers hypothesized that a specific gene, which affects the neurotransmitter dopamine, might play a role in the development of attention-deficit/hyperactivity disorder (ADHD). They suspected this particular gene because abnormalities in dopamine have been identified as a specific cause for ADHD. Furthermore, medications that affect dopamine in the brain can reduce ADHD symptoms. The researchers identified a group of children with and without ADHD. Then, they examined whether the two groups of children had different alleles for the candidate gene. The researchers found that a certain allele for this gene was much more common among youths with ADHD compared to youth without the disorder. Consequently, they concluded that the gene may be partially responsible for ADHD (Langley, 2019).
Of course, molecular genetics research is much more complicated than has been described here. Nearly all mental disorders are influenced by multiple genes; there is almost never a one-to-one relationship between the presence of a specific allele and the emergence of a given disorder. Furthermore, genes never affect behavior directly; their influence on behavior is always influenced by environmental experience (Kornilov & Grigorenko, 2016).
Review
Genes come in different variants, called alleles. The alleles we inherit from our parents can influence our physical attributes (e.g., hair and eye color) as well as our risk