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Behavioral genetics is an area of research that examines the relationship between genes and behavior. Behavioral geneticists use three approaches to identify the relative contributions of genetic and environmental influences on development. The first, and simplest, approach is by conducting a family study. In a family study, researchers determine whether a certain characteristic is shared by members of the same family. If the characteristic is partially determined by genetics, biologically related individuals are more likely to share the characteristic than unrelated individuals.

For example, researchers have examined the heritability of children’s intelligence using family studies. If we look at the light bars in Figure 2.2, we see that the correlations of IQ scores are higher among biological relatives than among nonbiological relatives. Behavioral concordance is expressed as the correlation between 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.

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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).