Behavioral geneticists conduct family studies to compare people who live together and share varying degrees of relatedness. Two kinds of family studies are common: twin studies and adoption studies (Koenen, Amstadter, & Nugent, 2012). Twin studies compare identical and fraternal twins to estimate how much of a trait or behavior is attributable to genes. If genes affect the attribute, identical twins should be more similar than fraternal twins because identical twins share 100% of their genes, whereas fraternal twins share about only 50%. Adoption studies, on the other hand, compare the degree of similarity between adopted children and their biological parents whose genes they share (50%) and their adoptive parents with whom they share no genes. If the adopted children share similarities with their biological parents, even though they were not raised by them, it suggests that the similarities are genetic.
Adoption studies also shed light on the extent to which attributes and behaviors are influenced by the environment. For example, the degree to which two genetically unrelated adopted children reared together are similar speaks to the role of environment. Comparisons of identical twins reared in the same home with those reared in different environments can also illustrate environmental contributions to phenotypes. If identical twins reared together are more similar than those reared apart, an environmental influence can be inferred.
Genetic Influences on Personal Characteristics
Research examining the contribution of genotype and environment to intellectual abilities has found a moderate role for heredity. Twin studies have shown that identical twins consistently have more highly correlated scores than do fraternal twins. For example, a classic study of intelligence in over 10,000 twin pairs showed a correlation of .86 for identical and .60 for fraternal twins (Plomin & Spinath, 2004). Table 2.7 summarizes the results of comparisons of intelligence scores from individuals who share different genetic relationships with each other. Note that correlations for all levels of kin are higher when they are reared together, supporting the role of environment. Average correlations also rise with increases in shared genes.
Genes contribute to many other traits, such as sociability, temperament, emotionality, and susceptibility to various illnesses such as obesity, heart disease and cancer, anxiety, poor mental health, and a propensity to be physically aggressive (Esposito et al., 2017; McRae et al., 2017; Ritz et al., 2017). Yet even traits that are thought to be heavily influenced by genetics can be modified by physical and social interventions. For example, growth, body weight, and body height are largely predicted by genetics, yet environmental circumstances and opportunities influence whether genetic potentials are realized (Dubois et al., 2012; Jelenkovic et al., 2016). Even identical twins who share 100% of their genes are not 100% alike. Those differences are due to the influence of environmental factors, which interact with genes in a variety of ways.
Table 2.7
Source: Adapted from Bouchard and McGue (1981).
Note: MZ = monozygotic; DZ = dizygotic.
a Estimated correlation for individuals sharing neither genes nor environment =.0.
Gene–Environment Interactions
“You two are so different. Edward and Evan, are you sure you’re twins?” kidded Aunt Joan. As fraternal twins, Edward and Evan share 50% of their genes and are reared in the same home. One might expect them to be quite similar, but their similar genes are not the whole story. Genes and the environment work together in complex ways to determine our characteristics, behavior, development, and health (Chabris et al., 2015; Ritz et al., 2017; Rutter, 2012). Gene–environment interactions refer to the dynamic interplay between our genes and our environment. Several principles illustrate these interactions.
Range of Reaction
Everyone has a different genetic makeup and therefore responds to the environment in a unique way. In addition, any one genotype can be expressed in a variety of phenotypes. There is a range of reaction (see Figure 2.8), a wide range of potential expressions of a genetic trait, depending on environmental opportunities and constraints (Gottlieb, 2000, 2007). For example, consider height. Height is largely a function of genetics, yet an individual may show a range of sizes depending on environment and behavior. Suppose that a child is born to two very tall parents. She may have the genes to be tall, but unless she has adequate nutrition, she will not fulfill her genetic potential for height. In societies in which nutrition has improved dramatically over a generation, it is common for children to tower over their parents. The enhanced environmental opportunities, in this case nutrition, enabled the children to fulfill their genetic potential for height. Therefore, a genotype sets boundaries on the range of possible phenotypes, but the phenotypes ultimately displayed vary in response to different environments (Manuck & McCaffery, 2014). In this way, genetics sets the range of development outcomes and the environment influences where, within the range, that person will fall.
Figure 2.8 Range of Reaction
Source: Adapted from Gottlieb (2007).
Canalization
Some traits illustrate a wide reaction range. Others are examples of canalization, in which heredity narrows the range of development to only one or a few outcomes. Canalized traits are biologically programmed, and only powerful environmental forces can change their developmental path (Flatt, 2005; Posadas & Carthew, 2014; Waddington, 1971). For example, infants follow an age-related sequence of motor development, from crawling, to walking, to running. Around the world, most infants walk at about 12 months of age. Generally, only extreme experiences or changes in the environment can prevent this developmental sequence from occurring. For example, children reared in impoverished international orphanages and exposed to extreme environmental deprivation demonstrated delayed motor development, with infants walking 5 months to a year later than expected (Chaibal, Bennett, Rattanathanthong, & Siritaratiwat, 2016; Miller, Tseng, Tirella, Chan, & Feig, 2008).
Motor development is not entirely canalized, however, because some minor changes in the environment can subtly alter its pace and timing. For example, practice facilitates stepping movements in young infants, prevents the disappearance of stepping movements in the early months of life, and leads to an earlier onset of walking (Adolph & Franchak, 2017; Ulrich, Lloyd, Tiernan, Looper, & Angulo-Barroso, 2008).
These observations demonstrate that even highly canalized traits, such as motor development, which largely unfolds via maturation, can be subtly influenced by contextual factors.
Gene–Environment Correlations
Heredity and environment are each powerful influences on development. Not only do they interact, but heredity and environmental factors also are often correlated with each other (Plomin et al., 2016; Scarr & McCartney, 1983). Gene–environment correlation refers to the idea that many of our traits are supported by both our genes and environment (Lynch, 2016). That is, genes give rise to behaviors, which are associated with the environment (Knafo & Jaffee, 2013). There are three types of gene–environment correlations—passive, reactive, and activ—as shown in Figure 2.9.
Figure 2.9 Gene–Environment Correlation
The availability of instruments in the home