href="#fb3_img_img_b6a8f632-e66c-5ca7-9515-6cd8a9c19ab4.jpg"/>
Source: Adapted from Bouchard and McGue (1981).
a Estimated correlation for individuals sharing neither genes nor environment = .0.
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.
Gene–Environment Interactions
We have seen that 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 2.7), 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 2.7 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 powerful influences on development. Not only do they interact, but environmental factors often support hereditary traits (Plomin et al., 2016; Scarr & McCartney, 1983). Gene–environment correlation refers to the finding that many genetically influenced traits tend to be associated with environmental factors that promote their development (Lynch, 2016). That is, genetic traits influence children’s behavior, which is often supported or encouraged by the environment (Knafo & Jaffee, 2013). There are three types of gene–environment correlations—passive, evocative, and active.
Figure 2.8 Gene–Environment Correlation
Parents create homes that reflect their own genotypes. Because parents are genetically similar to their children, the homes that parents create support their own preferences but also correspond to their child’s genotype—an example of a passive gene–environment correlation (Wilkinson, Trzaskowski, Haworth, & Eley, 2013). It is a passive gene–environment correlation because it occurs regardless of the child’s behavior. For example, a parent might provide genes that predispose a child to develop music ability and create a home environment that reflects the parent’s interest and ability in music, which then also happens to support the child’s musical ability, as shown in the top photo in Figure 2.8. This type of gene–environment correlation tends to occur early in life because parents create rearing environments for their infants and young children.
Children naturally evoke responses from others and the environment, just as the environment and the actions of others evoke responses from the individual. In an evocative gene–environment correlation, a child’s genetic traits (e.g., personality characteristics, including openness to experience) influence the social and physical environment, which in turn shape development in ways that support the genetic trait (Burt, 2009; Klahr, Thomas, Hopwood, Klump, & Burt, 2013). For example, active, happy infants tend to receive more adult attention than do passive or moody infants (Deater-Deckard & O’Connor, 2000), and even among infant twins reared in the same family, the more outgoing and happy twin receives more positive attention than does the more subdued twin (Deater-Deckard, 2001). Why? Babies who are cheerful and smile often influence their social world by evoking smiles from others, which in turn supports the tendency to be cheerful. In this way, genotypes influence the physical and social environment to respond in ways that support the genotype. Children who engage in disruptive play tend to later experience problems with peers (Boivin et al., 2013). To return to the music example, a child with a genetic trait for music talent will evoke pleasurable responses (e.g., parental approval) when she plays music; this environmental support, in turn, encourages further development of the child’s musical trait. In addition, individuals vary in their sensitivity to environmental stimuli; some children may be more affected by environmental stimuli due to their genetic makeup (Belsky & Hartman, 2014; Pluess, 2015).
Children also take a hands-on role in shaping their development. Recall from Chapter 1 that a major theme in understanding human development is the finding that individuals are active in their development. Here we have an example of this theme. As children grow older, they have increasing freedom in choosing their own activities and environments. An active gene–environment correlation occurs when the child actively creates experiences and environments that correspond to and influence his genetic predisposition. For example, the child with a genetic