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Parents create homes that reflect their own genotypes. Because parents are genetically similar to their children, the homes that they create are not only in line with their own interests and preferences but also correspond with the child’s genotyp—an example of a passive gene–environment correlation (Wilkinson, Trzaskowski, Haworth, & Eley, 2013). For example, parents might provide genes that predispose a child to develop music ability and also provide a home environment that supports the development of music ability, such as by playing music in the home and owning musical instruments. This type of gene–environment correlation is seen early in life because children are reared in environments that are created by their parents, who share their genotype.
People 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 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 support the genetic 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, some individuals may be more affected by environmental stimuli due to their genetic makeup (Belsky & Hartman, 2014).
Figure 2.10 Development Stage and Gene–Environment Correlations
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 pattern. 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 trait for interest and ability in music actively seeks experiences and environments that support that trait, such as friends with similar interests and after-school music classes. This tendency to actively seek out experiences and environments compatible and supportive of our genetic tendencies is called niche-picking (Corrigall & Schellenberg, 2015; Scarr & McCartney, 1983).
The strength of passive, evocative, and active gene–environment correlations changes with development, as shown in Figure 2.10 (Scarr, 1992). Passive gene–environment correlations are common at birth as caregivers determine infants’ experiences. Correlations between their genotype and environment tend to occur because their environments are made by genetically similar parents. Evocative gene–environment correlations also occur from birth, as infants’ inborn traits and tendencies influence others, evoking responses that support their own genetic predispositions. In contrast, active gene–environment correlations take place as children grow older and more independent. As they become increasingly capable of controlling parts of their environment, they engage in niche-picking by choosing their own interests and activities, actively shaping their own development. Niche-picking contributes to the differences we see in siblings, including fraternal twins, as they grow older. But identical twins tend to become more similar over time perhaps because they are increasingly able to select the environments that best fit their genetic propensities. As they age, identical twins—even those reared apart—become alike in attitudes, personality, cognitive ability, strength, mental health, and preferences, as well as select similar spouses and best friends (McGue & Christensen, 2013; Plomin et al., 2016; Plomin & Deary, 2015; Rushton & Bons, 2005).
Epigenetic Influences on Development
We have seen that development is influenced by the dynamic interaction of biological and contextual forces. Genes provide a blueprint for development, but phenotypic outcomes, individuals’ characteristics, are not predetermined. Our genes are expressed as different phenotypes in different contexts or situations, known as epigenetics (Moore, 2017). The term epigenetics literally means “above the gene.” The epigenome is a molecule that stretches along the length of DNA and provides instructions to genes, determining how they are expressed, whether they are turned on or off. Epigenetic mechanisms determine how genetic instructions are carried out to determine the phenotype (Lester, Conradt, & Marsit, 2016). At birth, each cell in our body turns on only a fraction of its genes. Genes continue to be turned on and off over the course of development and also in response to the environment (Meaney, 2017). Environmental factors such as toxins, injuries, crowding, diet, and responsive parenting can influence the expression of genetic traits. In this way, even traits that are highly canalized can be influenced by the environment.
For example, consider brain development. Providing an infant with a healthy diet and opportunities to explore the world will support the development of brain cells, governed by genes that are switched on or off. Brain development influences motor development, further supporting the infant’s exploration of the physical and social world, thereby promoting cognitive and social development. Active engagement with the world encourages connections among brain cells. Exposure to toxins or extreme trauma might suppress the activity of some genes, potentially influencing brain development and its cascading effects on motor, cognitive, and social development. In this way, an individual’s neurological capacities are the result of epigenetic interactions among genes and contextual factors that determine his or her phenotype (Lerner & Overton, 2017). These complex interactions are illustrated in Figure 2.11 (Dodge & Rutter, 2011). Interactions between heredity and environment change throughout development as does the role we play in constructing environments that support our genotypes, influence our epigenome, and determine who we become (Lickliter & Witherington, 2017). For a striking example of epigenetics, see the Applying Developmental Science feature.
Figure 2.11 Epigenetic Framework
Source: Gottlieb (2007). With permission from John Wiley & Sons.
Applying Developmental Science
Altering the Epigenome
These two mice are genetically identical. Both carry the agouti gene but in the yellow mouse the agouti gene is turned on all the time. In the brown mouse it is turned off.
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One of the earliest examples of epigenetics is the case of agouti mice, which carry