As you have previously read, behavioral genetics is the scientific study of the interplay between genetic and environmental contributions to behavior. Often referred to as the nature/nurture debate, Gottlieb (1998, 2000, 2002) suggests an analytic framework for this debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay indicates that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Additionally, environmental circumstances can trigger symptoms of a genetic disorder. For example, a person who has sickle cell anemia, a recessive gene linked disorder, can experience a sickle cell crisis under conditions of oxygen deprivation. Or, someone predisposed genetically to type-two diabetes can trigger the disease through poor diet and little exercise.

Research has shown how the environment and genotype interact in several ways. Genotype-Environment Correlations refer to the processes by which genetic factors contribute to variations in the environment (Plomin et al., 2013). There are three types of genotype-environment correlations:

Passive genotype-environment correlations occur when children passively inherit both the genes and the environments their family provides. Certain behavioral characteristics, such as being athletically inclined, may run in families. Children in these families have inherited both the genes that would enable success at these activities, and an environment that encourages them to engage in sports. Figure 4.8 highlights this correlation by demonstrating how a family passes on water skiing skills through both genetics and environmental opportunities.

Evocative genotype-environment correlations refer to how the social environment reacts to individuals based on their inherited characteristics. For example, whether children have a more outgoing or shy temperament will affect how they are treated by others.

Active genotype-environment correlations occur when individuals seek out environments that support their genetic tendencies. This is also referred to as niche picking. For example, children who are musically inclined seek out music instruction and opportunities that then facilitate their natural musical ability.

Conversely, genotype-environment interactions involve genetic susceptibility to the environment. Adoption studies provide evidence for genotype-environment interactions. For example, the Early Growth and Development Study (Leve et al., 2010) followed 360 adopted children and their adopted and biological parents in a longitudinal study. Results have shown that children whose biological parents exhibited psychopathology, exhibited significantly fewer behavior problems when their adoptive parents used more structured (vs. unstructured) parenting. Additionally, higher levels of psychopathology in adoptive parents increased the risk that children would develop behavior problems, but only when the biological parents’ psychopathology was also high. Consequently, these results show that environmental effects on behavior differ based on the individual’s genotype, especially the effects of stressful environments on genetically at-risk children.

Lastly, the study of epigenetics examines modifications in DNA that affect gene expression and are passed on when the cells divide. Environmental factors, such as nutrition, stress, and teratogens are thought to change gene expression by switching genes on and off. These gene changes can then be inherited by daughter cells. This would explain why monozygotic or identical twins may increasingly differ in gene expression with age. For example, Fraga et al. (2005) found that when examining differences in DNA, a group of monozygotic twins were indistinguishable during the early years. However, when the twins were older there were significant discrepancies in their gene expression, most likely due to different experiences. These differences included a range of personal characteristics, including susceptibilities to disease.

Conclusions

As you’ve read in this section, there is a genetic component to nearly every human trait, but our genes do not determine every trait we express. The complex relationships between genes and the environment contribute to diversity in our personalities and traits, meaning that even if two individuals are genetically identical (as in the case of monozygotic twins) they will have some differences between them. These complex interactions begin as soon as a genetically unique zygote (or zygotes, in the case of twins) is created at conception. Our next chapter will explore the ways that the prenatal environment begins to shape the course of development.