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).
Molecular Genetics
Another way to study the effects of genes on behavior is to examine children’s genes at the molecular (rather than the behavioral) level. Recent advances in our knowledge of the human genome and in gene research technology have allowed scientists to search for specific genes that might be partially responsible for certain disorders (Kornilov & Grigorenko, 2016).
Recall that in neurotypical individuals, genes show natural variation, called alleles. Molecular genetics is the scientific field in which researchers attempt to link the presence of specific alleles with certain attributes, behaviors, or disorders. One way to identify which alleles might be responsible for specific disorders is to conduct a linkage study. In a linkage study, researchers search the entire genetic structure of individuals (i.e., perform a “genome scan”), looking for the presence of certain alleles and the existence of a specific disorder. If researchers find certain alleles in individuals with the disorder and do not find these alleles in people without the disorder, they hypothesize that the allele is partially responsible for the disorder (Schulze & McMahon, 2019).
Researchers tend to use linkage studies when they do not know exactly where to look for genes responsible for the disorder. Given the magnitude of the human genome, it is difficult to identify links between certain alleles and specific disorders. However, researchers have successfully used linkage studies to identify alleles responsible for disorders caused by single genes, such as Huntington’s disease. Linkage studies have been less successful in identifying the causes of disorders that depend on the presence or absence of multiple genes.
An alternative technique is to conduct an association study. In an association study, researchers select a specific gene that they believe might play a role in the emergence of a disorder. Then, they examine whether there is an association between a particular allele of this “candidate” gene and the disorder (Jaffee, 2016).
For example, researchers hypothesized that a specific gene, which affects the neurotransmitter dopamine, might play a role in the development of attention-deficit/hyperactivity disorder (ADHD). They suspected this particular gene because abnormalities in dopamine have been identified as a specific cause for ADHD. Furthermore, medications that affect dopamine in the brain can reduce ADHD symptoms. The researchers identified a group of children with and without ADHD. Then, they examined whether the two groups of children had different alleles for the candidate gene. The researchers found that a certain allele for this gene was much more common among youths with ADHD compared to youth without the disorder. Consequently, they concluded that the gene may be partially responsible for ADHD (Langley, 2019).
Of course, molecular genetics research is much more complicated than has been described here. Nearly all mental disorders are influenced by multiple genes; there is almost never a one-to-one relationship between the presence of a specific allele and the emergence of a given disorder. Furthermore, genes never affect behavior directly; their influence on behavior is always influenced by environmental experience (Kornilov & Grigorenko, 2016).
Review
Genes come in different variants, called alleles. The alleles we inherit from our parents can influence our physical attributes (e.g., hair and eye color) as well as our risk for developing certain disorders.
Behavioral geneticists conduct family, adoption, and twin studies to determine the heritability of psychological characteristics like intelligence, personality, and mental health problems.
Molecular geneticists conduct linkage and association studies to identify specific genes that may underlie certain disorders.
How Do Genetic and Environmental Factors Interact?
The Diathesis–Stress Model
Genes guide our maturation, but they do not determine our development. Our genotype refers to the genetic code that we inherit from our parents. In contrast, our phenotype is the observable expression of our genetic endowment. Our phenotype is determined by the complex interaction between our genes and our environment (Grigorenko et al., 2016).
The diathesis–stress model can be used to explain the way genes and environments interact and affect development. According to this model, a child exhibits a disorder when an underlying genetic risk for the disorder is triggered by a stressful experience or life event. Both genetic risk and an environmental stressor are necessary for the disorder to emerge; the genetic risk or environmental experience alone is insufficient to bring about the disorder (Plomin et al., 2017).
We can see the usefulness of the diathesis–stress model in a famous study conducted by Avshalom Caspi and colleagues (2003). The researchers followed a large group of children from early childhood through early adulthood in order to examine the relationship between child maltreatment and depression later in life. As we might expect, children exposed to maltreatment were at risk for depression later in life. However, whether a maltreated child developed depression depended on his or her genotype (Figure 2.3).
Children who did not experience maltreatment were at low risk for depression later in life, regardless of their genes. However, children exposed to severe maltreatment displayed different outcomes, depending on their genotypes. Specifically, children who inherited one or two short alleles of the serotonin transporter gene were likely to develop depression in adulthood. Interestingly, this gene regulates the neurotransmitter serotonin, a chemical that plays an important role in mood regulation. The short version of this gene seems to place children at risk for depression if they also experience maltreatment. In contrast, children who inherited two long alleles of the serotonin transporter gene were not more likely to develop depression in adulthood, even if they were exposed to maltreatment. The long version of this gene seems to protect children from the effects of stressful life events.
Figure 2.3 ■ The Diathesis–Stress Model
Note: Children’s likelihood of depression depends on their genetic risk and an environmental stressor (i.e., maltreatment). SS = two short alleles, SL = one short, one long allele, LL = two long alleles. Based on Caspi and colleagues (2003).
The diathesis–stress model is especially helpful in explaining multifinality, the tendency of children exposed to the same environmental stressor to show different developmental outcomes. In Caspi and colleagues’ (2003) study, maltreated children showed divergent outcomes depending on their genetic risk.
Gene–Environment Correlation
The diathesis–stress model shows that both genes and environment influence development. A second influential model, developed by Sandra Scarr and Kathleen McCartney (1983), shows that genes and environments are not independent. According to the gene–environment correlation model, we sometimes select environments that complement our genotypes. Specifically,
