Our genetic makeup is not necessarily a permanent fixture; it can be influenced by several factors. Among these factors are the social and environmental influences in which we live, including anything from light and temperature to exposure to chemicals. The environment in which a person is raised can trigger the expression of behavior for which a person is genetically predisposed, while the same person raised in a different environment may exhibit different behavior.
Long-standing debates have taken place over the idea of which factor is more important – genes or environment. Is a person destined to have a particular outcome in life because of his or her genetic makeup, or can environment work to change what might be considered "bad" genes? Today, it is generally agreed upon that neither genes nor environment work alone; rather, these two work in tandem to create the person we ultimately become.
Environmental elements like light and temperature have been shown to induce certain changes in genetic expression; additionally, exposure to drugs and chemicals can significantly affect how genetics are expressed. People often inherit sensitivity to the effects of various environmental risk factors, and different individuals may be differently affected by exposure to the same environment in medically significant ways. For example, sunlight exposure has a much stronger influence on skin cancer risk in fair-skinned humans than in individuals with an inherited tendency to darker skin. The color of a person's skin is largely genetic, but the influence of the environment will affect these genes in different ways.
Gene-environment correlations, known as rGE, can be explained in 3 particular ways – passive, evocative, or active. In passive gene-environment correlation, an association exists between a person's genetic makeup and the environment in which he or she is raised. In other words, the person's environment, particularly in the case of children, is largely determined by the parent's genetic characteristics. Parents create a home environment that is influenced by their own heritable characteristics. When the children's own genotype influences their behavior or cognitive outcomes, the result can be a misleading relationship between environment and outcome. For example, an intelligent parent is likely to create a home environment rich in educational materials and experience. Since intelligence is moderately heritable, it can be argued that intelligence in the child is inherited rather than a factor of the home environment created by the parents. It is relatively unclear whether the genetic or environmental factors had more to do with the child's development.
Evocative gene-environment correlation happens when an individual's (heritable) behavior evokes an environmental response. For example, the association between marital conflict and depression may reflect the tensions that arise when engaging with a depressed spouse rather than a causal effect of marital conflict on risk for depression.
In active gene-environment correlation, the person's genetic makeup may lead them to select particular environments. For example, a shy person is likely to choose quiet activities and less boisterous environments than an extroverted individual. He or she may be more likely to be found at the library rather than a dance club.
Adoption and Twin Studies
Adoption and twin studies can help make sense of the influence of genes and the environment. Studies of adult twins are used to investigate which traits are heritable. Identical twins raised apart tend to be similar in intelligence and, in some cases, life events and circumstance when studied years later, even when raised separately. For adoption studies, since the child being raised by parents genetically different from his or her biological parents, the influence of the environment shows in how similar the child is to his or her adoptive parents or siblings. Adoption studies make a case for the strong influence of environment, whereas twin studies make a strong case for genetic influence.