Psychologists believe human behavior is influenced by a combination of genes and the environment. Heredity plays an important role in many disorders, including anxiety, depression, and schizophrenia. Whether a predisposition becomes expressed in a person’s life may depend on environmental factors such as the quality of the family relationship or the level of life stress. No individual gene accounts for complex psychological traits by itself; genes do not dictate our personalities, writes Mark Goldfeder. The role of genetics is not a certainty that particular behaviors, abilities, or psychological traits or disorder will develop, he says. The genotype, or genetic code, is a kind of recipe for determining the features or traits of an organism, he writes. But whether the genotype is expressed in the organism’s observable traits, or phenotype, depends on a complex interaction of genes, he adds. Goldfeder: Genes are not the sole arbiter of personality.
Familial Association Studies
The more closely related people are, the more genes they have in common. More distant relatives have fewer genes in common. The study method is called a familial association study, used to examine family linkages in disorders such as schizophrenia. Researchers find a greater risk of the disorder among closer blood relatives of schizophrenia patients than among more distant relatives. The closer their blood relationship, the more likely people are to share common environments.
In identical twins, a fertilized egg cell splits into two cells, and each one develops into a separate person. Fraternal twins share only 50 percent of their genetic make-up. Researchers find identical twins are more likely than fraternal twins to share some psychological traits. Researchers compare concordance rates, or percentages of shared traits or disorders, in twin studies. The problem is that identical twins may be treated more alike than fraternity twins. The higher rate of concordances suggests a genetic contribution to the disorder or trait.
The clearest way to separate the roles of environment and heredity is to conduct adoptee studies. When identical twins are separated at an early age and reared apart in separate adoptive families, we can attribute any differences to environmental factors since their genetic make-up is the same. This natural experiment does not happen often, but when it does, it provides a special opportunity to study the role of the environment.