Researchers do different kinds of studies to see whether and to what extent a characteristic might be genetically transmitted.
In family studies, researchers look at similarities among members of a family with respect to a particular trait. If the trait is genetically inherited, it should be similar in blood relatives. The closer the blood relationship, the more similar people should be.
Family studies alone don’t reveal whether a trait is genetically inherited. A family shares genes, but they also share similar environments. When researchers find trait similarity in a family study, their findings may suggest that the trait is genetically inherited, but the study can’t prove it.
Compared to family studies, twin studies give researchers more solid evidence about whether a trait is inherited. In twin studies, researchers compare pairs of identical twins to fraternal, or nonidentical, twins. When doing these studies, researchers assume that identical twin pairs share the same environment, just as fraternal twin pairs do. However, identical twins share all of their genes with each other, while fraternal twins share only half of their genes. When a trait shows more similarity between identical twins than between fraternal twins, the greater similarity probably comes from shared genes, not shared environment.
One problem with this type of study is that identical twins may not in fact share an identical environment while fraternal twins do. People tend to treat identical twins in unusual ways. For example, people may treat identical twins as if they are similar in every respect, or they might focus intensely on differences between them.
In order to avoid uncertain environmental factors, researchers sometimes study separated twins. Twins who are separated when they are very young and brought up in different families have different environmental influences but identical genes. Trait similarities between separated twins result mostly from genes.