Twin Studies

The study of twins in scientific research allows researchers to attempt to disentangle the effects of genetic and environmental effects on biology and psychology. Twin studies have provided valuable insight into detecting and treating various diseases and psychological disorders, as they reveal the importance of genetic and environmental influences on traits, phenotypes, and disorders. Twin research is a key tool in the field of behavioral genetics, and twin studies are part of the broader methodology used in behavior genetics, which uses genetically informative data to track a variety of traits ranging from personal behavior to the presentation of severe mental illness such as schizophrenia. In attempting to describe, understand, and explain how learning takes place throughout a person’s life through educational research, twin studies provide an ideal framework to distinguish the effects of genetics from that of the environment.

Monozygotic (MZ, or identical) twins share 100% of their genetic material and as such are genetically identical, whereas dizygotic (DZ, or fraternal) twins share, on average, 50% of their genes, which is the same as a normal sibling relationship. Because MZ twins are genetically identical, most of their differences on certain traits (e.g., height, intelligence, depression) are due to environmental experiences that vary between the twins. Comparing the phenotypic expression of MZ twins and DZ twins on a specific trait can provide insight into the degree of genetic and environmental influence of that trait. If MZ twins are more similar on a specific trait than DZ twins, then this provides evidence that this trait is largely influenced by genes. However, if MZ and DZ twins share a trait to an equal extent, then it is likely that the environment influences the trait more than genetic factors.

Twins also share many aspects of their environment, including their uterine environment, parenting style, education, socioeconomic status, and community because they are born into the same family. However, twin studies are still useful to study trait presentation in twins when there are unique environmental differences between them, such as an event or occurrence that has only affected one twin, like a head injury or birth defect. The presence of a given trait in only one identical twin (called discordance) provides powerful insight into environmental effects on that trait.

To maximize the available data for twin studies, large, worldwide registers of data on twins and their relatives have been established as resources. These registers no longer focus on the assessment of a single phenotype but collect a wide range of traits and environmental factors in twins and their family members. These registers make it possible to conduct analyses of many different variables in relatives, such as using multivariate analysis or including covariates when assessing for the interaction between genotype and environment in influencing a certain trait. This entry reviews the history and methodology of twin studies.