Why Twin Studies?

Twins provide a valuable source of information for health and psychological research, as their unique relationship allows researchers to pull apart and examine genetic and environmental influences.  Twin study findings have been influential in detecting and treating various diseases and psychological disorders. 

How are they able to do this?  Twin studies allow researchers to examine the overall role of genes in the development of a trait or disorder.  Comparisons between monozygotic (MZ or identical) twins and dizygotic (DZ or fraternal) twins are conducted to evaluate the degree of genetic and environmental influence on a specific trait.  MZ twins are the same sex and share 100% of their genes.  DZ twins can be the same- or opposite-sex and share, on average, 50% of their genes.

If MZ twins show more similarity on a given trait compared to DZ twins, this provides evidence that genes significantly influence that trait.  However, if MZ and DZ twins share a trait to an equal extent, it is likely that the environment influences the trait more than genetic factors.  For example, Figure 1 presents MZ and DZ twin correlations for several physical and psychological characteristics.  As shown in the figure, MZ twins, on average, are twice as similar as DZ twins for these characteristics, suggesting that genes influence the development of these traits.

In addition to traditional twin studies, children-of-twins studies are a particularly useful way to examine genetic and environmental influences on traits and disorders and their transmission across generations.  For example, since MZ (identical) twins share 100% of their genes, the children of MZ twins are as genetically related to their twin parent’s twin sister or twin brother as they are to their parent.  Given this, their cousins through that aunt or uncle are like genetic half-siblings.  As these cousins typically live in different households, this relationship allows for a unique perspective into possible interactions between genetic influences and environmental factors. 

Results from twin studies may suggest that a particular trait or disorder has a genetic component, but this does not provide information about the location of this gene or genes.  Nevertheless, twin designs and methods are extremely useful for understanding the extent to which psychological and medical disorders, as well as behaviors and traits, are influenced by genetic factors.  This information can then be used to develop better ways to prevent and treat disorders and maladaptive behaviors.  Indeed, some of the most effective treatments for medical disorders (e.g., early onset breast cancer) have been developed partly as a result of twin study research.