Knowledge gained from studies of twins may help researchers improve the diagnosis, treatment, and prevention of many health conditions. Twin studies are the ideal way for scientists to discover the relative contribution of heredity (genes) and environment to certain health conditions.

Hereditary information is passed from parents to their children through their genes, which are made of a complex molecule called DNA. Environmental factors include family, parental behaviors, peers, lifestyles, exposures to various substances or phenomena (such as air, noise, and chemicals), and built and social environmental factors such as neighborhood walkability and social deprivation.

Both identical and fraternal twins are essential to twin studies. Twins are always the same age, and they usually share a similar environment, both in the womb and while growing up. Identical twins share 100% of their genes, while fraternal twins share only about half their genes (just like regular siblings).

The concept of concordance is important in twin studies. Concordance between twins means that both members of the pair share the condition under study. To see how much heredity (genetics) contributes to a certain condition, scientists compare the concordance in identical vs. fraternal twins. If the concordance is greater in identical twins than in fraternal twins, a strong hereditary (genetic) contribution to the condition is suggested. If the concordance in identical and fraternal twins is similar, then environmental factors may play an important role.

The concept of discordance is equally important. The growing field of epigenetics has given researchers a new tool for understanding how small changes in our genes can influence our health and wellbeing. These differences can develop in the womb, or they can be triggered by the environment.  Epigenetic twin studies typically involve identical twins pairs in which one twin has a condition, such as depression, and the other does not. Since identical twins share 100% of their genes, studying the differences can help researchers identify what factors may trigger a condition. For more about epigenetics, click here.