As each day is perfectly unique, no two people are ever perfectly identical. How much our genetics and our environment influence our individuality is a question Scientists have been exploring for centuries. Nature is our genetic make-up, which we get from our parents, while nurture is a combination of our environment and our choices. Each choice we make and the environment in which we live has an effect on who we are and what happens to us. Therefore, our genetics are not our destiny. The infinite possibilities of nature and nurture make it difficult to figure out what causes increased risk of disease; nature or nurture? How can we isolate the few variables which provide causal relationships with diseases arising from our environment and lifestyle or our genetics?
One such ways is through studying twins. Twin Studies have been performed since 5th century BCE and are significant in that they “help researchers tease apart nature, our genes, genetic code and nurture, our environment.” The modern father of twin study, Francis Galton, explored the effects of nature verses nurture in twins in 1875. To understand how twin studies can be differentiated between nature and nurture, one must understand that there are two types of twins; monozygotic twins (identical twins) and dizygotic twins (non-identical twins). Monozygotic twins, come from a single fertilized egg, (one egg and one sperm) and results in the twins sharing 100% of their genes, and the monozygotic twins are thus genetically identical. Dizygotic twins, come from two individually fertilized eggs (2 eggs, 2 sperm) which are fertilized and in the womb at the same time, and result in the twins sharing 50% of their genes just like regular siblings.
Researchers from the medical, psychological and behavioral fields try to establish the relative contributions of the environmental and genetic factors which contribute to the risk for developing a given disease or behavior. Identical twins are identical in genetic make-up and thus any difference in diseases or behaviors is (almost) solely due to differences in the environment. The environment can be behaviors like exercise, nutrition, social group influence, intrauterine conditions or any number of things. While monozygotic twins often share much of their environment in the early stages of their lives as they ideally grow up in the same house at the same time with the same parents and social circle, they also differ in specific ways throughout life. Those environmental differences can then be considered to explain different susceptibility to disease, different traits and different emotions.
Scientists are interested in finding monozygotic twins who were raised in different environments or have different lifestyle habits. Such an approach allows scientists to isolate lifestyle factors by controlling for genetics. For example, in one study done on monozygotic twins, who moved apart and one of which developed dementia focused on the contribution of lifestyle to developing the disease in a paper titled Potentially modifiable risk factors for dementia in identical twins. Scientists retrospectively found differences between the sets of twins with regards to lifestyle habits, such as education and physical activity diet and exercise. The results suggest that the monozygotic twin with dementia was significantly less educated and participated in less physical activity than their twin without dementia.
The use of monozygotic twins are the closest we are today, to having two individuals be genetically identical. With these unique individuals, we have gained knowledge leading to the underlying non-genetic causes of diseases, traits and behaviors as well as further understanding and possible treatments. For instance, using monozygotic twins where one is physically active and the other is sedentary, can show the positive effects and necessity of exercise, independent of genetics, in preventing chronic diseases. Exercise promotes overall health and healthy lifespan. Twin studies help us to understand the bigger picture of the effects of our genetics and our environment.
Fraga, M. F., Ballestar, E., Paz, M. F., Ropero, S., Setien, F., Ballestar, M. L., . . . Esteller, M. (2005). From The Cover: Epigenetic differences arise during the lifetime of monozygotic twins. Proceedings of the National Academy of Sciences, 102(30), 10604-10609.
Gatz, Margaret et al. (2006). Potentially modifiable risk factors for dementia in identical twins. Alzheimer’s & Dementia. The Journal of the Alzheimer’s Association, Volume 2, Issue 2, 110-117.