Explain how twin studies were used to note the differences in the brain between people who have this diagnoses and the co-twins who do not. (what looks different about the brains of the twins where only one had the disease?

The studies of monozygotic or identical twins are used to analyze the degree of genetic influence versus the environmental contribution on a specific pathology, this is because monozygotic twins share 100% of their genes, meanwhile the dizygotic twins share only the 50% of their genes, approximately.  

The studies conducted on twins are a special type of epidemiological study because this relation allows the researchers to evaluate and compare the influence of genetic and environmental variables.  

Explanation:

Researchers have conducted twin’s studies to discuss the “nature and nurture” question, it is well known that genetics are associated with the environmental factors to produce the human individualities.  

However, the twin studies are a perfect model to evaluate the shared genetic and environmental factors for the trait of interest; in addition, it can help to estimate the proportion of variance in a particular genetic variation versus the amount of influence of shared or unshared environment. It also can reduce the amount of genetic and/or environmental variability in a genetic study.

Sahu, M., & Prasuna, J. G. (2016). Twin studies: A unique epidemiological tool. Indian journal of community medicine: official publication of Indian Association of Preventive & Social Medicine, 41(3), 177.