Twin studies are the most favorable ones to see how and to what extend our genes affect our behaviors. This is because it is almost impossible to take into account infinite number of environmental factors but we have certain information to statistically analyze about the kinship (the rate of common gene). For instance, while identical twins have the same genes, fraternal twins have 50 percent of their genes in common. In addition, to compare, adopted children could be examined since they do not have kinship but are raised in the same environment.
In the case of Jim twins, after their birth they were separated and adopted and raised by different families. They found each other at the age of 39. They realized they have many physical characteristics and also social preferences in common. For instance, they both drive Chevrolets, they both smoke the same brands of cigarettes, they both are interested in woodworking. And also they are both heavy drinkers and heavy smokers. They got the almost the same scores on some social and psychological tests, such as intelligence and extraversion tests. These similarities might derive from genes. According to behavioral genetics theory, genes determine not only the behavior or personality but also the environment. So, the case of the Jim twins demonstrates this determination clearly since they had different environments.