You may not be shocked when you hear that many siblings share similarities, such as similar intelligence levels, occupational styles, and psychological behavioral traits. You may conclude that these strong similarities are due not to similarity of genes but because of how those siblings were raised in the same family household. However, research has shown that genetically identical twins share stronger similarities than generic siblings, and while some believe that this could be due to the fact that twins are generally raised more similar than other siblings, separated twin studies have shown that much of the similarities found in siblings is a result of genetics rather than external environmental factors such as childhood upbringing.
These separated twin studies allow for psychologists to disregard external factors because of the fact that the twins were raised in separate households. Because of this researchers are able to isolate genetics and their effect of life outcomes in twins. The findings are astounding, and there are numerous stories of twins who were separated at birth yet have extremely similar lives. For instance, Gerald Levey and Mark Newman were both separated at birth, and when they reunited they discovered that they were both firefighters, and shared similar personalities. Such research is difficult to conduct because it requires for a situation in which separated twins can be analyzed many years later, yet whenever the research is properly conducted, similar and astonishing findings are seen.
I personally know two older twins from high school who were raised under a similar household, but then were sent off to different colleges having external experiences during a time of great personal development. Despite the great differences in their physical experiences and mental experiences, both twins still have very similar lives, and even now as they are separated I expect their lives to progress in a similar fashion.