I recently read an intriguing article on twin studies comparing monozygotic twins to dizygotic twins to opposite sex dizygotic twins. The study was interested in finding whether the monozygotic twins who shared 100% of their genes shared more similar traits than the dizygotic twin sets that had nearly half that amount of shared genes. Obviously, this is a twin study to test whether genetics are influential in characteristics, such as extraversion or intelligence, or whether environmental experiences are more influential. If the monozygotic (MZ) twins illustrate stronger correlation in characteristic similarity than the dizygotic (DZ) twins, it can be assumed genetics are influencing characteristics.
This was a controlled experiment, but did contain a third variable, "equal environments". The researchers found that similarity of treatment for each twin during childhood was a possible contributing factor in development. For instance, the MZ twins resembled one another more than the DZ twins and therefore were treated as one compared to DZ twins being treated different. They provided three possible explanations to their findings. Explanation A, "there are no direct effects whatsoever of treatment similarity on resemblance in behavior or vice versa". Explanation B, "there are direct effects, but they run from the more similar behavior of MZ twins to their more similar environments, implying that the more similar environment of MZ twins is a child effect". Lastly, explanation C, "there are direct effects of the more similar treatment of MZ twins on their more similar behavior." (BorkenauLast, Riemann, Angleitner & Spinath, 2001)
The results indicated a stronger correlation of similar traits between MZ twins than same sex DZ twins and even more so than DZ opposite sex. The third type of twins adds to my belief that "equal environments" are a contributing factor. Sociology has proven that girls and boys are put into gender roles growing up and are majority of the time treated differently. Since the DZ opposite sex twins had the lowest correlation of similar traits, this could support the environment developing who we are. However, I must also consider males and females as generally having differing traits.
Results concerning the MZ twins and their stronger positive correlation of trait similarity are supportive to genetics developing who we are. However, the MZ twins also spent more time together, resembled each other more, were mistaken for the other twin more, shared more similar experiences and were generally dressed alike more than DZ twins. These are factors one must consider when observing data because they are environmental influences that could be contributing to the MZ twin's similar genetics. With all these factors and statistical evidence that supports both sides of the nature vs. nurture debate, it's difficult to give a point blank answer.